L-Histidine decarboxylase in the human brain: properties and localization

The properties of the histamine-forming enzyme in human brain samples were studied utilizing a radiochromatographic procedure. The influence of postmortem conditions was checked with rat brains, and the results indicated that the enzyme activity is not altered in situ for a delay not exceeding 4 h at ambient temperature. Moreover, tissue blocks or homogenates can be stored at temperatures for up to 3 months with a good preservation of the enzyme activity. The data indicate that histamine synthesis in the human brain involves the "specific" histidine decarboxylase (HD, EC 4.1.1.22) and not the aromatic L-amino acid decarboxylase: (1) the optimum pH is 7.4 at 10(-6) M-L-histidine; (2) the apparent Km is about 3.10(-5) M; (3) it is inhibited by alpha-hydrazino histidine and brocresine but not affected by alpha-methyl DOPA. Moreover, a major portion of the enzyme is localized in a subcellular fraction containing nerve terminals and it shows an uneven regional distribution which parallels that observed in the brain of other mammalian species. Taken together these data strongly suggest that histamine could play a neurotransmitter role in the human brain.