The interactions between biogenic amines and pyridoxal, pyridoxal phosphate, and pyridoxal kinase

The effect of biogenic amines on pyridoxal kinase activity has been investigated in vitro. Experiments with a purified beef brain pyridoxal kinase indicate that 3,4-dihydroxyphenylalanine (dopa), dopamine, norepinephrine, and serotonin inhibit the reaction, while tyrosine, tryptophan, and 5-hydroxytryptophan (5-HTP) have no effect. However, nonenzymatio studies under conditions similar to those of the enzymatic studies reveal that most of the inhibition by dopa, dopamine and norepinephrine is due to reaction of these compounds with pyridoxal phosphate while reaction between pyridoxal phosphate and the tryptophan derivatives, 5-HTP and serotonin, does not occur under the conditions employed. Additional nonenzymatic experiments indicate that the inhibition by serotonin is not due to removal of substrate (ATP, Zn²⁺, or pyridoxal). This evidence suggests that serotonin exerts its inhibitory effect by interacting with pyridoxal kinase. The enzyme inhibition observed with serotonin may represent a form of metabolic regulation.