beta-elimination of beta-halo substrates by D-amino acid transaminase associated with inactivation of the enzyme. Trapping of a key intermediate in the reaction

Emerging Moonlighting Functions of the Branched-Chain Aminotransferase Proteins

Significance: Unique to the branched-chain aminotransferase (BCAT) proteins is their redox-active CXXC motif. Subjected to post-translational modification by reactive oxygen species and reactive nitrogen species, these proteins have the potential to adopt numerous cellular roles, which may be fundamental to their role in oncogenesis and neurodegenerative diseases. An understanding of the interplay of the redox regulation of BCAT with important cell signaling mechanisms will identify new targets for future therapeutics. Recent Advances: The BCAT proteins have been assigned novel thiol oxidoreductase activity that can accelerate the refolding of proteins, in particular when S-glutathionylated, supporting a chaperone role for BCAT in protein folding. Other metabolic proteins were also shown to have peroxide-mediated redox associations with BCAT, indicating that the cellular function of BCAT is more diverse. Critical Issues: While the role of branched-chain amino acid metabolism and its metabolites has dominated aspects of cancer research, less is known about the role of BCAT. The importance of the CXXC motif in regulating the BCAT activity under hypoxic conditions, a characteristic of tumors, has not been addressed. Understanding how these proteins operate under various cellular redox conditions will become important, in particular with respect to their moonlighting roles. Future Directions: Advances in the quantification of thiols, their measurement, and the manipulation of metabolons that rely on redox-based interactions should accelerate the investigation of the cellular role of moonlighting proteins such as BCAT. Given the importance of cross talk between signaling pathways, research should focus more on these "housekeeping" proteins paying attention to their wider application. Antioxid. Redox Signal. 34, 1048-1067.

Chemistry and Biochemistry of Sulfur Natural Compounds: Key Intermediates of Metabolism and Redox Biology

Sulfur contributes significantly to nature chemical diversity and thanks to its particular features allows fundamental biological reactions that no other element allows. Sulfur natural compounds are utilized by all living beings and depending on the function are distributed in the different kingdoms. It is no coincidence that marine organisms are one of the most important sources of sulfur natural products since most of the inorganic sulfur is metabolized in ocean environments where this element is abundant. Terrestrial organisms such as plants and microorganisms are also able to incorporate sulfur in organic molecules to produce primary metabolites (e.g., methionine, cysteine) and more complex unique chemical structures with diverse biological roles. Animals are not able to fix inorganic sulfur into biomolecules and are completely dependent on preformed organic sulfurous compounds to satisfy their sulfur needs. However, some higher species such as humans are able to build new sulfur-containing chemical entities starting especially from plants' organosulfur precursors. Sulfur metabolism in humans is very complicated and plays a central role in redox biochemistry. The chemical properties, the large number of oxidation states, and the versatile reactivity of the oxygen family chalcogens make sulfur ideal for redox biological reactions and electron transfer processes. This review will explore sulfur metabolism related to redox biochemistry and will describe the various classes of sulfur-containing compounds spread all over the natural kingdoms. We will describe the chemistry and the biochemistry of well-known metabolites and also of the unknown and poorly studied sulfur natural products which are still in search for a biological role.

Stereochemistry of reactions of the inhibitor/substrates l- and d-β-chloroalanine with β-mercaptoethanol catalysed by l-aspartate aminotransferase and d-amino acid aminotransferase respectively

Two members of the alpha-family of PLP-dependent enzymes, L-aspartate aminotransferase and D-amino acid aminotransferase, have been shown to catalyse beta-substitution of L- and D-beta-chloroalanine respectively with beta-mercaptoethanol, reactions typical of the beta-family of PLP-dependent enzymes. The reaction catalysed by L-aspartate aminotransferase has been shown to occur with retention of stereochemistry, a typical outcome for reactions catalysed by beta-family enzymes. There are also indications that the reaction catalysed by D-amino acid aminotransferase may involve retention of stereochemistry. Both enzymes have been shown to catalyse exchange at C-3 when the appropriate enantiomer of beta-chloroalanine is the substrate.

L-alanine-glyoxylate aminotransferase II of rat kidney and liver mitochondria possesses cysteine S-conjugate beta-lyase activity: a contributing factor to the nephrotoxicity/hepatotoxicity of halogenated alkenes?

