A bifunctional 3,5-epimerase/4-keto reductase for nucleotide-rhamnose synthesis in Arabidopsis

l-Rhamnose is a component of plant cell wall pectic polysaccharides, diverse secondary metabolites, and some glycoproteins. The biosynthesis of the activated nucleotide-sugar form(s) of rhamnose utilized by the various rhamnosyltransferases is still elusive, and no plant enzymes involved in their synthesis have been purified. In contrast, two genes (rmlC and rmlD) have been identified in bacteria and shown to encode a 3,5-epimerase and a 4-keto reductase that together convert dTDP-4-keto-6-deoxy-Glc to dTDP-beta-l-rhamnose. We have identified an Arabidopsis cDNA that contains domains that share similarity to both reductase and epimerase. The Arabidopsis gene encodes a protein with a predicated molecular mass of approximately 33.5 kD that is transcribed in all tissue examined. The Arabidopsis protein expressed in, and purified from, Escherichia coli converts dTDP-4-keto-6-deoxy-Glc to dTDP-beta-l-rhamnose in the presence of NADPH. These results suggest that a single plant enzyme has both the 3,5-epimerase and 4-keto reductase activities. The enzyme has maximum activity between pH 5.5 and 7.5 at 30 degrees C. The apparent K(m) for NADPH is 90 microm and 16.9 microm for dTDP-4-keto-6-deoxy-Glc. The Arabidopsis enzyme can also form UDP-beta-l-rhamnose. To our knowledge, this is the first example of a bifunctional plant enzyme involved in sugar nucleotide synthesis where a single polypeptide exhibits the same activities as two separate prokaryotic enzymes.     

The identification and characterization of two separate carboxylesterases in guinea-pig serum.

The esterase activity of guinea-pig serum was investigated. A 3-fold purification was achieved by removing the serum albumin by Blue Sepharose CL-6B affinity chromatography. The partially purified enzyme preparation had carboxylesterase and cholinesterase activities of 1.0 and 0.22 mumol of substrate/min per mg of protein respectively. The esterases were labelled with [3H]di-isopropyl phosphorofluoridate (DiPF) and separated electrophoretically on sodium dodecyl sulphate/polyacrylamide gels. Two main labelled bands were detected: band I had Mr 80 000 and bound 18-19 pmol of [3H]DiPF/mg of protein, and band II had Mr 58 000 and bound 7 pmol of [3H]DiPF/mg of protein. Bis-p-nitrophenyl phosphate (a selective inhibitor of carboxylesterase) inhibited most of the labelling of bands I and II. The residual labelling (8%) of band I but not band II (4%) was removed by preincubation of partially purified enzyme preparation with neostigmine (a selective inhibitor of cholinesterase). Paraoxon totally prevented the [3H]DiPF labelling of the partially purified enzyme preparation. Isoelectrofocusing of [3H]DiPF-labelled and uninhibited partially purified enzyme preparation revealed that there were at least two separate carboxylesterases, which had pI3.9 and pI6.2, a cholinesterase enzyme (pI4.3) and an unidentified protein that reacts with [3H]DiPF and has a pI5.0. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of these enzymes showed that the carboxylesterase enzymes at pI3.9 and pI6.2 corresponded to the 80 000-Mr subunit (band I) and 58 000-Mr subunit (band II). The cholinesterase enzyme was also composed of 80 000-Mr subunits (i.e. the residual labelling in band I after bis-p-nitrophenyl phosphate treatment). The unidentified protein at pI5.0 corresponded to the residual labelling in band II (Mr 58 000), which was insensitive to neostigmine and bis-p-nitrophenyl phosphate. These studies show that the carboxylesterase activity of guinea-pig serum is the result of at least two separate and distinct enzymes.     

