Biochemical properties and kinetic parameters of dihydroxyphenyialanine – 5-hydroxytryptophan decarboxylase in brain, liver, and adrenals of cat

Abstract

Biochemical properties and kinetic parameters of nonpurified dihydroxyphenylaianine –5-hydroxytryptophan decarboxylase extracted from brain and two peripheral organs, liver and adrenals, were studied in the cat. This study shows that decarboxylase activity in brain is lower than in peripheral organs and that 5-hydroxytryptophan can be decarboxylated without exogenous addition of pyridoxal-5′-phosphate (PLP). However, the addition of PLP substantially increases the enzyme activity. Excess of coenzyme (>60 μM) induces inhibition in adrenals and liver but not in the central nervous system (CNS). The observed inhibition might be related to the presence of a tetrahydroisoquinoline derivative formed in the medium. Differentiation between mechanisms of action of decarboxylase in the CNS and peripheral organs is suggested.