Glycosidases induced in Aspergillus tamarii. Secreted alpha-D-galactosidase and beta-D-mannanase.

An alpha-D-galactosidase (EC 3.2.1.22) and a beta-D-mannanase (EC 3.2.1.78), which were secreted into the growth medium when Aspergillus tamarii was cultivated in the presence of galactomannan, were purified by a procedure including chromatography on hydroxyapatite and DEAE-cellulose columns. Each of these enzymes showed a single protein band, corresponding to their respective activities, on polyacrylamide-gel electrophoresis. Both enzymes were shown to be glycoproteins containing N-acetylglucosamine, mannose and galactose, with molar proportions of 1:6:1.5 for alpha-D-galactosidase and 1:13:8 for beta-D-mannanase. Mr values as determined by polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate and by the electrophoretic method of Hedrick & Smith [(1968) Arch. Biochem. Biophys. 126, 155-164] were 56000 and 53000 respectively. The alpha-D-galactosidase differed markedly from the mycelial forms I and II studied in the preceding paper [Civas, Eberhard, Le Dizet & Petek (1984) Biochem. J. 219, 849-855] with regard to both its kinetic and structural properties.     

A novel method for the synthesis of nucleoside triphosphates labelled with inorganic [32P]phosphate specifically in the beta-position.

A method was developed for the introduction of [32p]Pi specifically into the beta-position of ATP and GTP. The method is based on two separate reactions involving (a) phosphorolysis of poly(A) or poly(G) [Soreq, Nudel, Salomon, Revel & Littauer (1974) J. Mol Biol. 88, 233-245] in the presence of [32P]Pi and (b) conversion of the labelled diphosphate into the corresponding triphosphate by transferring the active phosphate group from 1,3-diphosphoglycerate in a coupled reaction as decribed by Glynn & Chappell [(1964) Biochem. J. 90, 147-149]. Radioactivity in the beta- and gamma-phosphate groups of the labelled triphosphate was measured by using polynucleotide kinase. No detectable radioactivity was found in the gamma-phosphate group. The suitability of this method for the synthesis of other nucleoside triphosphates specifically labelled in the beta-position is discussed.     

Heme degradation during autoxidation of oxyhemoglobin

Two fluorescent heme degradation compounds are detected during autoxidation of oxyhemoglobin. These fluorescent compounds are similar to fluorescent compounds formed when hydrogen peroxide reacts with hemoglobin [E. Nagababu and J. M. Rifkind, Biochem. Biophys. Res. Commun. 247, 592-596 (1998)]. Low levels of heme degradation in the presence of superoxide and catalase are attributed to a reaction involving the superoxide produced during autoxidation. The inhibition of most of the degradation by catalase suggests that the hydrogen peroxide generated during autoxidation of oxyhemoglobin produces heme degradation by the same mechanism as the direct addition of hydrogen peroxide to hemoglobin. The formation of the fluorescent degradation products was inhibited by the peroxidase substrate, ABTS, which reduces ferrylhemoglobin to methemoglobin, indicating that ferrylhemoglobin is produced during the autoxidation of hemoglobin. It is the transient formation of this highly reactive Fe(IV) hemoglobin, which is responsible for most of the heme degradation during autoxidation.     

The source of oxygen in the reaction catalysed by collagen lysyl hydroxylase.

