On the inhibitory activity of 4-vinyl analogues of pyridoxal: enzyme and cell culture studies.

Analogues of pyridoxal and of pyridoxal phosphate in which the 4-CHO group is replaced with CH = CH2 were synthesized and were found to be potent inhibitors of pyridoxal kinase and pyridoxine phosphate oxidase of rat liver. They also inhibited the growth of mouse Sarcoma 180 and mammary adenocarcinoma TA3 in cell culture. Saturation of the vinyl double bond, replacement of the 5-CH2OH with methyl, methylation of the phenolic hydroxyl, or conversion to the N-oxide resulted in diminution or loss of all these activities. Similarly, the introduction of a beta-methyl group into the vinyl analogues of pyridoxal reduced all these inhibitory activities. The 4-vinyl anatogue of pyridoxal was shown to be a substrate of pyridoxal kinase and the product a potent inhibitor of pyridoxine oxidase, competing with pyridoxal phosphate. The affinity of this phosphorylated pyridoxal analogue to some apoenzymes varied greatly, indicating striking differences among the cofactor binding sites of these enzymes. The growth inhibitory effects of these analogues on cells in culture correlated well with their effects on pyridoxal kinase and pyridoxine phosphate oxidase in cell-free systems.