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Biomedical subjects

K Tanizawa

Publications and source records attributed to K Tanizawa.

At least 127 records · Page 7Linked to original sources
PubMed1985

Stereochemistry of the kynureninase reaction.

The steric course of the replacement of the anthranilyl group of kynurenine by hydrogen was determined by conversion of (2S,3R)- and (2S,3S)-[3-3H]kynurenine into alanine with kynureninase in D2O, followed by chirality analysis of the alanine methyl group. To minimize enolization, the labeled substrates were generated in situ from the corresponding stereospecifically tritiated tryptophan species. The result, replacement in a retention mode, together with the finding that tritium from the alpha-position of the substrate is recycled and appears both at C alpha and C beta of the product suggests a single base mechanism and an active site geometry of the pyridoxal phosphate-substrate complex in which H alpha and the beta-substituent are syn oriented.

Chemical Phenomena↗
PubMed1985

Oxygen and sulfur esters of "inverse substrates": different responses of amidinophenol and amidinothiophenol in the activation of the rate of tryptic hydrolysis of the inverse esters.

Kinetic parameters for the trypsin-catalyzed hydrolysis of the oxygen and sulfur "inverse substrates," p-amidinophenyl and p-amidinothiophenyl acetates and trimethylacetates, have been compared. The results suggest that both series of compounds are hydrolyzed via an identical pathway. Appreciable differences, however, were observed in the efficiency of the acylation process in both series, possibly reflecting the spatial requirements of the enzyme's active site toward these substrates. As reported previously, acceleration in deacylation by a positively charged molecule is a characteristic feature of trypsin-catalyzed hydrolysis of "inverse substrates." In the present investigation, it was shown that p-amidinothiophenol is ineffective as an activator, whereas its oxygen counterpart behaves as a potent activator toward oxygen and sulfur substrates. It is assumed that some ionic interaction between the enzyme and the ligand molecule could prevent the rate enhancement.

Acetates↗
PubMed1985

Purification and characterization of selenocysteine beta-lyase from Citrobacter freundii.

The purification and characterization of bacterial selenocysteine beta-lyase, an enzyme which specifically catalyzes the cleavage of L-selenocysteine to L-alanine and Se0, are presented. The enzyme, purified to near homogeneity from Citrobacter freundii, is monomeric with a molecular weight of ca. 64,000 and contains 1 mol of pyridoxal 5'-phosphate as a cofactor per mol of enzyme. L-Selenocysteine is the sole substrate (Km, 0.95 mM). L-Cysteine is a competitive inhibitor of the enzyme (Ki, 0.65 mM). The enzyme also catalyzes the alpha, beta elimination of beta-chloro-L-alanine to form NH3, pyruvate, and Cl- and is irreversibly inactivated during the reaction. The physicochemical properties, e.g., amino acid composition and subunit structure, of the bacterial enzyme are fairly different from those of the pig liver enzyme (Esaki et al., J. Biol. Chem. 257:4386-4391, 1982). However, the catalytic properties of both enzymes, e.g., substrate specificity and inactivation by the substrate or a mechanism-based inactivator, beta-chloro-L-alanine, are very similar.

Amino Acids↗
PubMed1985

Transition-state inhibition of thrombin and trypsin by amidinophenylpyruvates.

The interactions of human thrombin and bovine trypsin with 4-amidinophenylpyruvate (p-APPA), 3-amidinophenylpyruvate (m-APPA) and benzamidine were studied by equilibrium binding and by stopped-flow kinetics with proflavin displacement. The excellent inhibitory properties of p-APPA with both enzymes are explained by a two-step transition-state mechanism in which the initial Michaelis complex E.I reacts rapidly to form a fairly stable, chemically bonded complex E-I, in which the keto group of p-APPA forms a hemiketal with the gamma-O-atom of Ser195 at the active site. The hemiketal complex of thrombin and p-APPA may be further stabilized by a hydrogen bond between a carboxylate oxygen of p-APPA and the N tau-atom (= N epsilon 2) of His57, as was previously shown for the trypsin-p-APPA complex by X-ray crystal structure analysis by J. Walter and W. Bode. m-APPA is apparently sterically incapable of forming a hemiketal with Ser195 O gamma; it does not bind to thrombin or trypsin in a time-dependent manner, and it displays KI values with both enzymes close to those obtained for benzamidine itself. In the p-APPA-thrombin reaction the overall binding constant KI (E-I) is 1.3 microM, while the initial binding displays a KM (E.I) estimated at least 100-fold higher (700 microM). The half-time for the formation of E-I is about 0.6 s at a p-APPA concentration of 1 microM.