Purification and characterization of an enzymically active cleavage product of pig kidney aldehyde reductase

During the purification of pig kidney aldehyde reductase by an established procedure [Flynn, Cromlish & Davidson (1982) Methods Enzymol. 89, 501-506] a second enzyme with aldehyde reductase activity may be purified. When the procedure was performed in the presence of 5 mM-EDTA, only traces of the second reductase, pig kidney aldehyde reductase (minor form), were present. By the criterion of sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, pig kidney aldehyde reductase (minor form) had Mr 35 000, in comparison with Mr 40 200 found for pig kidney aldehyde reductase. Amino acid analysis of both enzymes and tryptic-peptide-map comparisons indicated differences in primary structure. The N-terminus of pig kidney aldehyde reductase (minor form) had the sequence Lys-Val-Leu, in contrast with the blocked (acetylated) N-terminus of pig kidney aldehyde reductase. The C-terminal sequence of both enzymes was the same. Both reductases were immunologically identical by double immunodiffusion and rocket immunoelectrophoresis. Pig kidney aldehyde reductase (minor form) had 50% of the specific activity of pig kidney aldehyde reductase when tested with a variety of aldehyde substrates. Michaelis constants of both enzymes for these substrates and for NADPH were similar, but values for kcat. and kcat./Km indicated that catalytically pig kidney aldehyde reductase was the more efficient enzyme. Typical aldehyde reductase inhibitors, such as phenobarbital and sodium valproate, had the same effect on both enzymes. It was concluded that pig kidney aldehyde reductase (minor form) is an enzymically active cleavage product of pig kidney aldehyde reductase which is formed when the latter is purified in the absence of the metalloproteinase inhibitor EDTA.     

A peptidase in human platelets that deamidates tachykinins. Probable identity with the lysosomal "protective protein".

We discovered an enzyme in human platelets that deamidates substance P and other tachykinins. Because an amidated carboxyl terminus is important for biological activity, we purified and characterized this deamidase. The enzyme, released from human platelets by thrombin, was purified to homogeneity by ammonium sulfate precipitation, followed by chromatography on an octyl-Sepharose column and chromatofocusing on PBE 94. The purified enzyme exhibits esterase, peptidase, and deamidase activities. The peptidase activity (with furylacryloyl-Phe-Phe) is optimal at pH 5.0 while the esterase (benzoyl-tyrosine ethyl ester) and deamidase (D-Ala2-Leu5-enkephalinamide) activities are optimal at pH 7.0. With biologically important peptides, the enzyme acts both as a deamidase (substance P, neurokinin A, and eledoisin) and a carboxy-peptidase (with bradykinin, angiotensin I, substance P-free acid, oxytocin-free acid) at neutrality, although the carboxypeptidase action is faster at pH 5.5. Enkephalins, released upon deamidation of enkephalinamides, were not cleaved. Gly9-NH2 of oxytocin was released without deamidation. Peptides with a penultimate Arg residue were not hydrolyzed. Some properties of the deamidase are similar to those reported for cathepsin A. The deamidase is inhibited by diisopropylfluorophosphate, inhibitors of chymotrypsin-type enzymes, and mercury compounds while other inhibitors of catheptic enzymes, trypsin-like enzymes, and metalloproteases were ineffective. In gel filtration, the native enzyme has an Mr = 94,000 while in non-reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis the Mr = 52,000 indicating it exists as a dimer. After reduction, deamidase dissociates into two chains of Mr = 33,000 and 21,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. [3H]diisopropylfluorophosphate labeled the active site serine in the Mr = 33,000 chain. The first 25 amino acids of both chains were sequenced. They are identical with the sequences of the two chains of lysosomal "protective protein" which, in turn, has sequence similarity to the KEX1 gene product and carboxypeptidase Y of yeast. This protective protein complexes with beta-galactosidase and neuraminidase in lysosomes and is vitally important in maintaining their activity and stability. A defect in this protein is the cause of galactosialidosis, a severe genetic disorder. The ability of physiological stimuli (e.g. thrombin or collagen) to release the deamidase from platelets indicates that it may also be involved in the local metabolism of bioactive peptides.     

