Subcellular distribution of a factor inactivating tyrosine aminotransferase. Study of its mechanism and relationship to different forms of the enzyme.

The subcellular distribution of a tyrosine aminotransferase inactivating factor in rat liver has been investigated. Most of its activity is associated with plasma membranes, with minor amounts in mitochondria and endoplasmatic reticulum. The factor is also found in kidney and inactivates the enzyme reversibly in presence of cysteine, most likely by modification of -SH groups. ATP counteracts this inactivation only, when crude enzyme extracts are inactivated by purified subcellular fractions or when the purified enzyme is inactivated in presence of liver or kidney cortex homogenates. The relationship of this inactivation to reported different forms of the enzyme has been investigated. Form I of three different forms, that can be obtained by hydroxyl-apatite chromatography, is readily inactivated, form III can be partly converted to form I by incubation in presence of purified plasma membranes. The relationship of these findings to a possible multistep mechanism in the turnover of the enzyme discussed.     

Myeloperoxidase-dependent oxidative inactivation of neutrophil neutral proteinases and microbicidal enzymes.

The susceptibility of a number of human neutrophil granule enzymes to oxidative inactivation was investigated. Addition of H2O2 to the cell-free medium from stimulated neutrophils resulted in inactivation of all enzymes tested. This was inhibited by azide and methionine, indicating that inactivation was due to myeloperoxidase-derived oxidants. Lysozyme was more than 50% inactivated by one addition of 100 nmol of H2O2/ml, whereas myeloperoxidase, beta-glucuronidase, gelatinase and collagenase were almost completely inactivated by three additions. Cathepsin G was slightly less susceptible, whereas elastase was extremely resistant to oxidative attack. Myeloperoxidase-dependent enzyme inactivation may be a means whereby the neutrophil can terminate the activity of its granule enzymes and control the release of degradative enzymes into the tissues.     

Tyrosine Hydroxylase Activation and Inactivation by Protein Phosphorylation Conditions

Tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, catalyzes the conversion of tyrosine to DOPA, Cyclic AMP-dependent protein phosphorylation conditions alter tyrosine hydroxylase activity in rat striatal homogenates. In agreement with other laboratories, we find that short-term pre-incubation (3 min) of extracts under phosphorylating conditions (Mg . ATP, cAMP) increases enzyme activity two- to tenfold over control as measured during a subsequent 15-min assay. We now report that preincubation under phosphorylating conditions for longer periods (30 min) results in a loss of activity to levels equal to or below that of the control enzyme. Addition of purified bovine brain protein kinase catalytic subunit and Mg . ATP enhances activation and increases the rate of inactivation. To demonstrate that inactivation is not associated with proteolytic degradation or irreversible denaturation, the inactivated form of the enzyme can be reactivated. The protein kinase inhibitor protein decreases the activation process and prevents inactivation of the enzyme to below control values. The sedimentation coefficient is not changed by phosphorylation conditions (S = 8.8 +/- 0.1). Although the apparent Km of the enzyme for the 6-methyltetrahydropterine (6-MPH4) cofactor is reduced (0.86 mM, control; 0.32 mM, activated), it is also reduced in the inactivated form (0.38 mM). The Ki for dopamine is increased from 4.5 microM for the control to 28 microM for the activated enzyme, whereas the inactivated form of the enzyme exhibits a Ki of 10 microM. Removal of catecholamines by gel filtration fails to alter activity and the apparent cofactor Km. Moreover, both the activated and the inactivated states persist following gel filtration. It therefore appears that the activation-inactivation process is not mediated solely by the modulation of enzyme feedback inhibition or changes in the Km for 6-MPH4. We also describe a coupled decarboxylase assay in which labeled dopamine is resolved from the precursors tyrosine and DOPA by low-voltage paper electrophoresis.     