Several halogenated alkenes are metabolized in part to cysteine S-conjugates, which are mitochondrial toxicants of kidney and, to a lesser extent, other organs. Toxicity is due to cysteine S-conjugate beta-lyases, which convert the cysteine S-conjugate into pyruvate, ammonia and a reactive sulphur-containing fragment. A section of the human population is exposed to halogenated alkenes. To understand the health effects of such exposure, it is important to identify cysteine S-conjugate beta-lyases that contribute to mitochondrial damage. Mitochondrial aspartate aminotransferase [Cooper, Bruschi, Iriarte and Martinez-Carrion (2002) Biochem. J. 368, 253-261] and mitochondrial branched-chain aminotransferase [Cooper, Bruschi, Conway and Hutson (2003) Biochem. Pharmacol. 65, 181-192] exhibit beta-lyase activity toward S -(1,2-dichlorovinyl)-L-cysteine (the cysteine S-conjugate of trichloroethylene) and S -(1,1,2,2-tetrafluoroethyl)-L-cysteine (the cysteine S-conjugate of tetrafluoroethylene). Turnover leads to eventual inactivation of these enzymes. Here we report that mitochondrial L-alanine-glyoxylate aminotransferase II, which, in the rat, is most active in kidney, catalyses cysteine S-conjugate beta-lyase reactions with S -(1,1,2,2-tetrafluoroethyl)-L-cysteine, S -(1,2-dichlorovinyl)-L-cysteine and S -(benzothiazolyl-L-cysteine); turnover leads to inactivation. Previous workers showed that the reactive-sulphur-containing fragment released from S -(1,1,2,2-tetrafluoroethyl)-L-cysteine and S -(1,2-dichlorovinyl)-L-cysteine is toxic by acting as a thioacylating agent - particularly of lysine residues in nearby proteins. Toxicity, however, may also involve 'self-inactivation' of key enzymes. The present findings suggest that alanine-glyoxylate aminotransferase II may be an important factor in the well-established targeting of rat kidney mitochondria by toxic halogenated cysteine S-conjugates. Previous reports suggest that alanine-glyoxylate aminotransferase II is absent in some humans, but present in others. Alanine-glyoxylate aminotransferase II may contribute to the bioactivation (toxification) of halogenated cysteine S-conjugates in a subset of individuals exposed to halogenated alkenes.

Human mitochondrial and cytosolic branched-chain aminotransferases are cysteine S-conjugate beta-lyases, but turnover leads to inactivation

The mitochondrial and cytosolic branched-chain aminotransferases (BCAT(m) and BCAT(c)) are homodimers in the fold type IV class of pyridoxal 5'-phosphate-containing enzymes that also contains D-amino acid aminotransferase and 4-amino-4-deoxychorismate lyase (a beta-lyase). Recombinant human BCAT(m) and BCAT(c) were shown to have beta-lyase activity toward three toxic cysteine S-conjugates [S-(1,1,2,2-tetrafluoroethyl)-L-cysteine, S-(1,2-dichlorovinyl)-L-cysteine, and S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine] and toward beta-chloro-L-alanine. Human BCAT(m) is a much more effective beta-chloro-L-alanine beta-lyase than two aminotransferases (cytosolic and mitochondrial isozymes of aspartate aminotransferase) previously shown to possess this activity. BCAT(m), but not BCAT(c), also exhibits measurable beta-lyase activity toward a relatively bulky cysteine S-conjugate [benzothiazolyl-L-cysteine]. Benzothiazolyl-L-cysteine, however, inhibits the L-leucine-alpha-ketoglutarate transamination reaction catalyzed by both enzymes. Inhibition was more pronounced with BCAT(m). In the presence of beta-lyase substrates and alpha-ketoisocaproate (the alpha-keto acid analogue of leucine), no transamination could be detected. Therefore, with an amino acid containing a good leaving group in the beta position, beta-elimination is greatly preferred over transamination. Both BCAT isozymes are rapidly inactivated by the beta-lyase substrates. The ratio of turnover to inactivation per monomer in the presence of toxic halogenated cysteine S-conjugates is approximately 170-280 for BCAT(m) and approximately 40-50 for BCAT(c). Mitochondrial enzymes of energy metabolism are especially vulnerable to thioacylation and inactivation by the reactive fragment released from toxic, halogenated cysteine S-conjugates such as S-(1,1,2,2-tetrafluoroethyl)-L-cysteine. The present results suggest that BCAT isozymes may contribute to the mitochondrial toxicity of these compounds by providing thioacylating fragments, but inactivation of the BCAT isozymes might also block essential metabolic pathways.