The synthetic peptides (Pro-Pro-Gly)5 and (Ile-Lys-Gly)5-Phe were hydroxylated with collagen prolyl hydroxylase and lysyl hydroxylase in an 18O2 atmosphere. The oxygen atoms in the hydroxy groups of hydroxyproline and hydroxylysine were 87% and 6.5% respectively derived from the atmospheric 18O2. The results are consistent with those reported previously for proline hydroxylation in vivo [Fujimoto & Tamiya (1962) Biochem. J. 84, 333-335; Prockop, Kaplan & Udenfriend (1962) Biochem. Biophys. Res. Commun. 9, 192-196; Fujimoto & Tamiya (1963) Biochem. Biophys. Res. Commun. 10, 498-501; Prockop, Kaplan & Udenfriend (1963) Arch. Biochem. Biophys. 101, 499-503] and in vitro [Cardinale, Rhoads & Udenfriend (1971) Biochem. Biophys. Res. Commun. 43, 537-543] and for lysine hydroxylation in vivo [Fujimoto & Tamiya (1963) Biochem. Biophys. Res. Commun. 10, 498-501]. In view of the similarities of these two oxygenase-type hydroxylation reactions the participation of intermediates is proposed, the oxygen atoms of which are exchangeable with those of water. The atmospheric oxygen atoms incorporated into the intermediate must be equilibrated with water oxygen atoms in the slower lysyl hydroxylase reaction.     

Generation of the superoxide radical during autoxidation of oxymyoglobin.

Autoxidation of bovine oxymyoglobin to metmyoglobin induces co-oxidation of epinephrine to adrenochrome. This co-oxidation is markedly inhibited by superoxide dismutase [EC 1.15.1.1]. Electron transfer from oxymyoglobin to ferricytochrome c is partially inhibited by superoxide dismutase. These results indicate that autoxidation of oxymyoglobin results in generation of superoxide radicals. Autoxidation of oxymyoglobin is accelerated by superoxide dismutase and partially inhibited by catalase [EC 1.11.1.6].     

Oxidation--reduction potentials of turkey liver xanthine dehydrogenase and the origins of oxidase and dehydrogenase behaviour in molybdenum-containing hydroxylases.

Redox potentials for the various centres in the enzyme xanthine dehydrogenase (EC 1.2.1.37) from turkey liver determined by potentiometric titration in the presence of mediator dyes, with low-temperature electron-paramagnetic-resonance spectroscopy. Values at 25 degrees C in pyrophosphate buffer, pH 8.2, are: Mo(VI)/Mo(V)(Rapid),-350 +/- 20mV; Mo(V) (Rapid)/Mo(IV), -362 +/- 20mV; Fe-S Iox./Fe-S Ired., -295 +/- 15mV; Fe-S IIox./Fe-S IIred., -292 +/- 15mV; FAD/FADH,-359+-20mV; FADH/FADH2, -366 +/- 20mV. This value of the FADH/FADH2 potential, which is 130mV lower than the corresponding one for milk xanthine oxidase [Cammack, Barber & Bray (1976) Biochem. J. 157, 469-478], accounts for many of the differences between the two enzymes. When allowance is made for some interference by desulpho enzyme, then differences in the enzymes' behaviour in titration with xanthine [Barber, Bray, Lowe & Coughlan (1976) Biochem. J. 153, 297-307] are accounted for by the potentials. Increases in the molybdenum potentials of the enzymes caused by the binding of uric acid are discussed. Though the potential of uric acid/xanthine (-440mV) is favourable for full reduction of the dehydrogenase, nevertheless, during turnover, for kinetic reasons, only FADH and very little FADH2 is produced from it. Since only FADH2 is expected to react with O2, lack of oxidase activity by the dehydrogenase is explained. Reactivity of the two enzymes with NAD+ as electron acceptor is discussed in relation to the potentials.     

Autoxidation of native oxymyoglobin. Kinetic analysis of the pH profile.

The rate of autoxidation of native oxymyoglobin to metmyoglobin has been examined over the pH range of 4.8--12.6 in 0.1 M buffer at 25 degrees C, and some 40 values of the observed first-order rate constant, kobs, are plotted against pH of the solution. In order to understand the kobs--pH profile thus obtained, some mechanistic models are proposed for the autoxidation reaction. The fitting of their rate equations as a function of pH has been examined to the experimental kobs-pH plot by a least-squares method with the use of a digital computer. The complicated pH-profile can be best explained by the 'acid-base catalyzed three states model', which reveals not only the catalytic role of hydrogen ions and hydroxyl ions, but also the involvement of two dissociation groups of myoglobin molecule in the autoxidation reaction.     