Regulation of glutamine synthetase from Saccharomyces cerevisiae by repression, inactivation and proteolysis

Glutamine synthetase activity is modulated by nitrogen repression and by two distinct inactivation processes. Addition of glutamine to exponentially grown yeast leads to enzyme inactivation. 50% of glutamine synthetase activity is lost after 30 min (a quarter of the generation time). Removing glutamine from the growth medium results in a rapid recovery of enzyme activity. A regulatory mutation (gdhCR mutation) suppresses this inactivation by glutamine in addition to its derepressing effect on enzymes involved in nitrogen catabolism. The gdhCR mutation also increases the level of proteinase B in exponentially grown yeast. Inactivation of glutamine synthetase is also observed during nitrogen starvation. This inactivation is irreversible and consists very probably of a proteolytic degradation. Indeed, strains bearing proteinase A, B and C mutations are no longer inactivated under nitrogen starvation.     

Involvement of Reversible Inactivation in the Regulation of Nitrate Reductase Enzyme Levels in Chlamydomonas reinhardtii

All nitrate reductase-related activities of Chlamydomonas reinhardtii wild-type and mutant 305 cells were degraded in vivo under conditions in which the reversible inactivation could take place. When the enzyme was in the inactive form, half-lives of all nitrate reductase-related activities in wild and mutant 305 strains decreased significantly. The only nitrate reductase-related activity present in mutant 104, nitrate reductase-diaphorase, was incapable of undergoing reversible inactivation and was not degraded under any of the conditions tested. Addition of nitrate to inactive nitrate reductase of mutant 305 caused the in vivo reactivation of the enzyme and halted its degradation. Our results indicate that reversibly inactivated nitrate reductase from C. reinhardtii is the main target for a degradation system, and that nitrate reductase related diaphorase must be integrated in a reversibly inactive nitrate reductase complex to undergo degradation. A physiological role for the interconversion process of nitrate reductase can be understood on the basis of these facts.     

Biochemical and functional properties of serine esterases in acidic cytoplasmic granules of cytotoxic T lymphocytes

Percoll gradient fractions of homogenates of murine cloned cytotoxic T lymphocytes (CTL) were analyzed for the trypsin-like enzyme alpha-N-benzyloxy-carbonyl-L-lysinethiobenzyl ester (BLT) esterase recently described in CTL homogenates. Enzymatic activity was found in three areas of the gradient: the dense cytolysin containing granules; a light granule fraction; and a variable amount in the soluble fraction at the top of the gradient. Gel filtration columns showed a major peak of BLT esterase activity eluted at the position of a 60-kDa protein, and an additional, minor BLT esterase peak eluting at about 27 kDa. The separated enzymes were both significantly inhibited by the serine protease inhibitors diisopropylfluorophosphate and phenylmethyl sulfonyl fluoride (PMSF), indicating they are both serine proteases, but showed different patterns of inhibition by a series of inhibitors, suggesting the larger enzyme is not a simple dimer of the smaller. pH activity profiles of both CTL BLT esterases showed an optimum at about pH 8. PMSF inactivation of BLT esterase in detergent extracts of CTL diminished sharply as the pH was dropped below 7. Agents which raise the pH of acidic intracellular compartments were found to markedly enhance the PMSF inactivation of BLT esterase in intact CTL, showing that the granules have a low internal pH. Similarly, [3H]diisopropylfluorophosphate labeling of intact CTL gave four protein bands on non-reduced gels, of which two were labeled threefold more effectively in the presence of chloroquine. In parallel studies of inactivation of CTL lytic activity, PMSF pretreatment caused a 50% reduction of the lytic activity under conditions where greater than 90% of the BLT esterase activity was inactivated. Addition of agents raising the intragranular pH dramatically enhanced the BLT esterase inactivation but did not concomitantly reduce CTL lytic activity. These results indicate that inactivation of lytic function by PMSF is unlikely to be due to its reaction with protease in acidic granules, and suggest that the activity of these enzymes may not be required for cytotoxicity.     

The structure and function of acid proteases. IV. Inactivation of the acid protease from Mucor pusillus by acid protease-specific inhibitors.