Mitochondrial aspartate aminotransferase catalyses cysteine S-conjugate beta-lyase reactions

Rat liver mitochondrial aspartate aminotransferase (a homodimer) was shown to catalyse a beta-lyase reaction with three nephrotoxic halogenated cysteine S-conjugates [ S -(1,1,2,2-tetrafluoroethyl)-L-cysteine, S -(1,2-dichlorovinyl)-L-cysteine and S -(2-chloro-1,1,2-trifluoroethyl)-L-cysteine], and less effectively so with a non-toxic cysteine S-conjugate [benzothiazolyl-L-cysteine]. Transamination competes with the beta-lyase reaction, but is not favourable. The ratio of beta elimination to transamination in the presence of S -(1,1,2,2-tetrafluoroethyl)-L-cysteine and 2-oxoglutarate is >100. Syncatalytic inactivation by the halogenated cysteine S-conjugates is also observed. The enzyme turns over approx. 2700 molecules of halogenated cysteine S-conjugate on average for every monomer inactivated. Kidney mitochondria are known to be especially sensitive to toxic halogenated cysteine S-conjugates. Evidence is presented that 15-20% of the cysteine S-conjugate beta-lyase activity towards S -(1,1,2,2-tetrafluoroethyl)-L-cysteine in crude kidney mitochondrial homogenates is due to mitochondrial aspartate aminotransferase. The possible involvement of mitochondrial aspartate aminotransferase in the toxicity of halogenated cysteine S-conjugates is also discussed.

Mechanisms of cysteine S-conjugate beta-lyases

Mercapturic acids are conjugates of S-(N-acetyl)-L-cysteine formed during the detoxification of xenobiotics and during the metabolism of such endogenous agents as estrogens and leukotrienes. Many mercaturates are formed from the corresponding glutathione S-conjugates. This chapter focuses on (a) the discovery of the cysteine S-conjugate beta-lyases; (b) the involvement of pyridoxal-5-phosphate; (c) the influence of the electron-withdrawing properties of the group attached to the sulfur atom; and (d) the potential of cysteine S-conjugates as pro-drugs.

A stereo-inverting D-phenylglycine aminotransferase from Pseudomonas stutzeri ST-201: purification, characterization and application for D-phenylglycine synthesis

D-phenylglycine aminotransferase (D-PhgAT) from a newly isolated soil bacterium, Pseudomonas stutzeri ST-201, was purified to electrophoretic homogeneity and characterized. The molecular weight (M(r)) of the native enzyme was estimated to be 92,000. It is composed of two subunits identical in molecular weight (M(r)) = 47,500). The isoelectric point (pI) of the native enzyme was 5.0. The enzyme catalyzed reversible transamination specific for D-phenylglycine or D-4-hydroxyphenylglycine in which 2-oxoglutarate was an exclusive amino group acceptor and was converted into L-glutamic acid. Neither the D- nor L-isomer of phenylalanine, tyrosine, alanine, valine, leucine, isoleucine or serine could serve as a substrate. The enzyme was most active at alkaline pH with maximum activity at pH 9-10. The temperature for maximum activity was 35-45 degrees C. The apparent K(m) values for D-phenylglycine and for 2-oxoglutarate at 35 degrees C, pH 9.5 were 1.1 and 2.4 mM, respectively. The enzyme activity was strongly inhibited by typical inhibitors of pyridoxal phosphate-dependent enzymes. Possible application of this enzyme for synthesis of enantiomerically pure D-phenylglycine was demonstrated.

The ubiquitous cofactor NADH protects against substrate-induced inhibition of a pyridoxal enzyme

In the usual reaction catalyzed by D-amino acid transaminase, cleavage of the alpha-H bond is followed by the reversible transfer of the alpha-NH2 to a keto acid cosubstrate in a two-step reaction mediated by the two vitamin B6 forms pyridoxal 5'-phosphate (PLP) and pyridoxamine 5'-phosphate (PMP). We report here a reaction not on the main pathway, i.e., beta-decarboxylation of D-aspartate to D-alanine, which occurs at 0.01% the rate of the major transaminase reaction. In this reaction, beta-C-C bond cleavage of the single substrate D-aspartate occurs rather than the usual alpha-bond cleavage in the transaminase reaction. The D-alanine produced from D-aspartate slowly inhibits both transaminase and decarboxylase activities, but NADH or NADPH instantaneously prevent D-aspartate turnover and D-alanine formation, thereby protecting the enzyme against inhibition. NADH has no effect on the enzyme spectrum itself in the absence of substrates, but it acts on the enzyme.D-aspartate complex with an apparent dissociation constant of 16 microM. Equivalent concentrations of NAD or thiols have no such effect. The suppression of beta-decarboxylase activity by NADH occurs concomitant with a reduction in the 415-nm absorbance due to the PLP form of the enzyme and an increase at 330 nm due to the PMP form of the enzyme. alpha-Ketoglutarate reverses the spectral changes caused by NADH and regenerates the active PLP form of the enzyme from the PMP form with an equilibrium constant of 10 microM. In addition to its known role in shuttling electrons in oxidation-reduction reactions, the niacin derivative NADH may also function by preventing aberrant damaging reactions for some enzyme-substrate intermediates. The D-aspartate-induced effect of NADH may indicate a slow transition between protein conformational studies if the reaction catalyzed is also slow.

