Activities of 3,4-dihydroxy-L-phenylalanine and 5-hydroxy-L-tryptophan decarboxylases in rat brain: assay characteristics and distribution.

Abstract

Abstract— Optimal assay conditions for decarboxylation of 3,4‐dihydroxy‐l‐phenylalanine (DOPA) and 5‐hydroxy‐l‐tryptophan (5‐HTP) were determined in homogenates of rat brain by use of a sensitive, precise microradiometric technique. The two activities exhibited widely different optima for pH, temperature and substrate concentrations. The activity of 5‐HTP decarboxylase was stimulated 2‐fold by added pyridoxal‐5‐phosphate and was relatively resistant to antagonists of pyridoxal‐P. By contrast, the activity of DOPA decarboxylase was stimulated 20‐fold by added coenzyme and could be completely inhibited by carboxyl trapping agents. DOPA decarboxylase activity in subcellular fractions of brain was associated predominately with the soluble fractions and its distribution in the various fractions closely paralleled that of lactic acid dehydrogenase. 5‐HTP decarboxylase activity in brain was distributed almost equally between soluble and particulate fractions, and its distribution within the particulate fractions differed from that of succinic acid dehydrogenase. The two decarboxylases in brain exhibited a 7‐fold divergence in relative specific activity when their respective distributions in subcellular fractions were compared. Similarly, the regional distributions of the two decarboxylases in rat brain did not parallel one another; e.g. there was a 4‐fold difference between the ratio of the two activities in cerebellum and that found in the corpus striatum.