Purification and properties of alpha-D-mannosidase from the germinated seeds of Medicago sativa (alfalfa).

alpha-Mannosidase of Medicago sativa (alfalfa) was purified 1340-fold. The purification method included dialysis of the crude extract against a citrate/phosphate buffer, pH 3.9, (NH4)SO4 precipitation, hydroxyapatite chromatography, chromatography on Sephadex G-200 and finally a preparatory electrophoresis on polyacrylamide-gel gradient by Doly & Petek's [(1977) J. Chromatogr. 137. 69--81] method. Each step of purification was checked by polyacrylamide-gel disc electrophoresis. The purified enzyme showed a single band, corresponding to alpha-mannosidase activity. alpha-Mannosidase has a mol.wt. 230 000 as estimated by Hedrick & Smith's [(1968) Arch. Biochem. Biophys. 126, 155--164] method and also by polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate by Weber & Osborn [(1969) J. Biol. Chem. 244, 4406--4412]. The enzyme comprises four subunits of different molecular weight. Optimum pH and Km values were determined with p-nitrophenyl alpha-D-mannoside as substrate. When incubated at a temperature between 20 and 62 degrees C before assay, alpha-mannosidase initially shows an increase in activity. alpha-Mannosidase is stable when the pH is about neutrality. It can be inactivated by several metal ions, including Zn2+. At a pH below 5 the enzyme undergoes irreversible inactivation. The presence of EDTA at acid pH considerably enhances the inactivation of the enzyme. This inactivation due to EDTA can be specifically reversed by incubation with Zn2+.     

Binding of ouabain to Na+, K+-dependent ATPase during the ATPase reaction. Evidence for a dimer structure of the ATPase.

Na+, K+-dependent ATPase [EC 3.6.1.3] was purified from porcine kidney by the method of Lane et al. [(1973) J. Biol. Chem. 248, 7197-7200] with slight modifications [Yamaguchi, M. & Tonomura, Y., (1979) J. Biochem. 86, 509-523]. The amounts of a phosphorylated intermediate (EP) and ouabain bound to the enzyme during the ATPase reaction were measured in 2.1 mM MgCl2 and various concentrations of NaCl and KCl at pH 7.5 and 20 degrees C. In presence of NaCl and the absence of KCl, the molar ratio of the amounts of EP and bound ouabain was 1 : 2. In the presence of both NaCl and KCl, it was 1 : 1. In both cases, the amount of bound ouabain was equal to that of EP in the absence of ouabain. These findings suggest that the functional unit of the transport ATPase is a dimer.     

Cartilage proteoglycans. Assembly with hyaluronate and link protein as studied by electron microscopy.

Aggregates formed by the interaction of cartilage proteoglycan monomers and fragments thereof with hyaluronate were studied by electron microscopy by use of rotary shadowing [Wiedemann, Paulsson, Timpl, Engel & Heinegård (1984) Biochem. J. 224, 331-333]. The differences in shape and packing of the proteins bound along the hyaluronate strand in aggregates formed in the presence and in the absence of link protein were examined in detail. The high resolution of the method allowed examination of the involvement in hyaluronate binding of the globular core-protein domains G1, G2 and G3 [Wiedemann, Paulsson, Timpl, Engel & Heinegård (1984) Biochem. J. 224, 331-333; Paulsson, Mörgelin, Wiedemann, Beardmore-Gray, Dunham, Hardingham, Heinegård, Timpl & Engel (1987) Biochem. J. 245, 763-772]. Fragments comprising the globular hyaluronate-binding region G1 form complexes with hyaluronate with an appearance of necklace-like structures, statistically interspaced by free hyaluronate strands. The closest centre-to-centre distance found between adjacent G1 domains was 12 nm. Another fragment comprising the binding region G1 and the adjacent second globular domain G2 attaches to hyaluronate only by one globule. Also, the core protein obtained by chondroitinase digestion of proteoglycan monomer binds only by domain G1, with domain G3 furthest removed from the hyaluronate. Globule G1 shows a statistical distribution along the hyaluronate strands. In contrast, when link protein is added, binding is no longer random, but instead uninterrupted densely packed aggregates are formed.     