The mechanism of high-yielding chiral syntheses catalysed by wild-type and mutant forms of aspartate aminotransferase

The ability of aspartate aminotransferase to catalyse beta-elimination of alpha-amino acids that have a good leaving group at C beta has been exploited in the synthesis of novel amino acids by the inclusion of appropriate nucleophiles as co-substrates. Two compounds, L-serine O-sulphate and 3-chloro-L-alanine, were used as beta-elimination substrates. Nucleophiles used successfully as co-substrates were thiosulphate, 2-mercaptoethanol, mercaptoacetate and aminoethylthiopseudourea. The synthesis achieved using serine O-sulphate and thiosulphate was found to produce sulphocysteine with a yield of 70%. Circular dichroism demonstrated that the compound was a single enantiomer and, therefore, that nucleophilic addition had taken place on the enzyme. The initial rate of synthesis was 10% of the rate at which the enzyme catalyses its normal transamination reaction. The synthetic reaction was accompanied by minor side reactions that led to small amounts of additional amino acid and oxo acid products through partitions of the main reaction at two stages in the mechanism. By mutating Arg292, which is the residue that binds the distal carboxyl group of natural substrates, the wild-type enzyme was converted to a form that could discriminate completely between serine O-sulphate and chloroalanine as beta-eliminating substrate. Similar alterations in nucleophile cosubstrate specificity were also observed. Whereas, for example, the wild-type enzyme catalysed syntheses between 3-chloroalanine and either mercaptoethanol or mercaptoacetate with equal facility, the Arg292Asp enzyme showed complete preference for mercaptoethanol. The system should be of general use in the synthesis of novel amino acids as single enantiomers with potentially interesting biological activities.

Interaction of pyridoxal 5'-phosphate with tryptophan-139 at the subunit interface of dimeric D-amino acid transaminase

The crystal structure of dimeric bacterial D-amino acid transaminase shows that the indole rings of the two Trp-139 side chains face each other in the subunit interface about 10 angstroms from the coenzyme, pyridoxal 5'-phosphate. To determine whether it has a role in the catalytic efficiency of the enzyme or interacts with the coenzyme, Trp-139 has been substituted by several different types of amino acids, and the properties of these recombinant mutant enzymes have been compared to the wild-type enzyme. In the native wild-type holoenzyme, the fluorescence of one of the three Trp residues per monomer is almost completely quenched, probably due to its interaction with PLP since in the native wild-type apoenzyme devoid of PLP, tryptophan fluorescence is not quenched. Upon reconstitution of this apoenzyme with PLP, the tryptophan fluorescence is quenched to about the same extent as it is in the native wild-type enzyme. The site of fluorescence quenching is Trp-139 since the W139F mutant in which Trp-139 is replaced by Phe has about the same amount of fluorescence as the wild-type enzyme. The circular dichroism spectra of the holo and the apo forms of both the wild-type and the W139F enzymes in the far-ultraviolet show about the same degree of ellipticity, consistent with the absence of extensive global changes in protein structure. Furthermore, comparison of the circular dichroism spectrum of the W139F enzyme at 280 nm with the corresponding spectral region of the wild-type enzyme suggests a restricted microenvironment for Trp-139 in the latter enzyme. The functional importance of Trp-139 is also demonstrated by the finding that its replacement by Phe, His, Pro, or Ala gives mutant enzymes that are optimally active at temperatures below that of the wild-type enzyme and undergo the E-PLP --> E-PMP transition as a function of D-Ala concentration with reduced efficiency. The results suggest that a fully functional dimeric interface with the two juxtaposed indole rings of Trp-139 is important for optimal catalytic function and maximum thermostability of the enzyme and, furthermore, that there might be energy transfer between Trp-139 and coenzyme PLP.

