Identity of rat liver mitochondrial asparagine-pyruvate transaminase with phenylalanine-pyruvate transaminase

Identification of rat liver mitochondrial asparagine-pyruvate transaminase with phenylalanine-pyruvate transaminase has been done. When a mitochondria extract was subjected to isoelectric focusing, the two enzyme activities were identically focused. This procedure and DEAE-Sepharose chromatography revealed multiple forms of the enzyme, in which the main form was purified. In the various purification steps the two enzyme activities appeared in the same fraction. The enzyme of the final preparation step gave a single band in polyacrylamide gel electrophoresis in the presence and absence of sodium dodecyl sulfate. During the purification, a similar increase of the specific activity and yield were obtained in the two activities. Phenylalanine was found to be a competitive inhibitor of asparagine transaminase. These results suggest the identity of the two enzymes.     

Asparagine transaminase from rat liver.

Asparagine transaminase has been purified about 200-fold from rat liver. The enzyme has a broad specificity toward both amino acids and alpha-keto acids. Thus, amino acids substituted in the beta position such as asparagine, S-methylcysteine, phenylalanine, cysteine, serine, and aspartate are substrates. The enzyme is also active with alanine, methionine, homoserine, alpha-aminobutyrate, glutamine, and leucine. The enzyme has a high affinity for glyoxylate but the affinity falls off markedly through the series glyoxylate, pyruvate, alpha-ketoburyrate, alpha-Keto acids substituted in the beta or gamma position, such as alpha-ketosuccinamate, phenylpyruvate, p-hydroxyphenylpyruvate, alpha-keto-gamma-methiolburyrate, and alpha-keto-gamma-hydroxybutyrate, are substrates for the enzyme. Amino acids or alpha-keto acids possessing a branch point at the beta carbon are inactive. Kinetic analysis of the asparagine glyoxylate transamination reaction is consistent with a ping-pong mechanism.     

Synthesis of glutamic oxaloacetic transaminase isozymes in rat liver cells

The synthesis of glutamic oxaloacetic transaminase isozymes in rat liver explants was studied using specific antisera against the cytosolic and mitochondrial isozymes. The pulse-labeled cytosolic isozyme was detected in the cytosolic fraction and remained there in pulse-chase experiments. On the other hand, the pulse-labeled mitochondrial isozyme was detected as a larger precursor in the cytosolic fraction. During chase, the amount of pulse-labeled precursor of the mitochondrial isozyme decreased and labeled mature mitochondrial isozyme appeared in the mitochondrial fraction.     

Purification and partial characterization of cysteine-glutamate transaminase from rat liver.

Cysteine-glutamate transaminase (cysteine aminotransferase; EC 2.6.1.3) has been purified 149-fold to an apparent homogeneity giving a specific activity of 2.09 IU per milligram of protein with an overall yield of 15%. The isolation procedures involve the preliminary separation of a crude rat liver homogenate which was submitted sequentially to ammonium sulfate fractionation, TEAE-cellulose column chromatography, ultrafiltration, and isoelectrofocusing. The final product was homogenous when examined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS). A minimal molecular weight of 83 500 was determined by Sephadex gel chromatography. The molecular weight as estimated by polyacrylamide gel electrophoresis in the presence of SDS was 84 000. The purified enzyme exhibited a pH optimum at 8.2 with cysteine and alpha-ketoglutarate as substrates. The enzyme is inactivated slowly when kept frozen and is completely inactivated if left at room temperature for 1 h. The enzyme does not catalyze the transamination of alpha-methyl-DL-cysteine, which, when present to a final concentration of 10 mM, exhibits a 23.2% inhibition of transamination of 30 mM of cysteine. The mechanism apparently resembles that of aspartate-glutamate transaminase (EC 2.6.1.1) in which the presence of a labile hydrogen on the alpha-carbon in the substrate is one of the strict requirements.     

Deuterium isotope effect in the transamination of p-tyrosine by rat liver tyrosine transaminase

The rate of transamination of p-tyrosine catalyzed by rat liver soluble tyrosine aminotransferase (E.C. 2.6.1.5.) was significantly reduced when the hydrogen at the alpha-carbon position is replaced by deuterium or when the reactions were conducted in 2H2O. The cleavage of carbon-hydrogen bond at alpha-carbon position is at least partly involved in the rate-limiting step of tyrosine transamination. In 2H2O solvent the reduction of the overall rates of transamination of both p-tyrosine and alpha-2H1-p-tyrosine occurred uncompetitively which suggests that the deuterium solvent effect is involved in the tautomerization of the external Schiff's base.