Interaction of Pyridoxal 5'-Phosphate Form of Aspartate Aminotransferase with Vitamin B-6 Compounds and Antagonists in Rabbit Erythrocytes

Abstract

Intracellular interaction of the pyridoxal 5'-phosphate (PLP) form of aspartate aminotransferase (AspAT) with vitamin B-6 and antagonists of vitamin B-6 in rabbit erythrocytes was measured in situ. In the erythrocytes, about 75% of the total AspAT was saturated with PLP. On the basis of the concentration of PLP in the erythrocytes, the result showed that about 13% of the total PLP was bound to AspAT in the erythrocytes. The form of the residual approximately 25% of the total AspAT was not identified: the residual AspAT was not converted to the PLP form even when a high amount of PLP was accomulated in the erythrocytes. Neither the PLP-AspAT level nor concentrations of PLP and pyridoxamine 5'-phosphate (PMP) were changed by incubation of the erythrocytes with the rabbit plasma and crude extracts of liver and kidney which were dialyzed or treated with dinitrophenylhydrazine. The modified form of PLP-AspAT with D-cycloserine was converted to PLP-AspAT in the hemolysate but was not in the erythrocyte. In contrast, the modified form of PLP-AspAT with DL-penicillamine was converted to PLP-AspAT both in the hemolysate and the erythrocyte. The concentration of vitamin B-6 compounds in the erythrocytes and the effects of the antagonists on the concentration were also measured after the erythrocytes were incubated with free vitamin B-6 compounds.