The reducing activity of S-aminoethylcysteine ketimine and similar sulfur-containing ketimines

S-aminoethylcysteine ketimine and other sulfur-containing similar ketimines reduce molecular oxygen and phospho-18-tungstate (Folin Marenzi reagent), although the sulfur atom is formally present in the non reducing thioether form. We have now found that 2,6-diclorophenolindophenol, some ferrihemoproteins and other reagents are also reduced by this group of ketimines. Ferricytochrome c is reduced faster than methemoglobin, metmyoglobin and free hematin, whereas horse radish peroxidase compound I is reduced at once. These results indicate a wider reducing activity of this type of ketimine. The oxidation of ketimines by ferric cytochrome c appears a relevant finding pointing to a new possible way of enzymatic modification of sulfur-ketimines in tissues.

Sulfur-containing cyclic ketimines and imino acids. A novel family of endogenous products in the search for a role

Aminoethylcysteine, lanthionine, cystathionine and cystine are mono-deaminated either by L-amino-acid oxidase or by a transaminase exhibiting the properties described for glutamine transaminase. The deaminated products cyclize producing the respective ketimines. Authentic samples of each ketimine were prepared by reacting the appropriate aminothiol compound with bromopyruvate, except cystine ketimine which required the interaction of thiopyruvate with cystine sulfoxide. Reduction of the first three mentioned ketimines with NaBH4 yields the respective derivatives with the saturated rings of thiomorpholine and hexahydrothiazepine. The same reduction is carried out enzymically by a reductase extracted from mammalian tissues. Properties of the members of this family of compounds are described. Gas chromatography followed by mass spectrometry permits the identification of most of these products. HPLC is very useful for the determination of the ketimines by taking advantage of specific absorbance at 380 nm obtained by prior derivatization with phenylisothiocyanate. Adaptation of these and other analytical procedures to biological samples disclosed the presence of most of these compounds in bovine brain and in human urine. By using [35S]lanthionine ketimine as a representative member of the ketimine group, the specific, high-affinity, saturable and reversible binding to bovine brain membranes has been demonstrated. The binding is removed by aminoethylcysteine ketimine and by cystathionine ketimine indicating the occurrence in bovine brain of a common binding site for ketimines. The reduced ketimines are totally ineffective in competing with [35S]lanthionine ketimine. Alltogether these findings are highly indicative for the existence in mammals of a novel class of endogenous sulfur-containing cyclic products provided with a possible neurochemical function to be investigated further.

Effect of substitution of a lysyl residue that binds pyridoxal phosphate in thermostable D-amino acid aminotransferase by arginine and alanine

Lys-145 of the thermostable D-amino acid aminotransferase, which binds pyridoxal phosphate, was replaced by Ala or Arg by site-directed mutagenesis. Both mutant enzymes were purified to homogeneity; their absorption spectra indicated that both mutant enzymes contained pyridoxal phosphate bound non-covalently. Even though the standard assay method did not indicate any activity with either mutant, addition of an amino donor, D-alanine, to the Arg-145 mutant enzyme led to a slow decrease in absorption at 392 nm with a concomitant increase in absorption at 333 nm. This result suggests that the enzyme was converted into the pyridoxamine phosphate form. The amount of pyruvate formed was almost equivalent to that of the reactive pyridoxal phosphate in the mutant enzyme. Thus, the Arg-145 mutant enzyme is able to catalyze slowly the half-reaction of transamination. Exogenous amines, such as methylamine, had no effect on the half-reaction with the Arg-145 mutant enzyme. In contrast, the Ala-145 mutant enzyme neither underwent the spectral change by addition of D-alanine nor catalyzed pyruvate formation, in the absence of added amine. However, the Ala-145 mutant enzyme catalyzed the half-reaction significantly in the presence of added amine. These findings suggest that a basic amino acid residue, such as lysine or arginine, is required at position 145 for catalysis of the half-reaction. The role of the exogenous amines differs with various active-site mutant enzymes.

Identification of 1,4-thiomorpholine-3-carboxylic acid (TMA) in normal human urine

The sulfur-containing cyclic imino acid 1,4-thiomorpholine-3-carboxylic acid (TMA) has been identified in normal human urine. After the enrichment procedure with ion-exchange chromatography, the urine extracts were reacted with diazomethane. Gas-liquid chromatography revealed the presence of two peaks with the same retention times exhibited by authentic TMA after the same derivatization. The two compounds have been identified by mass-spectrometry as the monomethylated and dimethylated derivatives of TMA. This result represents the first indication of the occurrence of TMA in a mammalian sample.