Isolation of rat liver microsomal short-chain beta-ketoacyl-coenzyme A reductase and trans-2-enoyl-coenzyme A hydratase: evidence for more than one hydratase

An enzyme preparation (IIIB) isolated from liver microsomes of untreated male rats was found to contain two activities--short-chain trans-2-enoyl-CoA hydratase and beta-ketoacyl-CoA reductase. The hydratase was purified more than 1000-fold, while the reductase activity was purified over 600-fold. Employing sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, a single band with a molecular weight of 76,000 was observed. Although attempts to separate these two activities have failed, it remains to be established whether the final preparation contains a single enzyme with two activities or two separate enzymes. The hydratase was most active toward crotonyl-CoA, followed by trans-2-hexenoyl-CoA (6:1) and -octenoyl-CoA (8:1); the enzyme was essentially inactive toward substrates containing more than eight carbon atoms. The Vmax for crotonyl-CoA was 2117 mumol/min/mg protein, while the Km was 59 microM. Using acetoacetyl-CoA as substrate, the Vmax for the beta-ketoacyl-CoA reductase was over 60 mumol/min/mg protein and the Km was 37 microM; the Vmax for beta-ketopalmitoyl-CoA was only 15% of that observed with acetoacetyl-CoA, although the Km was 6 microM. During the course of purification, a second short-chain hydratase was discovered (fraction IVA); unlike IIIB, this fraction catalyzed the hydration of 4:1, 6:1, and 8:1 at similar rates. The partially purified preparation yielded maximal activity with 8:1 CoA (apparent Vmax 35 mumol/min/mg), followed by 6:1 CoA, 4:1 CoA, and 10:1 CoA; longer chain CoA's were relatively poor substrates, with trans-2-hexadecenoyl CoA about 0.1 as active as 8:1 CoA. On SDS-gels, fraction IVA contained four bands, all of which were below 60,000 Mr. Proteases, such as trypsin, chymotrypsin, and subtilisin, were found to completely inactivate both enzyme fractions.     

Isolation from pig lens of two proteins with dihydrodiol dehydrogenase and aldehyde reductase activities.

Dimeric and monomeric proteins containing dihydrodiol dehydrogenase and aldehyde reductase activities were purified from pig lens. The dimeric enzyme of Mr 65,000 specifically oxidized the trans-dihydrodiols of naphthalene and benzene with NADP+ as a strict cofactor, and reduced alpha-diketones, aromatic aldehydes and glyceraldehyde with NADPH as a cofactor. The monomeric enzyme of Mr 35,000, although identical with aldose reductase, oxidized the trans-dihydrodiol of naphthalene at a pH optimum of 7.6. These results suggest that the two enzymes are involved in the pathogenesis of naphthalene cataract.     

Purification and some characteristics of nitric oxide reductase-containing vesicles from Paracoccus denitrificans

Nitric oxide reductase of Paracoccus denitrificans was purified, with the use of 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO) detergent, as membrane vesicles of apparent Mr = 2-3 x 10(6). Fifty percent of the protein was a peptide of Mr = 34,000. Further fractionation with sodium dodecyl sulfate (SDS) resulted in vesicles in which the peptide constituted 90-95% of the protein. This peptide, which is rich in Ala, Gly, Ser, Asx, and Glx, is considered to be the peptide of nitric oxide reductase. The CHAPSO- and SDS-fractionated preparations lost activity at 4 degrees C, pH 7.4, with half-times, respectively, of about 6 days and 4 h. Specific activities at 32 degrees C, pH 7.4, of about 0.33 mumol of NO x min-1 x mg-1 were realized after fractionation with CHAPSO in a phenazine methosulfate/ascorbate-based assay. The Km(NO) was less than or equal to 17 microM at pH 7.4. Rates decreased substantially below pH 5 and above pH 7.6. The preparations were free or almost free of cytochromes, exhibited otherwise no absorption bands in the visible region, contained no redox metals except for very small amounts of iron, were not inhibited by EDTA or some other common inhibitors of redox-metal enzymes, and were not observed to catalyze the reduction of nitrate, nitrite, or N2O. An absorption band at 274 nm in both the CHAPSO- and SDS-fractionated preparations was attributed to the presence of a solvent-soluble chromophore. N-Bromosuccinimide (NBS) inactivated the enzyme and bleached the chromophore both in the enzyme preparation and, after its purification, in 95% ethanol. NBS-inactivated enzyme could be reconstituted with purified chromophore, which alone seemed to have no nitric oxide reductase activity, but not with purified chromophore that had been reacted with NBS. Spectral changes interpretable as due to changes in redox state were not observed when enzyme was exposed to NO or certain reducing agents.     