Mucor pusillus acid protease was rapidly inactivated with 1 : 1 stoichiometry by reaction with diazoacetyl-DL-norleucine methyl ester (DAN) in the presence of cupric ions. Cupric ions were essential for this inactivation. The rate of inactivation was maximal at around pH 6 when the enzyme was mixed with DAN and cupric ions without prior mixing of the reagents, and at pH 5.3 when DAN and cupric ions were mixed and incubated before addition to the enzyme solution. In both cases, the rate of inactivation decreased as the pH was either increased or decreased. The amino acid composition of an acid hydrolysate of the DAN-Modified enzyme was indistinguishable from that of the native enzyme except for the incorporation of about one norleucine residue per molecule of protein. The enzyme was also inactivated by reaction with 1,2-epoxy-3-(p-nitrophenoxy)-propane (EPNP). At the stage of about 90% inactivation, 1.50 residues of EPNP were incorporated per molecule of protein and the rate of inactivation followed pseudo-first order kinetics. The optimal pH for the inactivation was pH 3.0 and the rate of inactivation decreased as the pH was either increased or decreased. Furthermore, the enzyme was strongly inhibited by pepstatin, and the reactions of DAN and of EPNP was also inhibited significantly by prior treatment of the enzyme with pepstatin. These results suggest that the enzyme may have two essential carboxyl groups at the active site, one reactive with DAN in the presence of cupric ions and the other with EPNP, and that pepstatin binds part of the active site to inhibit the reactions with DAN and EPNP as well as the enzyme activity.     

The structure and function of acid proteases. VI. Effects of acid protease-specific inhibitors on the acid proteases from Aspergillus niger var. macrosporus.

1. The Type B acid protease from Aspergillus niger var. macrosporus was inactivated by reaction with diazoacetyl-DL-norleucine methyl ester (DAN), DL-1-diazo-3-tosylamido-2-heptanone (DTH), and L-1-diazo-3-tosylamido-4-phenyl-2-butanone (DTPB) in the presence of cupric ions. The reaction with DAN took place with 1:1 stoichiometry. The enzyme was also inactivated by reaction with 1, 2-epoxy-3-(p-nitrophenoxy)-propane (EPNP) with concomitant incorporation of approximately two EPNP molecules per molecule of protein. Moreover, these reactions of DAN and of EPNP were markedly inhibited by pepstatin. These results seem to indicate that, as in the case of porcine pepsin [EC 3.4.23.1] and related acid proteases, the enzyme has two essential carboxyl groups at the active site, one reactive with DAN and related diazo reagents in the presence of cupric ions and the other reactive with EPNP, and that pepstatin binds in the vicinity of these residues. 2. The Type A acid protease from the same mold, on the other hand, was found to be markedly less sensitive to these specific inhibitors. Under conditions where the Type B enzyme was completely inactivated by DAN and related diazo reagents, only partial inactivation of this enzyme occurred. The effect of prior mixing of DAN and cupric ions on the pH profile of inactivation was also different from that for the Type B enzyme. Moreover, the Type A enzyme was not inactivated by EPNP. These results thus indicate that the nature of the active site of the Type A enzyme is rather different from that of the Type B enzyme and hence that the Type A enzyme belongs to a different class of acid proteases from the Type B enzyme.     

Reaction of serine proteases with substituted 3-alkoxy-4-chloroisocoumarins and 3-alkoxy-7-amino-4-chloroisocoumarins: new reactive mechanism-based inhibitors