Animals↗
PubMed1984

Deacylation rate constants of acylated human and porcine plasmins.

Deacylation rate constants, k3, were measured for the human acyl-plasmins which contain in the acyl portion, RCO, the R groups methyl, cyclohexyl, p-nitrophenyl, isobutyl, trans-isopropenyl, phenyl, alpha-naphthyl, anisoyl and p-guanidinophenyl. Values of k3 were also determined for the porcine acyl-plasmins which have R = methyl, cyclohexyl, phenyl, anisoyl and p-guanidinophenyl. In general, for both the human and porcine acyl-plasmins, k3 decreased as the electron-donating ability of the acyl group increased. At 25 degrees C, the human acyl-plasmin with R = methyl has the highest k3, 1.06 +/- 0.05 X 10(-2) s-1, and the porcine acyl-plasmin with R = p-guanidinophenyl the lowest k3, 1.4 +/- 0.1 X 10(-6) s-1.

Acylation↗
PubMed1984

Stereochemistry of ornithine decarboxylase reaction.

To determine the steric course of the reaction of bacterial ornithine decarboxylase [EC 4.1.1.17], we have carried out the decarboxylation of L-ornithine in 2H2O and that of DL-[2-2H]ornithine in H2O, and obtained putrescine bearing a single deuterium atom in the C-1 position. The stereochemistry of [1-2H]putrescine was established by conversion to 1-(2-pyrrolidinyl)-2-propanone with acetoacetate and the pro-S hydrogen-specific diamine oxidase from pea seedlings. Analysis of deuterium content by gas chromatography-mass spectrometry showed that the deuterium label was fully retained during the conversion of [1-2H]putrescine produced by the decarboxylation of L-ornithine in 2H2O to 1-(2-pyrrolidinyl)-2-propanone, in contrast with the considerable loss of label from [1-2H]putrescine which was produced by the decarboxylation of DL-[2-2H]ornithine in H2O. The extent of loss of the deuterium label was in good agreement with the estimated value based on the isotope effect in the diamine oxidase reaction. These results indicate that the introduced deuterium (or hydrogen) is in the pro-R position at C-1 of putrescine, and consequently the ornithine decarboxylase reaction proceeds with retention of configuration.

Chemical Phenomena↗
PubMed1984

The effect of carboxylates and halides on L-lysine 6-aminotransferase-catalyzed reactions.

L-Lysine:2-oxoglutarate 6-aminotransferase catalyzes very slow transamination between L-alanine and 2-oxoglutarate. A high concentration of anions such as formate, acetate and halides greatly accelerated this transamination without affecting the affinity of the enzyme for L-alanine. In contrast, the anions strongly inhibited the normal L-lysine 6-transamination in a competitive manner with L-lysine and in a non-competitive manner with 2-oxoglutarate. This result suggests that the enzyme has an anion binding site which normally binds the carboxyl group of L-lysine. The binding of halides or carboxylates to this site probably induces a conformational change of the enzyme, and results in the inhibition of L-lysine 6-transamination, and in the stimulation of L-alanine transamination. Treatment of the enzyme with an arginine-specific dicarbonyl reagent, phenylglyoxal, led to a loss of the enzyme activity for L-lysine. The activity for L-alanine was not affected, but the stimulating effect of anions on L-alanine transamination was impaired. Thus, it is suggested that an arginine residue(s) plays an important role in the anion binding site.

Alanine↗
PubMed1984

Purification and properties of amino acid racemase from Aeromonas punctata subsp. caviae.

An amino acid racemase, which occurs in the cytoplasmic fraction of Aeromonas punctata subsp. caviae, has been purified to homogeneity by the criteria of electrophoresis and ultracentrifugation. The enzyme has a molecular weight of about 80,000 and consists of two subunits identical in molecular weight (about 40,000). The enzyme contains 2 mol of pyridoxal 5'-phosphate per mol of enzyme, and exhibits absorption maxima at 280 nm and 420 nm. The holoenzyme is resolved by dialysis against hydroxylamine to yield the inactive apoenzyme, which is reconstituted by the addition of pyridoxal 5'-phosphate to recover the full activity. The enzyme catalyzes racemization of a number of amino acids, e.g. lysine, ornithine, ethionine, arginine, glutamine, and methionine. The Michaelis constants were determined: 1 mM for L-lysine; 0.9 mM for D-lysine; 0.9 mM for L-ornithine; 1 mM for L-arginine; and 2.6 microM for pyridoxal 5'-phosphate. This enzyme is similar in enzymological properties to the racemase of Pseudomonas putida, but is distinct from it in immunochemical properties.

