5-Hydroxytryptamine (5HT) and 5-hydroxytryptophan (5HTP) form cyclic compounds (probably of the tetrahydro-beta-carboline type) with pyridoxal phosphate (PLP). In the first step of reaction a Schiff's base is formed; during incubation it is transformed into a cyclic compound with a maximum absorption spectrum at 330 nm. The degree of cyclization depends on pH and substrate concentrations. One mole of 5HT or 5HTP reacts with one mole of coenzyme. The velocity of cyclization increased with an excess of either 5HT (5HTP) or PLP, without any change in the mole to mole ratio. The formation of cyclic compounds was confirmed by the use of isotopes, separation from substrates being achieved by high-voltage electrophoresis.
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