The time-dependent inactivation of several serine proteases including human leukocyte elastase, cathepsin G, rat mast cell proteases I and II, and human skin chymase by a number of 3-alkoxy-4-chloroisocoumarins, 3-alkoxy-4-chloro-7-nitroisocoumarins, and 3-alkoxy-7-amino-4-chloroisocoumarins at pH 7.5 and the inactivation of several trypsin-like enzymes including human thrombin and factor XIIa by 7-amino-4-chloro-3-ethoxyisocoumarin and 4-chloro-3-ethoxyisocoumarin are reported. The 3-alkoxy substituent of the isocoumarin is likely interacting with the S1 subsite of the enzyme since the most reactive inhibitor for a particular enzyme had a 3-substituent complementary to the enzyme's primary substrate specificity site (S1). Inactivation of several enzymes including human leukocyte elastase by the 3-alkoxy-7-amino-4-chlorisocoumarins is irreversible, and less than 3% activity is regained upon extensive dialysis of the inactivated enzyme. Addition of hydroxylamine to enzymes inactivated by the 3-alkoxy-7-amino-4-chloroisocoumarins results in a slow (t1/2 greater than 6.7 h) and incomplete (32-57%) regain in enzymatic activity at pH 7.5. Inactivation by the 3-alkoxy-4-chloroisocoumarins and 3-alkoxy-4-chloro-7-nitroisocoumarins on the other hand is transient, and full enzyme activity is regained rapidly either upon standing, after dialysis, or upon the addition of buffered hydroxylamine. The rate of inactivation by the substituted isocoumarins is decreased when substrates or reversible inhibitors are present in the incubation mixture, which indicates active site involvement. The inactivation rates are dependent upon the pH of the reaction mixture, the isocoumarin ring system is opened concurrently with inactivation, and the reaction of 3-alkoxy-7-amino-4-chloroisocoumarins with porcine pancreatic elastase is shown to be stoichiometric. The results are consistent with a scheme where 3-alkoxy-7-amino-4-chloroisocoumarins react with the active site serine of a serine protease to give an acyl enzyme in which a reactive quinone imine methide can be released. Irreversible inactivation could then occur upon alkylation of an active site nucleophile (probably histidine-57) by the acyl quinone imine methide. The finding that hydroxylamine slowly catalyzes partial reactivation indicates that several inactivated enzyme species may exist. The 3-alkoxy-substituted 4-chloroisocoumarins and 4-chloro-7-nitroisocoumarins are simple acylating agents and do not give stable inactivated enzyme structures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inactivation of rat pineal hydroxyindole-O-methyltransferase by disulfide-containing compounds

Rat pineal hydroxyindole-O-methyltransferase activity in crude homogenates is reduced by treatment with disulfides. Cystamine (IC50 = 128 microM) and selenocystamine (IC50 = 13 microM) are the most potent compounds tested. Reduced cystamine (cysteamine) and diaminohexane are inactive. N,N'-Diacetylcystamine, penicillamine disulfide, and glutathione disulfide are less potent or inactive; but several peptides (oxytocin, vasopressin, and arginine vasotocin) are active. Inactivation by cystamine is time- and temperature-dependent and is accelerated at higher pH. Disulfide treatment of intact pinealocytes also inactivates the enzyme. Addition of dithiothreitol during the enzyme assay completely reactivates inactivated enzyme formed by disulfide treatment of homogenates or intact cells. Rat hydroxyindole-O-methyltransferase is also inactivated in the absence of added disulfides and dissolved O2. This spontaneous inactivation is time-, temperature-, and pH-dependent and can be completely prevented, but not reversed, by dithiothreitol. In contrast to the inhibitory effects of cystamine on the rat enzyme, cystamine does not alter bovine hydroxyindole-O-methyltransferase and increases ovine hydroxyindole-O-methyltransferase activity. The bovine and ovine enzymes do not become inactive in the absence of added disulfides. Together these observations indicate that rat pineal hydroxyindole-O-methyltransferase can be inactivated by a protein thiol:disulfide exchange mechanism. This mechanism may contribute to the physiological regulation of this enzyme in the rat pineal gland but does not appear to be a common feature of pineal hydroxyindole-O-methyltransferase regulation in all species.     

Irreversible inactivation of rat liver tyrosine aminotransferase

Homogenates prepared from rat livers irreversibly inactivate tyrosine aminotransferase, both endogenous and purified exogenous enzyme, in the presence of certain compounds which bind to pyridoxal 5′-P. The rate of inactivation ranged from a half-life of 0.72 to greater than 15 hr. The pyridoxal 5′-P binding compounds may be considered to be structural analogs for α-ketoglutarate or l-tyrosine, both of which are substrates for the enzyme. l-Cysteine and l-DOPA are the most effective compounds tested of each of the two structural analog classes, respectively. Absence of the carboxyl group from l-cysteine or l-DOPA has little effect on the half-life of the enzyme, whereas absence or substitution of the amino group results in an increased enzyme half-life. Absence of the —SH group from l-cysteine or of the 3′-OH group from l-DOPA results in little or no inactivation of the enzyme ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ increased to greater than 15 hr). Semicarbazide and hydroxylamine have little effect on the stability of the enzyme. Addition of pyridoxal 5′-P to homogenates incubated with l-cysteine or l-DOPA inhibits the inactivation of the enzyme. However, the addition of cofactor to inactivated enzyme does not restore lost activity.There is a disappearance of antigenic cross-reacting material during inactivation of the enzyme. This loss of specific cross-reacting material occurs at a slower rate than the loss of enzyme activity, indicating that enzymatic activity is lost prior to loss of antigenic recognition. A three-step proposal is presented to explain the data observed in which the first step is a reversible loss of pyridoxal 5′-P from the enzyme, followed by a specific irreversible inactivation of the enzyme, and ending with nonspecific proteolysis or degradation of the inactivated enzyme molecules.     