The radiosensitivity of rat thymocytes.

gamma-Irradiation in vitro apparently blocked a plasma-membrane associated, superoxide-producing, NADPH oxidase in rat thymocytes. Differential centrifugation of the mixed thymocytes indicated the smaller lymphocytes (approx. 6 microns diameter) to be the radiosensitive population. The oxidase system co-isolated in part with thymus nuclei and could be solubilized by detergent treatment [Bellavite, Jones, Cross, Papini & Rossi (1984) Biochem. J. 223, 639-648]. Endogenous NADPH was the rate-limiting component for superoxide formation in vitro. The level of NADPH was lowered by gamma-irradiation, an effect mimicked by GSSG in the presence of 50 microM-ZnCl2 to inhibit GSSG reductase. These findings are suggested as the metabolic basis for interphase death of small lymphocytes exposed to ionizing radiation.     

Autoxidation of oxymyoglobin. An overall stoichiometry including subsequent side reactions

Oxymyoglobin (MbO2) is oxidized easily to metmyoglobin (metMb) with generation of the superoxide anion, which can be converted by the spontaneous dismutation into H2O2, this being also a potent oxidant of MbO2. In the presence of sodium azide in stoichiometric amounts, however, the rate of autoxidation of MbO2 increased rapidly with increasing concentration of the anion, but soon reached a saturating level, the extent of which was about twice that of the normal autoxidation in buffer alone. Quantitative analysis has revealed that this enhancement is not due to the nucleophilic displacement of O2- from MbO2 by the anion (Satoh, Y., and Shikama, K. (1981) J. Biol. Chem. 256, 10272-10275), but is due to the additional oxidation of MbO2 by H2O2 freed from the metMb being occupied by the anion at the sixth coordination position. Based on these novel results and stoichiometric considerations, it is possible to propose a new view that H2O2 produced from O2- can be eliminated or decomposed mostly, if not completely, by the metMb resulting from the normal autoxidation reaction of MbO2, presumably via the formation of the ferryl species.     

Reappraisal of the quantitative importance of non-skeletal-muscle source of N tau-methylhistidine in urine.

The claim of Millward, Bates, Grimble, Brown, Nathan & Rennie [(1980) Biochem. J. 190. 225--228] that muscle actomyosin contributes as little as 25% of urinary N tau-methylhistidine is not consistent with other published data from that group [Bates, DeCoster, Grimble, Holloszy, Millward & Rennie (1980) J. Physiol. (London) 303, 41 P] or with literature values. It appears likely that the turnover rate of muscle actomyosin has been considerably underestimated and that when realistic rates of protein turnover are used, muscle tissue remains the major contributor of N tau-methylhistidine in urine.     

Ca2+-uptake properties of two populations of mitochondria from normal and denervated rat soleus muscle.

Ultraturrax- and Nagarse-released populations of mitochondria were characterized with respect to their Ca2+-uptake activities (i) by means of the indirect polarographic technique and (ii) directly by the 45Ca Ruthenium Red-quench method of Reed & Bygrave [(1974) Biochem. J. 140, 143-155]. The denervated-muscle subsarcolemmal and intermyofibrillar mitochondrial fractions displayed markedly decreased rates and capacities for Ca2+ uptake compared with their respective controls. The implications of these findings with respect to the process of cell necrosis are discussed.     

Application of topological methods in enzyme kinetics.

