NADH-coupled spectrophotometric assay of tyrosine aminotransferase

A rapid spectrophotometric method for estimation of tyrosine aminotransferase activity (TAT) is described, based on a coupled reaction with NADH-dependent aromatic ketoacid reductase. 3-iodo-L-tyrosine, upon TAT action, is transformed into 3-iodo-4-hydroxyphenylpyruvate which quickly reacts with NADH in the presence of aromatic ketoacid reductase; oxidation rates at 340 nm are linear with protein concentration over the whole range of purification steps of TAT. This new method, for its sensitivity, easy performance and possibility of a continuous monitoring of TAT reaction, may be considered comparable to the more diffuse spectrophotometric standard method, and also as an alternative, advantageous procedure in some instances. The method for purification of the coupled aromatic ketoacid reductase is also described.

Enzyme-enzyme interaction and the biosynthesis of aromatic amino acids in Escherichia coli

The technique of affinity chromatography has been used to demonstrate that enzymes involved in the biosynthesis of tyrosine and phenylalanine in Escherichia coli undergo reversible interactions. Thus it has been shown that the aromatic amino acid aminotransferase (aromatic-amino-acid: 2-oxoglutarate amino-transferase, EC 2.6.1.57) reacts specifically with chorismate mutaseprephenate dehydrogenase (chorismate pyruvate mutase, EC 5.4.99.5 and prephenate: NAD+ oxidoreductase (decarboxylating), EC 1.3.1.12) in the absence of reactants and with chorimate mutase-prephenatedehydratase (prephenate hydro-lyase (decarboxylating), EC 4.2.1.51) in the presence of phyenylpyruvate. Tyrosine causes dissociation of the aminotransferase: mutasedehydrogenase complex while dissociation of the aminotransferase-mutasedehydratase complex occurs on omission of phenylpyruvate. Only the active form of chorismate mutase-prephenate dehydrogenase participates in complex formation.

Use of p-aminophenyl D and L-lactic acids and p-aminophenyl pyruvic acid as effectors in the affinity chromatography of lactate dehydrogenase

p-Aminophenyl pyruvic acid and D-p-amino-phenyllactic acid were immobilized on a new synthetic acrylic carrier bearing acylating N-succinimidyl ester groups. The derivatives obtained were used successfully to purify lactate dehydrogenase (LDH) by affinity chromatography, the elution being carried out by means of NADH or preferably L-phenyllactic acid. Moreover, the specific activity of the LDH contained in a human blood serum was increased 270 times, using L-p-aminophenyllactic acid immobilized on a mixed polyacrylic agarose carrier.

The mitochondrial pyruvate carrier. Kinetics and specificity for substrates and inhibitors

1. Studies on the kinetics of pyruvate transport into mitochondria by an 'inhibitor-stop' technique were hampered by the decarboxylation of pyruvate by mitochondria even in the presence of rotenone. Decarboxylation was minimal at 6 degrees C. At this temperature the Km for pyruvate was 0.15 mM and Vmax. was 0.54nmol/min per mg of protein; alpha-cyano-4-hydroxycinnamate was found to be a non-competitive inhibitor, Ki 6.3 muM, and phenyl-pyruvate a competitive inhibitor, Ki 1.8 mM. 2. At 100 muM concentration, alpha-cyano-4-hydroxycinnamate rapidly and almost totally inhibited O2 uptake by rat heart mitochondria oxidizing pyruvate. Inhibition could be detected at concentrations of inhibitor as low as 1 muM although inhibition took time to develop at this concentration. Inhibition could be reversed by diluting out the inhibitor. 3. Various analogues of alpha-cyano-4-hydroxycinnamate were tested on rat liver and heart mitochondria. The important structural features appeared to be the alpha-cyanopropenoate group and the hydrophobic aromatic side chain. Alpha-Cyanocinnamate, alpha-cyano-5-phenyl-2,4-pentadienoate and compound UK 5099 [alpha-cyano-beta-(2-phenylindol-3-yl)acrylate] were all more powerful inhibitors than alpha-cyano-4-hydroxycinnamate showing 50% inhibition of pyruvate-dependent O2 consumption by rat heart mitochondria at concentrations of 200, 200 and 50 nM respectively. 4. The specificity of the carrier for its substrate was studied by both influx and efflux experiments. Oxamate, 2-oxobutyrate, phenylpyruvate, 2-oxo-4-methyl-pentanoate, chloroacetate, dichloroacetate, difluoroacetate, 2-chloropropionate, 3-chloropropionate and 2,2-dichloropropionate all exchanged with pyruvate, whereas acetate, lactate and trichloroacetate did not. 5. Pyruvate entry into the mitochondria was shown to be accompanied by the transport of a proton (or by exchange with an OH-ion). This proton flux was inhibited by alpha-cyano-4-hydroxycinnamate and allowed measurements of pyruvate transport at higher temperatures to be made. The activation energy of mitochondrial pyruvate transport was found to be 113 kJ (27 kcal)/mol and by extrapolation the rate of transport of pyruvate at 37 degrees C to be 42 nmol/min per mg of protein. The possibility that pyruvate transport into mitochondria may be rate limiting and involved in the regulation of gluconegenesis is discussed. 6. The transport of various monocarboxylic acids into mitochondria was studied by monitoring proton influx. The transport of dichloroacetate, difluoroacetate and oxamate appeared to be largely dependent on the pyruvate carrier and could be inhibited by pyruvate-transport inhibitors. However, many other halogenated and 2-oxo acids which could exchange with pyruvate on the carrier entered freely even in the presence of inhibitor.