Inactivation of Aspartic Transcarbamylase in Sporulating Bacillus subtilis: Demonstration of a Requirement for Metabolic Energy

The aspartic transcarbamylase (ATCase) activity of Bacillus subtilis cells disappears rapidly from stationary-phase cells prior to sporulation. ATCase activity does not appear in the culture fluid during the stationary phase; hence the enzyme appears to be inactivated in the cells. The enzyme is inactivated normally in two different mutants lacking proteases; the activity is very stable in crude extracts of cells or in the culture fluid. These results suggest that ATCase is not inactivated by the general proteolysis that occurs in sporulating bacteria. The inactivation of ATCase can be completely inhibited after it has begun by oxygen starvation or addition of fluoroacetate. Inhibitors of oxidative phosphorylation and electron transport also interrupt the inactivation of ATCase. The inactivation of ATCase is very slow in two mutant strains that are deficient in enzymes of tricarboxylic acid cycle. Addition of gluconate to stationary cultures of the mutant strains, which is known to restore depleted adenosine 5'-triphosphate pools in these bacteria, also restores inactivation of ATCase. These experiments support the conclusion that the generation of metabolic energy is necessary for the inactivation of ATCase in stationary cells. ATCase activity is stable in growing cells in which ATCase synthesis is repressed by addition of uracil; the enzyme is inactivated normally, however, when such cells cease growing.     

Vitamin B-6 requirement for irreversible inactivation of rat liver tyrosine aminotransferase.

In vitro inactivation of tyrosine aminotransferase at pH 7.0 did not occur in liver homogenates prepared from vitamin B-6-deficient rats, although it was previously demonstrated that the enzyme was inactivated in liver homogenates from vitamin B-6-adequate rats (R. D. Reynolds and S. D. Thompson, 1974, Arch. Biochem. Biophys.164, 43–51). Addition of 2 mm pyridoxine or pyridoxal-P to the incubated homogenate did not restore the inactivation, but injection of 1 mg of pyridoxine to deficient rats restored full inactivating activity by 12 h. All forms of vitamin B-6 injected restored inactivating activity in vitro. This effect appears to be specific for vitamin B-6, since no restoration of in vitro inactivation of tyrosine aminotransferase was observed following injection of riboflavin, thiamin, niacin, or folic acid. The restoration of inactivating activity in vitro following injection of pyridoxine was not inhibited by repeated injections of puromycin or cycloheximide. Apparently, in vivo protein synthesis is not required for the restoration of the in vitro inactivating activity. However, in vivo inactivation was similar in the vitamin B-6-adequate and -deficient rats. Inactivating activity is present in homogenates of liver and kidney, but not of abdominal muscle, small intestine, heart, testes, whole blood, or erythrocyte ghosts, and is found only in the plasma membrane fraction of liver. Similar to liver, the activity in the kidney homogenate requires the presence of l-cysteine and depends upon the vitamin B-6 status of the animal. Rapid inactivation in the liver occurs between pH 6.75 and 7.75 (final pH), with minimal inactivation above or below this range. No inhibition of inactivation was observed with homogenates incubated in the presence of several protease inhibitors.     