Aeromonas↗
PubMed1983

L-serine binds to arginine-148 of the beta 2 subunit of Escherichia coli tryptophan synthase.

Inactivation of the beta 2 subunit and of the alpha 2 beta 2 complex of tryptophan synthase of Escherichia coli by the arginine-specific dicarbonyl reagent phenylglyoxal results from modification of one arginyl residue per beta monomer. The substrate L-serine protects the holo beta 2 subunit and the holo alpha 2 beta 2 complex from both inactivation and arginine modification but has no effect on the inactivation or modification of the apo forms of the enzyme. This result and the finding that phenylglyoxal competes with L-serine in reactions catalyzed by both the holo beta 2 subunit and the holo alpha 2 beta 2 complex indicate that L-serine and phenylglyoxal both bind to the same essential arginyl residue in the holo beta 2 subunit. The apo beta 2 subunit is protected from phenylglyoxal inactivation much more effectively by phosphopyridoxyl-L-serine than by either pyridoxal phosphate or pyridoxine phosphate, both of which lack the L-serine moiety. The phenylglyoxal-modified apo beta 2 subunit binds pyridoxal phosphate and the alpha subunit but cannot bind L-serine or L-tryptophan. We conclude that the alpha-carboxyl group of L-serine and not the phosphate of pyridoxal phosphate binds to the essential arginyl residue in the beta 2 subunit. The specific arginyl residue in the beta 2 subunit which is protected by L-serine from modification by phenyl[2-14C]glyoxal has been identified as arginine-148 by isolating a labeled cyanogen bromide fragment (residues 135-149) and by digesting this fragment with pepsin to yield the labeled dipeptide arginine-methionine (residues 148-149). The primary sequence near arginine-148 contains three other basic residues (lysine-137, arginine-141, and arginine-150) which may facilitate anion binding and increase the reactivity of arginine-148. The conservation of the arginine residues 141, 148, and 150 in the sequences of tryptophan synthase from E. coli, Salmonella typhimurium, and yeast supports a functional role for these three residues in anion binding. The location and role of the active-site arginyl residues in the beta 2 subunit and in two other enzymes which contain pyridoxal phosphate, aspartate aminotransferase and glycogen phosphorylase, are compared.

Amino Acid Sequence↗
PubMed1983

A spectrophotometric method for the determination of aminopeptidase activity with leucine dehydrogenase.

L-Leucine dehydrogenase purified from Bacillus megaterium and Bacillus sphearicus was used for the determination of serum aminopeptidase activity with L-leucinamide as a substrate. L-Leucine produced by aminopeptidase was determined by measurement of the increase in absorbance at 340 nm caused by the formation of NADH. This method is useful for the kinetic studies of the aminopeptidase and the enzyme assay of a large number of samples. The serum aminopeptidase can be characterized to give some valuable information in clinical diagnosis by comparison of the results obtained by the present method with those by the conventional method with L-leucyl-p-nitroanilide as a substrate.

Amino Acid Oxidoreductases↗
PubMed1982

Stereochemistry of proton abstraction catalyzed by lysine and ornithine omega-aminotransferases.

(6R)-L-[6-3H]Lysine and (6S)-L-[6]3H]lysine were prepared to investigate the stereochemical aspects of the reaction catalyzed by bacterial L-lysine epsilon-aminotransferase. When (6R)-L-[6-3H]lysine was used as a substrate, the tritium label was retained in the product, delta 1-piperideine-6-carboxylate. In contrast, the radioactivity from (6S)-L-[6-3H]lysine was found in the solvent. Thus, the pro-S hydrogen at the prochiral C-6 carbon of L-lysine is specifically abstracted by L-lysine epsilon-aminotransferase. The proton exchange was observed by proton nuclear magnetic resonance analysis in the reaction of bacterial L-ornithine delta-aminotransferase with L-ornithine in 2H2O. The isolated L-[5-2H]ornithine was converted to dextrorotatory 4-phthalimido[4-2H]butyrate. This indicates that L-ornithine delta-aminotransferase catalyzes the stereospecific exchange of the pro-S hydrogen at the prochiral C-5 carbon of L-ornithine with the solvent.

Binding Sites↗