A criticism [Cornish-Bowden (1976) Biochem. J. 159, 167] of an algebraic method for deriving steady-state rate equations [Indge & Childs (1976) Biochem. J. 155, 567-570] is theoretically founded.     

Insulin exerts actions through a distinct species of guanine nucleotide regulatory protein: inhibition of adenylate cyclase.

Insulin failed to exert an effect on the basal and glucagon- and guanosine 5'-[beta, gamma-imido]-triphosphate-stimulated adenylate cyclase activities of hepatocyte membranes. In the presence of high GTP (0.1 mM) concentrations, however, insulin was shown to inhibit adenylate cyclase activity. This effect was dose-dependent, exhibiting an EC50 (median effective concentration) of 3 microM for GTP. Elevated glucagon concentrations blocked the inhibitory effect of insulin in a dose-dependent fashion, with an EC50 of 1 nM. The insulin inhibition was dose-dependent (EC50 = 90 pM). The inhibitory effects of insulin were abolished using membranes from either glucagon-desensitized hepatocytes or cholera-toxin-treated hepatocytes. If either Mn2+ replaced Mg2+ in adenylate cyclase assays or Na+ was removed from the assay mixtures then insulin failed to exert any inhibitory effect. It is suggested that insulin exerts its action on adenylate cyclase through an inhibitory guanine nucleotide protein. This is integrated with the proposal [Heyworth, Rawal & Houslay (1983) FEBS Lett. 154, 87-91; Heyworth, Wallace & Houslay (1983) Biochem. J. in the press] that insulin mediates a variety of cellular effects through a specific guanine nucleotide regulatory protein and associated protein kinase(s).     

Amino Acid Sequence, Spectral, Oxygen-Binding, and Autoxidation Properties of Indoleamine Dioxygenase-Like Myoglobin from the Gastropod Mollusc Turbo cornutus

Myoglobin was isolated from the radular muscle of the archaeogastropod mollusc Turbo cornutus (Turbinidae). This myoglobin is a monomer carrying one protoheme group; the molecular mass was estimated by SDS–PAGE to be about 40 kDa, 2.5 times larger than that of usual myoglobin. The cDNA-derived amino acid sequence of 375 residues was determined, of which 327 residues were identified directly by chemical sequencing of internal peptides. The amino acid sequence of Turbo myoglobin showed no significant homology with any other usual 16-kDa globins, but showed 36% identity with the myoglobin from Sulculus diversicolor (Haliotiidae) and 27% identity with human indoleamine 2,3-dioxygenase, a tryptophan-degrading enzyme containing heme. Thus, the Turbo myoglobin can be counted among the myoglobins which evolved from the same ancestor as that of indoleamine 2,3-dioxygenase. The absorbance ratio of γ to CT maximum (γ/CT) of Turbo metmyoglobin was 17.8, indicating that this myoglobin probably possesses a histidine residue near the sixth coordination position of heme iron. The Turbo myoglobin binds oxygen reversibly. Its oxygen equilibrium properties are similar to those of Sulculus myoglobin, giving P ₅₀ = 3.5 mm Hg at pH 7.4 and 20°C. The pH dependence of autoxidation of Turbo oxymyoglobin was quite different from that of mammalian myoglobin, suggesting a unique protein folding around the heme cavity of Turbo myoglobin. A kinetic analysis of autoxidation indicates that the amino acid residue with pK _a = 5.4 is involved in the reaction. The autoxidation reaction was enhanced markedly at pH 7.6, but not at pH 5.5 and 6.3 in the presence of tryptophan. We suggest that a noncatalytic binding site for tryptophan, in which several dissociation groups with pK _a ≥ 7.6 are involved, remains in Turbo myoglobin as a relic of molecular evolution.     

Purification and steady-state kinetics of adenosine 5''-pyrophosphate sulphurylase from baker''s yeast.