A new affinity labeling reagent for the active site of glycogen synthase. Uridine diphosphopyridoxal

A new affinity labeling reagent for glycogen synthase a from rabbit muscle, uridine diphosphopyridoxal, has been prepared. Incubation of the enzyme with this reagent resulted in a time-dependent, almost complete loss of activity. The inactivation was pseudo-first order, and the results of the kinetic analysis suggested the formation of a noncovalent enzyme-reagent complex prior to the covalent reaction, with a Kinact of 25 microM and a maximal rate constant of 0.22 min-1. The inactivation was pronouncedly protected by UDP-Glc and UDP, but not by the allosteric activator glucose 6-phosphate. The increase in a spectral peak at 425 nm and the decrease in enzymatic activity were well correlated, suggesting that the reagent causes the inactivation of the enzyme by the formation of a Schiff base. The rate of inactivation increased as the pH was raised, giving a pK of 8.85. Almost all the original activity was recovered by the treatment of the inactivated enzyme with cysteamine or any other aminothiol compound. No recovery of the activity, however, was observed with inactivated enzyme which had been treated with NaBH4. A peptide containing the labeled amino acid was isolated for inactivated enzyme after reduction with NaBH4, carboxymethylation, and chymotryptic digestion by fractionation on a Bio-Gel P-6 column and high performance liquid chromatographies. Manual Edman degradation established the sequence as Glu-Val-Ala-Asn-labeled Lys-Val-Gly-Gly-Ile-(Tyr). The introduction of an active site-directing moiety to pyridoxal 5'-phosphate makes the resultant reagent an effective probe for the active site of glycogen synthase.     

Amylase of the thermophilic actinomycete Thermomonospora vulgaris.

alpha-Amylase of the thermophilic actinomycete Thermomonospora vulgaris was partially purified. Maximal enzyme activity was obtained at 60degreeC and pH 6.0. KM value was l.4%. The effect of some metal salts on enzyme activity was studied. Enzyme activity was inhibited by by KCN, EDTA, and iodoacetate. Inhibition by EDTA was completely nullified by CaCl2, but the inhibition by iodoacetate was not overcome by 2-mercaptoethanol. Exposure of the enzyme to pH 7.0 and 9.0 for 2 hr. did not affect the enzyme, but exposure to pH 3.0 for few minutes completely inactivated the enzyme. Exposure of the enzyme to 60degreeC resulted in an appreciable inactivation and exposure to 80degreeC completely inactivated the enzyme. Addition of CaCl2, 2-mercaptoethanol, or enzyme substrate the 60degreeC exposed enzyme. However, bovine serym albumin had a protective effect when the enzyme was exposed to 60degreeC but not to 80degreeC. The enzyme was stable in the presence of 8 M urea.     

Evidence for the Degradation of Nicotinamide Adenine Dinucleotide Phosphate-Dependent Glutamate Dehydrogenase of Candida utilis During Rapid Enzyme Inactivation

The nicotinamide adenine dinucleotide phosphate-dependent glutamate dehydrogenase (NADP-GDH) from the food yeast Candida utilis was found to be rapidly inactivated when cultures were starved of a carbon source. The addition of glutamate or alanine to the starvation medium stimulated the rate of inactivation. Loss of enzyme activity was irreversible since the reappearance of enzyme activity, following the addition of glucose to carbon-starved cultures, was blocked by cycloheximide. A specific rabbit antibody was prepared against the NADP-GDH from C. utilis and used to quantitate the enzyme during inactivation promoted by carbon starvation. The amount of precipitable antigenic material paralleled the rapid decrease of enzyme activity observed after transition of cells from NH(4) (+)-glucose to glutamate medium. No additional small-molecular-weight protein was precipitated by the antibody as a result of the inactivation, suggesting that the enzyme is considerably altered during the primary steps of the inactivation process. Analysis by immunoprecipitation of the reappearance of enzyme activity after enzyme inactivation showed that increase of NADP-GDH activity was almost totally due to de novo synthesis, ruling out the possibility that enzyme activity modulation is achieved by reversible covalent modification. Enzyme degradation was also measured during steady-state growth and other changes in nitrogen and carbon status of the culture media. In all instances so far estimated, the enzyme was found to be very stable and not normally subject to high rates of degradation. Therefore, the possibility that inactivation was caused by a change in the ratio of synthesis to degradation can be excluded.     

An essential arginine residue at the substrate binding site of 4-hydroxyisophthalate hydroxylase

4-Hydroxyisophthalate hydroxylase was inactivated by treatment with phenylglyoxal by a process obeying pseudo-first order kinetics indicating the presence of an essential arginine located presumably in the active site. Addition of saturating amounts of 4-hydroxyisophthalate during the treatment resulted in complete protection of the enzyme from the inactivation, but addition of NADPH was totally ineffective. Analysis of the effect of various substrate analogs on the protection of the enzyme showed that carboxyl and hydroxyl groups at para positions on the aromatic ring are essential for substrate binding to the active site. It was also observed that analogs which protect the enzyme against phenylglyoxal inactivation are themselves effective inhibitors of the enzyme activity.     