Ribonucleotide reductase of Brevibacterium ammoniagenes is a manganese enzyme

Ribonucleotide reduction and not DNA replication is the site for the specific manganese requirement of DNA synthesis and cell growth in the coryneform bacterium Brevibacterium ammoniagenes. To characterize the metal effect we have isolated and purified ribonucleoside-diphosphate reductase from overproducing bacteria that were first deprived of and then reactivated by manganese ions. Purification on columns of Sephacryl S400, DEAE-cellulose and hydroxyapatite provided an apparently homogeneous enzyme consisting of two protein subunits. These were characterized by affinity chromatography on 2',5'-ADP-Sepharose as nucleotide-binding protein B1 (Mr = 80,000) and catalytic protein B2 (Mr = 100,000, composed of two Mr = 50,000 polypeptides), which were both necessary for activity. In vitro the purified enzyme does not require added metal ions except for an unspecific, twofold activity increase observed in the presence of Mg2+ and other divalent cations. Enzyme activity is inhibited by hydroxyurea (I50 = 2.5 mM). The electronic spectrum with maxima around 455 nm and 485 nm closely resembles that of manganese(III)-containing pseudocatalase and of oxo-bridged binuclear Mn(III) model complexes. Denaturation of the enzyme in trichloroacetic acid liberated an equimolar amount of Mn(II) which was detected by EPR spectroscopy. It was not possible to remove and reintroduce metal ions without loss of enzyme activity. Manganese-deficient cell cultures were also grown in the presence of 54MnCl2. Ribonucleotide reductase activity and radioactivity cochromatographed in several systems. Non-denaturing polyacrylamide gel electrophoresis showed that protein subunit B2 was specifically 54Mn-labeled. All these properties suggest that the ribonucleotide reductase of B. ammoniagenes is a manganese-containing analog of the non-heme-iron-containing reductases of Escherichia coli and eukaryotes.     

Molecular basis of the biological function of molybdenum. Developmental patterns of sulfite oxidase and xanthine oxidase in the rat

The developmental patterns of the molybdenum-containing enzymes sulfite oxidase and xanthine oxidase and of the mitochondrial enzymes adenylate kinase and succinate-cytochrome c reductase in rat liver are reported. Adenylate kinase and succinate-cytochrome c reductase develop in parallel with total liver protein and are maximal 5 days after birth. Sulfite oxidase, which is also a mitochondrial protein, shows its largest increase in activity between 5 and 11 days after birth. The appearance of sulfite oxidase and xanthine oxidase proteins parallels very closely the development of their respective activities. Xanthine oxidase activity is extremely low in rats prior to weaning at 21 days. Development of activity of this enzyme may be related to the protein nutritional status of the young animal. The development of both sulfite oxidase and xanthine oxidase activities is very much impaired by administration of tungsten to the pregnant rats for 20 days before birth of the litters. Apparently normal development of sulfite oxidase protein, however, leads to the accumulation of inactive molecules in the livers of offspring of tungsten-fed rats. Development of adenylate kinase and succinate-cytochrome c reductase activities is not affected by tungsten treatment.     

Epidermal and hair follicle transglutaminases. Partial characterization of soluble enzymes in newborn mouse skin

Treatment of skins of newborn mice with the neutral protease Dispase in order to separate dermis and epidermis causes pronounced changes in the levels of transglutaminase activity in the epidermis. Two soluble transglutaminases, one anionic enzyme and one cationic enzyme, of Mr approximately 90,000 and approximately 50,000, respectively, are extracted from epidermis; and the activities of both enzymes increase as a function of the time of Dispase treatment of skin. When the anionic Mr approximately 90,000 enzyme is incubated with Dispase after its chromatographic isolation from epidermal extracts, it is converted to a lower molecular weight enzyme. Hair follicles isolated from dermis prepared by a 12-h Dispase treatment of the skin of newborn mice contain two soluble cationic transglutaminases, one of which is indistinguishable from that of epidermis and the other which is not seen in epidermis. Both of these hair follicle enzymes are of Mr approximately 50,000 and appear to exist in monomeric form. They have been partially purified. Based upon these findings, we suggest that transglutaminase processing and control occur during normal differentiation of keratinocytes in epidermis and of hair follicle epidermal cells in dermis and that production of the proper forms of the enzyme may be essential to the formation of mature cornified envelopes and hair shafts, respectively.     