The use of ternary complexes to study ionizations and isomerizations during catalysis by lactate dehydrogenase

1. The binding of oxamate to pig heart and pig muscle isoenzymes of lactate dehydrogenase in the presence of NADH was studied by fluorescence titration. The dissociation constant of oxamate from the heart enzyme complex is 3mum and from the muscle isoenzyme 25mum at pH5. These values quantitatively increase with pH as predicted if oxamate can bind only to the enzyme-NADH complex if a group with pK6.9 is protonated. There are four non-interacting oxamate-binding sites per tetramer. 2. o-Nitrophenylpyruvate is a poor substrate for both isoenzymes but has a reasonable affinity to the heart isoenzyme. Initially, it forms an enzyme-NADH-substrate complex, which can be detected either by protein-fluorescence quenching or by NADH-fluorescence quenching. The pH-dependence of the dissociation constant of nitrophenylpyruvate also shows that this ternary complex can only form if a group with pK6.8 is protonated. Taken with the results of chemical-modification experiments, these results allow the pK of 6.8 to be assigned to a system probably involving the imidazole side chain of histidine-195. Formation of a ternary complex from a binary one at pH8 is predicted to result in a proton being taken up from solution. 3. Isotope-effect studies with NADH and its deuterium analogue show that the rapidly formed ternary complex with o-nitrophenylpyruvate slowly isomerizes to give an active ternary complex, which then rapidly decomposes to NAD(+). The isomerization is pH-independent, and it is suggested that histidine-195 is still protonated in the activated ternary complex, which is present before hydride transfer. 4. All four subunits of the enzyme are kinetically equivalent with respect to the oxidation of bound NADH by o-nitrophenylpyruvate. 5. A partial mechanism for the enzyme is described which emphasizes the isomerizations and ionizations involved in forming the reduced ternary complex at pH6 and 8.

The oxidation of Schiff bases of pyridoxamine with -oxo acids by manganous ions and peroxidase

1. Oxygen was taken up rapidly when pyruvate was added to mixtures of pyridoxamine and Mn(2+) ions after lag periods that were shortened by peroxidase (donor-hydrogen peroxide oxidoreductase, EC 1.11.1.7). 2. The total oxygen uptake was proportional to the pyridoxamine added and was accompanied by the disappearance of pyridoxamine; the pyruvate acted catalytically and hydrogen peroxide was not formed. 3. At pH6 more than half the pyridoxamine that disappeared was accounted for as pyridoxal and ammonia; it is suggested that the primary reaction is the oxidative deamination of the pyridoxamine. 4. Results were similar when alpha-oxobutyrate or glyoxylate were substituted for pyruvate, except that the reactions were slower and the yield of pyridoxal less. 5. The oxidative decarboxylations of alpha-oxoglutarate and phenylpyruvate are catalysed by Mn(2+) ions and these reactions are activated by peroxidase; pyridoxamine increased both the rates and total oxygen uptakes in these reactions, and ammonia was produced. 6. The lag periods in the oxidation of mixtures of pyridoxamine and alpha-oxo acids, catalysed by Mn(2+) ions, were also shortened by traces of colloidal manganese dioxide. 7. It is suggested that the activating effect of peroxidase depends on its catalysis of manganese oxidation.

The nature of the collagenolytic cathepsin of rat liver and its distribution in other rat tissues

1. An enzyme present in rat liver extracts degraded insoluble collagen maximally at pH3.5. Collagenolytic activity was more abundant in kidney, spleen and bone marrow and was also present in decreasing concentrations in ileum, lung, heart, skin and muscle. 2. The crude collagenolytic cathepsin was activated by cysteine and dithiothreitol, but not by 2-mercaptoethanol. Iodoacetamide, p-chloromercuribenzoate and 7-amino-1-chloro-3-l-tosylamidoheptan-2-one hydrochloride inhibited the enzyme. Zn(2+), Fe(3+) and Hg(2+) ions were strongly inhibitory, but Ca(2+), Co(2+), Mg(2+) and Fe(2+) ions had little or no effect. EDTA was an activator of the enzyme. Inhibitors of cathepsin B were found to enhance collagenolysis, but phenylpyruvic acid, a cathepsin D inhibitor, inhibited the enzyme. Di-isopropyl phosphorofluoridate had no effect. 3. Collagenolysis at pH3.5 and 28 degrees C was restricted to cleavage of the telopeptide region in insoluble collagen, and the material that was solubilized consisted mostly of alpha-chains. 4. The collagenolytic cathepsin was separated from cathepsins B2 and D by fractionation on Sephadex G-100 and a partial separation from cathepsin B1 was obtained by chromatography on DEAE-Sephadex. 5. The function of the collagenolytic cathepsin in the catabolism of collagen is discussed in relation to the action of the other lysosomal proteinases and the neutral collagenase.