Effect of pyridoxine deficiency on aromatic L-amino acid decarboxylase in adult rat brain.

We have investigated the effect of pyridoxine deficiency on aromatic L-amino acid decarboxylase (AADC) using both dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5HTP) as substrates in the rat brain. The activity ratios of DOPA decarboxylase/5HTP decarboxylase measured under optimal substrate and cofactor concentrations were different in the cerebellum, cerebral cortex, corpus striatum and hypothalamus of the normal rat. In pyridoxine deficiency, there were no parallel decreases in DOPA and 5HTP decarboxylase activities in various brain regions. Dialysis of brain homogenates, in the presence and absence of hydroxylamine, resulted in a total or near total loss of 5HTP decarboxylase activity compared to DOPA decarboxylase activity, indicating that pyridoxal phosphate may be more tightly bound to DOPA decarboxylase than to 5HTP decarboxylase. These results, indicating that pyridoxine deficiency has differential effects on the activity of AADC, are consistent with our earlier observation of non-parallel changes in dopamine and serotonin content in various brain regions of the pyridoxine-deficient rat.