Cytosolic l‐alanine:4,5‐dioxovalerate transaminase differs from the mitochondrial form

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Abstract
L‐Alanine:4,5‐dioxovalerate transaminase was detected in the kidney cytosolic fraction with a lower specific activity than the mitochondrial enzyme. The enzyme was purified from the cytosol to homogeneity with a yield of 32%, and comparative analysis with the mitochondrial form was performed. Both forms of the enzyme have identical pH and temperature optima and also share common antigenic determinants. However, differences in their molecular properties exist. The molecular mass of the native cytoplasmic enzyme is 260 kDa, whereas that of the mitochondrial enzyme is 210 kDa. In addition, the cytoplasmic l‐alanine:4,5‐dioxovalerate transaminase had a homopolymeric subunit molecular mass of 67 kDa compared to a subunit molecular mass of 50 kDa for the mitochondrial l‐alanine:4,5‐dioxovalerate transaminase. This is the first report of two forms of l‐alanine:4,5‐dioxovalerate transaminase. The different responses of cytosolic and mitochondrial l‐alanine:4,5‐dioxovalerate transaminases to hemin supplementation both in vitro and in vivo was demonstrated. Maximum inhibition of mitochondrial l‐alanine:4,5‐dioxovalerate transaminase activity was demonstrated with hemin injected at a dose of 1.2 mg/kg body mass, whereas the same dose of hemin stimulated the cytosolic enzyme to 150% of the control. A one‐dimensional peptide map of partially digested cytosolic and mitochondrial l‐alanine:4,5‐dioxovalerate transaminase shows that the two forms of the enzymes are structurally related. Partial digestion of the cytosolic form of the enzyme with papain generated a fragment of 50 kDa which was identical to that of the undigested mitochondrial form (50 kDa). Moreover, papain digestion resulted in a threefold increase in cytosolic enzyme activity over the native enzyme, and such enhancement was comparable to the activity of the mitochondrial form of the enzyme. Therefore, we conclude that the cytosolic form of l‐alanine:4,5‐dioxovalerate transaminase is different from the mitochondrial enzyme. Furthermore, immunoblot analysis indicated that the mitochondrial enzyme has antigenic similarity to the cytosolic enzyme as well as to the papain‐digested cytosolic enzyme 50‐kDa fragment.