Near-UV activation of enzymatic conversion of 5-hydroxytryptophan to serotonin.

Near-UV (337 nm) photoactivation of the 5-hydroxytryptophan decarboxylation reaction producing serotonin has been observed. The photoactivation effect was investigated as a function of fluence rate and fluence, and pH. Photoactivation of decarboxylase activity was found to occur at nearly neutral pH values (low activity of the enzyme in the dark). The findings indicate that the effect of light is similar to a pH shift toward the acid region, which causes the enzyme conversion from the inactive to active form. Pyridoxal phosphate, the decarboxylase cofactor, in the form of an adduct absorbing at 330-340 nm, is suggested as a candidate for the role of the photoactive chromophore of decarboxylase.