Nicotinamide inhibition of tri-and diphosphopyridine nucleotide-linked dehydrogenases.

In a previous study on the distribution of diphosphopyridine nucleotidase in the rabbit erythrocyte, we found that a nicotinamide-inhibitable DPNase is closely associated with the structural components of the cell. On hemolysis the enzyme remains firmly attached to the insoluble stroma and thus can be completely removed from the hemolysate by centrifugation. It was found, further, that the enzyme is largely confined, in its distribution, to the outer surface of the cell membrane. DPN,’ added to a suspension of red cells in the stroma-free hemolysate or to a stroma-containing hemolysate, is quickly destroyed unless nicotinamide has been previously added to inhibit the DPNase. When added to the stroma-free hemolysate, on the other hand, DPN remains unchanged (1). As anticipated, the lactic, malic, and glucose-Bphosphate dehydrogenases were found to be present in the soluble fraction of the rabbit hemolysate (1). Since the DPNase is confined to the stroma, the addition of nicotinamide to the stroma-containing hemolysate in amount 2 X 10” M, which is sufficient to suppress completely the action of DPNase, greatly increases the activity of the dehydrogenases. The activity, however, is never as great as with the stroma-free hemolysat,e. The addition of nicotinamide to the latter causes a slight decrease in the activity (l), due apparently to the inhibitory action of nicotinamide on the apodehydrogenases. While this study was in progress, Feigelson et al. (2) reported the inhibition of a DPN-linked dehydrogenase (malic) by nicotinamide. The present paper is concerned with manometric and spectrophotometric studies on the inhibition of TPNand DPN-linked dehydrogenases by nicotinamide.