The regulation of activity of the enzymes involved in the assimilation of nitrate by higher plants

1. Possible mechanisms regulating the activities of three enzymes involved in nitrate assimilation, nitrate reductase, nitrite reductase and glutamate dehydrogenase, were studied in radish cotyledons. 2. Nitrate-reductase and nitrite-reductase activities are low in nitrogen-deficient cotyledons, and are induced by their substrates. 3. Glutamate dehydrogenase is present regardless of the nitrogen status, and the enzyme can be increased only slightly by long-term growth on ammonia. 4. Although nitrate is the best inducer of nitrate reductase, lower levels of induction are also obtained with nitrite and ammonia. The experiments did not distinguish between direct or indirect induction by these two molecules. 5. Nitrite reductase is induced by nitrite and only indirectly by nitrate. 6. The induction of both nitrate reductase and nitrite reductase is prevented by the inhibitors actinomycin D, puromycin and cycloheximide, indicating a requirement for the synthesis of RNA and protein. 7. The decay of nitrate reductase, determined after inhibition of protein synthesis, is slower than the synthesis of the enzyme. Nitrite reductase is much more stable than nitrate reductase. 8. The synthesis of nitrate reductase is not repressed by ammonia, but is repressed by growth on a nitrite medium. 9. There is no inhibition of nitrate reductase, nitrite reductase or glutamate dehydrogenase by the normal end products of assimilation, but cyanate is a fairly specific inhibitor of nitrate reductase.     

Quaternary structure and composition of squash NADH:nitrate reductase

NADH:nitrate reductase (EC 1.6.6.1) was isolated from squash cotyledons (Cucurbita maxima L.) by a combination of Blue Sepharose and zinc-chelate affinity chromatographies followed by gel filtration on Bio-Gel A-1.5m. These preparations gave a single protein staining band (Mr = 115,000) on sodium dodecyl sulfate gel electrophoresis, indicating that the enzyme is homogeneous. The native Mr of nitrate reductase was found to be 230,000, with a minor form of Mr = 420,000 also occurring. These results indicate that the native nitrate reductase is a homodimer of Mr = 115,000 subunits. Acidic amino acids predominate over basic amino acids, as shown both by the amino acid composition of the enzyme and an isoelectric point for nitrate reductase of 5.7. The homogeneous nitrate reductase had a UV/visible spectrum typical of a b-type cytochrome. The enzyme was found to contain one each of flavin (as FAD), heme iron, molybdenum, and Mo-pterin/Mr = 115,000 subunit. A model is proposed for squash nitrate reductase in which two Mr = 115,000 subunits are joined to made the native enzyme. Each subunit contains 1 eq of FAD, cytochrome b, and molybdenum/Mo-pterin.     

Iron reductases from Pseudomonas aeruginosa

Cell-free extracts of Pseudomonas aeruginosa contain enzyme activities which reduce Fe(III) to Fe(II) when iron is provided in certain chelates, but not when the iron is uncomplexed. Iron reductase activities for two substrates, ferripyochelin and ferric citrate, appear to be separate enzymes because of differences in heat stabilities, in locations in fractions of cell-free extracts, in reductant specificity, and in apparent sizes during gel filtration chromatography. Ferric citrate iron reductase is an extremely labile activity found in the cytoplasmic fraction, and ferripyochelin iron reductase is a more stable activity found in the periplasmic as well as cytoplasmic fraction of extracts. A small amount of activity detectable in the membrane fraction seemed to be loosely associated with the membranes. Although both enzymes have highest activity reduced nicotinamide adenine dinucleotide, reduced glutathione also worked with ferripyochelin iron reductase. In addition, oxygen caused an irreversible loss of a percentage of the ferripyochelin iron reductase following sparge of reaction mixtures, whereas the reductase for ferric citrate was not appreciably affected by oxygen.     

