Characteristics of dihydroxyphenylalanine/5-hydroxytryptophan decarboxylase activity in brain and liver of cat.

Abstract

Abstract: Dihydroxyphenylalanine/5‐hydroxytryptophan (DOPA/5‐HTP) decarboxylase activity varied widely in different parts of the CNS, being highest in the neostriatum and lowest in the frontal cortex. The addition of 2.5 μm‐pyridoxal 5′‐phosphate (PLP), the coenzyme, increased enzyme activity in brainstem and liver, while higher concentrations led to a decrease in activity. In brainstem, the addition of 1000 μm PLP shows activity similar to that obtained without exogenous PLP. The effects of different monoamine oxidase (MAO) inhibitors on decarboxylase activity were demonstrated. Iproniazid phosphate and harmaline significantly decreased the decarboxylation in liver and brainstem, while pargyline inhibited only liver decarboxylation. Some decarboxylase inhibitors such as RO4–4602 and α‐methyl DOPA, as well as piribedil, a dopaminergic receptors agonist, were added in vitro to measure their action on decarboxylase with or without exogenous PLP or with double concentrations of substrate (5‐HTP). Piribedil (5000 μm) affected the enzymic reaction and triggered a higher inhibition in liver. Inhibition in brainstem needed less RO4–4602 (50 μm) than in liver (300 μm). Addition of PLP did not reverse this inhibition, while doubling the concentration of 5‐HTP nullified the inhibitory effect in liver only. Inhibition induced by α‐methyl DOPA (5 μm) was easily reversed by doubling the concentration of substrate. However, the presence of exogenous PLP restored the enzymic activity in liver only. We conclude from this work thus that the enzyme can decarboxylate its substrate without exogenous PLP, that MAO inhibitors might inhibit decarboxylase activity, and that decarboxylase inhibitors react differently when brain and liver are used as enzymic source. PLP seems to act as a protective agent on the active site of the enzyme in the brainstem and preferentially with the substrate in the liver.