The potential use of mechanism-based enzyme inactivators in medicine

Mechanism-based enzyme inactivator, alanine racemase, S-adenosylhomocysteine hydrolase, D-amino acid aminotransferase, gamma-aminobutyric acid aminotransferase, arginine decarboxylase, aromatase, L-aromatic amino acid decarboxylase, dihydrofolate reductase, dihydroorotate dehydrogenase DNA polymerase I, dopamine beta-hydroxylase, histidine decarboxylase, beta-lactamase, monoamine oxidase, ornithine decarboxylase, serine proteases, testosterone 5 alpha-reductase, thymidylate synthetase, xanthine oxidase.

Preliminary X-ray data for a D-amino acid amino-transferase from a novel thermophilic Bacillus

Crystals of the D-amino acid aminotransferase (D-ATA) from a novel thermophilic Bacillus species (Escherichia coli pICT113 cloned gene product) have been examined by X-ray analysis. The crystals grow as hexagonal prisms, with the symmetry of space group P61 or P65 (indistinguishable crystallographically). The cell dimensions are a = b = 135 A, c = 53 A, alpha = beta = 90 degrees, and gamma = 120 degrees. The unit cell has a volume of 850,000 A3 with six asymmetric units per unit cell. There is one dimer of molecular weight 62,000 per asymmetric unit, and the crystals diffract to 2.7 A.

Enzyme-activated inhibition of bacterial D-amino acid transaminase by beta-cyano-D-alanine

beta-Cyano-D-alanine is an efficient suicide substrate (Ki = 10 microM) of D-amino acid transaminase. This apparent inactivation is temperature dependent: it is irreversible at 10 degrees C or below and becomes progressively reversible at higher temperatures. Since at higher temperatures the apparent reactivation process predominates over the inactivation reaction, the reactivation process is considered to be endothermic. The nature of this reversibility suggests the formation of a heat labile bond between the inhibitor molecule and a nucleophilic group on the enzyme.

Inhibition of glutamate-aspartate transaminase by beta-methylene-DL-aspartate

beta-Methylene-DL-aspartate, a new beta, gamma-unsaturated amino acid, is an irreversible inhibitor of soluble pig heart glutamate-aspartate transaminase (Ki approximately 3 mM with respect to the L-form; limiting rate constant for inactivation approximately 0.4 min-1). The new amino acid is the most specific inhibitor of glutamate-aspartate transaminase thus far studied. It does not inactivate pig heart glutamate-alanine transaminase, soluble rat kidney glutamine transaminase K, gamma-aminobutyrate transaminase (from Pseudomonas fluorescens), glutamate decarboxylase (Escherichia coli), snake venom L-amino acid oxidase, or hog kidney D-amino acid oxidase. In addition, the following enzymes were not inhibited by beta-methylene-DL-aspartate in rat tissue homogenates: gamma-aminobutyrate transaminase (brain), tyrosine transaminase (liver), glutamine transaminase L (liver), asparagine, transaminase (liver), ornithine transaminase (liver) or branch-chain transaminase(s) (kidney). Intraperitoneal injection of beta-methylene-DL-aspartate into mice decreased kidney and liver glutamate-aspartate transaminase activities but had no effect on liver glutamate-alanine transaminase activity.

The slow kinetic transients of arylsulphatase A

A simple model is described to account for the anomalous time course of arylsulphatase A. In the case of the ox liver and human placental enzymes the enzyme-nitrocatechol sulphate complex can, in addition to forming products, slowly break down to form an inactive species which can turn in slowly regenerate active enzyme. When the inactive form binds sulphate the rate of reactivation is enhanced, 218-fold in the case of the ox enzyme. Rat liver arylsulphatase A is refractory to reactivation by sulphate.

Preparation of 3-mercaptolactic acid and S-aminoethylmercaptolactic acid

A simple and accurate method is described for synthesis of 3-mercaptolactic acid and its derivative, S-aminoethylmercaptolactic acid. Some spectrometric data of compounds are reported as well as their melting points, some colorimetric reactions and thin layer chromatographic behaviour. S-Aminoethylmercaptolactic acid is also determined by amino acid analyzer.

Kinetics of suicide substrates

When suicide substrates inactivate enzymes during catalysis, formation of product and inactivation of enzyme proceed concurrently. The steady-state hypothesis is applicable when catalytic quantities of enzyme are used. Equations for the rate of inactivation have been derived and integrated to obtain equations describing progress curves.