Regulation of lysine catabolism through lysine-ketoglutarate reductase and saccharopine dehydrogenase in Arabidopsis.

In plant and mammalian cells, excess lysine is catabolized by a pathway that is initiated by two enzymes, namely, lysine-ketoglutarate reductase and saccharopine dehydrogenase. In this study, we report the cloning of an Arabidopsis cDNA encoding a bifunctional polypeptide that contains both of these enzyme activities linked to each other. RNA gel blot analysis identified two mRNA bands-a large mRNA containing both lysine-ketoglutarate reductase and saccharopine dehydrogenase sequences and a smaller mRNA containing only the saccharopine dehydrogenase sequence. However, DNA gel blot hybridization using either the lysine-ketoglutarate reductase or the saccharopine dehydrogenase cDNA sequence as a probe suggested that the two mRNA populations apparently are encoded by the same gene. To test whether these two mRNAs are functional, protein extracts from Arabidopsis cells were fractionated by anion exchange chromatography. This fractionation revealed two separate peaks-one containing both coeluted lysine-ketoglutarate reductase and saccharopine dehydrogenase activities and the second containing only saccharopine dehydrogenase activity. RNA gel blot analysis and in situ hybridization showed that the gene encoding lysine-ketoglutarate reductase and saccharopine dehydrogenase is significantly upregulated in floral organs and in embryonic tissues of developing seeds. Our results suggest that lysine catabolism is subject to complex developmental and physiological regulation, which may operate at gene expression as well as post-translational levels.     

Purification and characterization of two aldose reductase isoenzymes from rabbit muscle

Using a modification of the procedure of Kormann et al. (Kormann, A. W., Hurst, R. O., and Flynn, T. G. (1972) Biochim. Biophys. Acta 258, 40-55) for the purification of glycerol dehydrogenase, two enzymes have been purified from the skeletal muscle of male rabbits. From a consideration of their properties these enzymes have been named aldose reductase 1 and aldose reductase 2, respectively. Both enzymes are monomeric by the criteria of gel filtration and polyacrylamide gel electrophoresis in sodium dodecyl sulfate and both reductases are immunologically identical as shown by double immunodiffusion and rocket immunoelectrophoresis. Aldose reductases 1 and 2 have almost identical amino acid compositions, their NH2 termini are blocked and the COOH termini of both enzymes are apparently identical. The enzymes differ, however, in molecular weight with aldose reductase 2 having Mr = 41,500 and aldose reductase 1 Mr 40,200. Both enzymes have the broad substrate specificity typical of the aldehyde reductase family of enzymes; Km values of aldose reductase 1 for aldo sugars were similar to those reported for rabbit lens aldose reductase, and both aldose reductase 1 and 2 were inhibited by the commercial aldose reductase inhibitors Alrestatin and Sorbinil. Two aldose reductases, immunologically and electrophoretically identical to the muscle enzymes, were found in rabbit lens. Two aldose reductases were also detected in the skeletal muscle of male rats and pigs and in pig and bovine lens. The presence of relatively large amounts of aldose reductase in muscle identifies a new and rich source of the enzyme.     

Two glutamyl-tRNA reductase activities in Escherichia coli

delta-Aminolevulinic acid (ALA) is the first committed precursor for tetrapyrrole biosynthesis. ALA formation in Escherichia coli occurs in a tRNA-dependent three-step conversion from glutamate. Glu-tRNA reductase is the key enzyme in this pathway. E. coli K12 contains two Glu-tRNA reductase activities which differ in their molecular weights. Here we describe the purification of one of these enzymes. Four different chromatographic separations yielded a nearly homogeneous protein. Its apparent molecular mass under denaturing (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and nondenaturing conditions (rate zonal sedimentation and gel filtration) is 85,000 +/- 5,000 Da. This indicates a monomeric structure for the active enzyme. Gel filtration and glycerol gradient centrifugation indicate that the other activity has a molecular mass of 45,000 +/- 5,000 Da. In the presence of NADPH both enzyme activities converted E. coli Glu-tRNA(2Glu) to glutamate 1-semialdehyde. Addition of GTP or hemin did not affect the reductase activity. Both enzymes display sequence-specific recognition of tRNA; E. coli Glu-tRNA(2Glu) is a good substrate while the Chlamydomonas reinhardtii, Bacillus subtilis, and Synechocystis Glu-tRNA(Glu) species are poorly recognized.     