ADP sulphurylase (EC 2.7.7.5) was purified by chromatography on Sephadex G-200 and DEAE-cellulose. The enzyme was assayed by measuring the incorporation of [32P]Pi into ADP in the presence of the substrate for the reverse reaction, adenosine 5'-sulphatophosphate. In the concentration ranges investigated, by using initial-velocity, product-inhibition and isotope-exchange studies, the data were consistent with a Ping Pong reaction mechanism, with Km for adenosine 5'-sulphatophosphate of 1.20 +/- 0.08 mM and a Km for Pi of 4.95 +/- 0.15 mM. Competitive substrate inhibition by Pi (Ki = 11.7 +/- 0.3 mM) was found. ADP sulphurylase catalyses a sulphate-independent Pi-ADP exchange reaction, the kinetics of which are consistent with the kinetics of the overall reaction, inconsistent with the assay of Burnell & Anderson [(1973) Biochem. J. 133, 417-428], which is based on a sulphate-dependent Pi-ADP exchange reaction.     

The mechanism of Klebsiella pneumoniae nitrogenase action. Simulation of the dependences of H2-evolution rate on component-protein concentration and ratio and sodium dithionite concentration.

The rate constants from Table 1 and Scheme 2 of Lowe & Thorneley [(1984) Biochem. J. 224, 877-886] were used to simulate the rate of H2 evolution, under various conditions, from nitrogenase isolated from Klebsiella pneumoniae. These rates depend on both the ratio and concentrations of the MoFe protein and Fe protein that comprise nitrogenase. The simulations explain the shapes of 'protein titration' and 'dilution effect' curves. The concept of an apparent Km for the reductant Na2S2O4 is shown to be invalid, since the dependence of H2-evolution rate on the square root of S2O4(2-) concentration is not hyperbolic and depends on the ratio and absolute concentrations of the MoFe protein and Fe protein.     

Amino Acid Sequence, Spectral, Oxygen-Binding, and Autoxidation Properties of Indoleamine Dioxygenase-Like Myoglobin from the Gastropod Mollusc Turbo cornutus

Myoglobin was isolated from the radular muscle of the archaeogastropod mollusc Turbo cornutus (Turbinidae). This myoglobin is a monomer carrying one protoheme group; the molecular mass was estimated by SDS–PAGE to be about 40 kDa, 2.5 times larger than that of usual myoglobin. The cDNA-derived amino acid sequence of 375 residues was determined, of which 327 residues were identified directly by chemical sequencing of internal peptides. The amino acid sequence of Turbo myoglobin showed no significant homology with any other usual 16-kDa globins, but showed 36% identity with the myoglobin from Sulculus diversicolor (Haliotiidae) and 27% identity with human indoleamine 2,3-dioxygenase, a tryptophan-degrading enzyme containing heme. Thus, the Turbo myoglobin can be counted among the myoglobins which evolved from the same ancestor as that of indoleamine 2,3-dioxygenase. The absorbance ratio of to CT maximum (/CT) of Turbo metmyoglobin was 17.8, indicating that this myoglobin probably possesses a histidine residue near the sixth coordination position of heme iron. The Turbo myoglobin binds oxygen reversibly. Its oxygen equilibrium properties are similar to those of Sulculus myoglobin, giving P ₅₀ = 3.5 mm Hg at pH 7.4 and 20°C. The pH dependence of autoxidation of Turbo oxymyoglobin was quite different from that of mammalian myoglobin, suggesting a unique protein folding around the heme cavity of Turbo myoglobin. A kinetic analysis of autoxidation indicates that the amino acid residue with pK _a = 5.4 is involved in the reaction. The autoxidation reaction was enhanced markedly at pH 7.6, but not at pH 5.5 and 6.3 in the presence of tryptophan. We suggest that a noncatalytic binding site for tryptophan, in which several dissociation groups with pK _a 7.6 are involved, remains in Turbo myoglobin as a relic of molecular evolution.