Enzymatic Phosphorylation of Vitamin B6 Analogues and Their Effect on Tyrosine Decarboxylase

Structural analogues of naturally occurring compounds may exert inhibitory action by direct competition or they may be metabolized to form inhibitory compounds (2).In the case of the vitamin Bg antagonist, deoxypyridoxine, Umbreit and Waddell (3) have shown that this compound must be phosphorylated before it becomes an inhibitor of tyrosine apodecarboxylase.It is of interest that arabitylflavin and dichloroflavin (4, 5), both of which are growth inhibitors for various organisms, are capable of being phosphorylated by yeast flavokinase (6).The experiments presented here are a further study of the mode of action of vitamin Ba analogues.It has been found that at least two modes of action of vitamin Be analogues are responsible for inhibitory action.Evidence will be presented demonstrating that inhibition is due to (1) competition between the phosphorylated analogues and pyridoxal phosphate for tyrosine apodecarboxylase, and (2) direct inhibition of pyridoxal kinase . ResultsUnder conditions in which the concentration of pyridoxal phosphate did not saturate tyrosine apodecarboxylase (5.5 X lO-* M (7)), a molar ratio of free analogue to pyridoxal phosphate of 45,000: 1 did not prevent the union of the coenzyme with tyrosine apodecarboxylase (Table I, Column 1).With pyridoxal, as much as 10 pmoles has been used with no inhibitory effect, and the ratio in this case was 182,000: 1.When certain of the analogues and adenosine triphosphate (ATP) were incubated for 15 minutes with a tyrosine apodecarboxylase preparation as indicated in Table I, Columns 2 and 3, the decarboxylation of tyrosine was