Genetic and biochemical studies of nitrate reduction in Aspergillus nidulans

1. In Aspergillus nidulans nitrate and nitrite induce nitrate reductase, nitrite reductase and hydroxylamine reductase, and ammonium represses the three enzymes. 2. Nitrate reductase can donate electrons to a wide variety of acceptors in addition to nitrate. These artificial acceptors include benzyl viologen, 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride, cytochrome c and potassium ferricyanide. Similarly nitrite reductase and hydroxylamine reductase (which are possibly a single enzyme in A. nidulans) can donate electrons to these same artificial acceptors in addition to the substrates nitrite and hydroxylamine. 3. Nitrate reductase can accept electrons from reduced benzyl viologen in place of the natural donor NADPH. The NADPH-nitrate-reductase activity is about twice that of reduced benzyl viologen-nitrate reductase under comparable conditions. 4. Mutants at six gene loci are known that cannot utilize nitrate and lack nitrate-reductase activity. Most mutants in these loci are constitutive for nitrite reductase, hydroxylamine reductase and all the nitrate-induced NADPH-diaphorase activities. It is argued that mutants that lack nitrate-reductase activity are constitutive for the enzymes of the nitrate-reduction pathway because the functional nitrate-reductase molecule is a component of the regulatory system of the pathway. 5. Mutants are known at two gene loci, niiA and niiB, that cannot utilize nitrite and lack nitrite-reductase and hydroxylamine-reductase activities. 6. Mutants at the niiA locus possess inducible nitrate reductase and lack nitrite-reductase and hydroxylamine-reductase activities. It is suggested that a single enzyme protein is responsible for the reduction of nitrite to ammonium in A. nidulans and that the niiA locus is the structural gene for this enzyme. 7. Mutants at the niiB locus lack nitrate-reductase, nitrite-reductase and hydroxylamine-reductase activities. It is argued that the niiB gene is a regulator gene whose product is necessary for the induction of the nitrate-utilization pathway. The niiB mutants either lack or produce an incorrect product and consequently cannot be induced. 8. Mutants at the niiribo locus cannot utilize nitrate or nitrite unless provided with a flavine supplement. When grown in the absence of a flavine supplement the activities of some of the nitrate-induced enzymes are subnormal. 9. The growth and enzyme characteristics of a total of 123 mutants involving nine different genes indicate that nitrate is reduced to ammonium. Only two possible structural genes for enzymes concerned with nitrate utilization are known. This suggests that only two enzymes, one for the reduction of nitrate to nitrite, the other for the reduction of nitrite to ammonium, are involved in this pathway.     

Gene dosage effect of L-proline biosynthetic enzymes on L-proline accumulation and freeze tolerance in Saccharomyces cerevisiae