Essential histidine residue in 3-ketosteroid-delta 1-dehydrogenase.

The variation with pH of kinetic parameters was examined for 3-ketosteroid-delta 1-dehydrogenase from Nocardia corallina. The Vmax/Km profile for 4-androstenedione indicates that activity is lost upon protonation of a cationic acid-type group with a pK value of 7.7. The enzyme was inactivated by diethylpyrocarbonate at pH 7.4 and the inactivation was substantially prevented by androstadienedione. Analyses of reactivation with neutral hydroxylamine, pH variation, and spectral changes of the inactivated enzyme revealed that the inactivation arises from modification of a histidine residue. Studies with [14C]diethylpyrocarbonate provided support for the idea that the 1-2 essential histidine residues are essential for the catalytic activity of the enzyme. Dye-sensitized photooxidation led to 50% inactivation of the enzyme with the decomposition of two histidine residues. This inactivation was also prevented by androstadienedione. Dancyl chloride caused a loss of the enzyme activity. Modifiers of glutamic acid, aspartic acid, cysteine, and lysine did not affect the enzyme activity. Butanedione and phenylglyoxal in the presence of borate rapidly inactivated the enzyme, indicating that arginine residues also have a crucial function in the active site. The data described support the previously proposed mechanism of beta-oxidation of 3-ketosteroid.     

Regulation of glutamine synthetase by metal-catalyzed oxidative modification in the marine oxyphotobacterium Prochlorococcus.

The inactivation of glutamine synthetase (GS; EC 6.3.1.2) by metal-catalyzed oxidation (MCO) systems was studied in several Prochlorococcus strains, including the axenic PCC 9511. GS was inactivated in the presence of various oxidative systems, either enzymatic (as NAD(P)H+NAD(P)H-oxidase+Fe(3+)+O(2)) or non-enzymatic (as ascorbate+Fe(3+)+O(2)). This process required the presence of oxygen and a metal cation, and is prevented under anaerobic conditions. Catalase and peroxidase, but not superoxide dismutase, effectively protected the enzyme against inactivation, suggesting that hydrogen peroxide mediates this mechanism, although it is not directly responsible for the reaction. Addition of azide (an inhibitor of both catalase and peroxidase) to the MCO systems enhanced the inactivation. Different thiols induced the inactivation of the enzyme, even in the absence of added metals. However, this inactivation could not be reverted by addition of strong oxidants, as hydrogen peroxide or oxidized glutathione. After studying the effect of addition of the physiological substrates and products of GS on the inactivation mechanism, we could detect a protective effect in the case of inorganic phosphate and glutamine. Immunochemical determinations showed that the concentration of GS protein significantly decreased by effect of the MCO systems, indicating that inactivation precedes the degradation of the enzyme.     

Folic acid deaminase activity during development in Dictyostelium discoideum.

Folic acid attracts vegetative amoebae of Dictyostelium discoideum. Secreted by bacteria, it may act as a food-seeking device. The inactivation of this attractant is catalyzed by a deaminase. As assay has been developed to measure the folic acid deaminase activity. In addition to cell-surface an intracellular deaminase, the amoebae of D. discoideum release the enzyme into the medium. The pH optimum of the extracellular enzyme was 6.0, and higher for the cell-associated deaminases. The extracellular enzyme was secreted maximally by vegetative amoebae, and its activity diminished during cell differentiation. The cell-surface bound enzyme was less active than the extracellular enzyme, and its activity decreased twofold during a 6-h starvation period. The enzyme activity of homogenates and 48,000 x g pellets diminished during this period 35 to 40%. The supernatant of a homogenate had a higher deaminase activity than the homogenate itself or its pellet; this suggests the presence of an inhibitor in the particulate fraction. The underlying mechanism for inactivation of folic acid has similar characteristics as that for inactivation of cyclic adenosine monophosphate.