We have previously reported that L-proline has cryoprotective activity in Saccharomyces cerevisiae. A freeze-tolerant mutant with L-proline accumulation was recently shown to carry an allele of the PRO1 gene encoding gamma-glutamyl kinase, which resulted in a single amino acid substitution (Asp154Asn). Interestingly, this mutation enhanced the activities of gamma-glutamyl kinase and gamma-glutamyl phosphate reductase, both of which catalyze the first two steps of L-proline synthesis and which together may form a complex in vivo. Here, we found that the Asp154Asn mutant gamma-glutamyl kinase was more thermostable than the wild-type enzyme, which suggests that this mutation elevated the apparent activities of two enzymes through a stabilization of the complex. We next examined the gene dosage effect of three L-proline biosynthetic enzymes, including Delta(1)-pyrroline-5-carboxylate reductase, which converts Delta(1)-pyrroline-5-carboxylate into L-proline, on L-proline accumulation and freeze tolerance in a non-L-proline-utilizing strain. Overexpression of the wild-type enzymes has no influence on L-proline accumulation, which suggests that the complex is very unstable in nature. However, co-overexpression of the mutant gamma-glutamyl kinase and the wild-type gamma-glutamyl phosphate reductase was effective for L-proline accumulation, probably due to a stabilization of the complex. These results indicate that both enzymes, not Delta(1)-pyrroline-5-carboxylate reductase, are rate-limiting enzymes in yeast cells. A high tolerance for freezing clearly correlated with higher levels of L-proline in yeast cells. Our findings also suggest that, in addition to its cryoprotective activity, intracellular L-proline could protect yeast cells from damage by oxidative stress. The approach described here provides a valuable method for breeding novel yeast strains that are tolerant of both freezing and oxidative stresses.     

Interaction and identification of ubiquinone-binding proteins in ubiquinol-cytochrome c reductase by azido-ubiquinone derivatives

Various azido-ubiquinone derivatives were synthesized and characterized. 3-Azido-2-methyl-5-methoxy-6-(3,7-dimethyloctyl)-1,4-benzoquinone was found to be suitable for the study of specific interaction between ubiquinone (Q) and protein. It was synthesized with high specific radioactivity and used to identify the Q-binding proteins in purified ubiquinol-cytochrome c reductase. This azido-Q derivative showed partial efficiency in restoring activity to the Q- and phospholipids-depleted ubiquinol-cytochrome c reductase in the absence of light. Azido-Q derivative treated samples, however, became completely inactivated upon photolysis, and the inactivation was not reversed by addition of Q derivatives. The redox state of the azido-Q derivative has little effect on the Q-binding affinity. Two protein subunits with Mr = 37,000 and 17,000 were found to be heavily labeled when depleted ubiquinol-cytochrome c reductase was treated with [3H] azido-Q derivative followed by photolysis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The amount of radioactive labeling of the Mr = 17,000 protein was proportional to the degree of inactivation and affected by the presence of phospholipids. The radioactive labeling of the Mr = 37,000 protein subunit, however, showed no correlation with degree of inactivation and was not affected by phospholipids. Since the radiolabeling at the Mr = 17,000 protein subunit was affected by phospholipids and correlated with the enzymatic activity, this subunit is probably the Q-binding protein in this enzyme complex (QPc). The inhibition of enzymatic activity by n-heptyl-4-hydroxyquinoline-N-oxide was easily reversed by addition of the azido-Q derivative. The distribution of radioactivity among the subunits of ubiquinol-cytochrome c reductase was not affected by the presence of antimycin A, 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole or n-heptyl-4-hydroxyquinoline-N-oxide, suggesting that the binding site(s) of these inhibitors are not the Q-binding site.     

Spontaneously active and ATPMg-dependent protein phosphatase activity in vascular smooth muscle

A spontaneously active (Mr greater than 350,000) and an ATPMg-dependent phosphatase (Mr congruent to 140,000) were identified in bovine aortic smooth muscle. The spontaneously active phosphatase was effective in dephosphorylating both phosphorylase a (240nmol32P/min/mg) and phosphorylated myosin light chains (1000nmol32P/min/mg). In contrast, the ATPMg-dependent phosphatase was only effective in dephosphorylating phosphorylase a (400nmol32P/min/mg). Phosphorylase phosphatase activity of the ATPMg-dependent enzyme was suppressed by the well-characterized modulator protein (inhibitor-2), whereas the activity of the spontaneously active enzyme was unaffected. The aortic spontaneously active phosphatase did not convert to an ATPMg-dependent form when it was stored at 4 degrees or incubated at 30 degrees C in either the presence or absence of modulator protein. These findings suggest that spontaneous and ATPMg-dependent phosphatase activities described in these studies are probably ascribable to different enzymes. Since both phosphorylase and myosin light chains are phosphorylated when smooth muscle contracts these phosphatases may participate in coordinating arterial contractility and metabolism.