Histidine decarboxylase in gastric tissues of primates

Abstract

Histidine decarboxylase activity was identified in the gastric mucosa of monkeys and man and in a human gastric carcinoid tumor. Crude homogenates of gastric mucosal tissue but not of the carcinoid tumor also demonstrated 5-hydroxytryptophan decarboxylase activity; this activity disappeared completely during purification procedures that resulted in increased specific activity of histidine decarboxylase. In all respects studied these histidine decarboxylases had properties that were very similar to or identical with histidine decarboxylases prepared from other mammalian sources: in particular, the apparent K m varied inversely with pH; the pH optimum varied inversely with substrate concentration; activity of the enzyme was enhanced by pyridoxal 5′-phosphate but not by benzene and inhibited by both brocresine and hydrazino-analog of histidine but not by α-methyl-dopa. In both monkeys and men histidine decarboxylase activity was greater in the fundus than it was in the antrum of the stomach. These findings indicate that the enzyme necessary for histamine biosynthesis is present in gastric tissues of primates and thus remove one theoretical objection to the hypothesis that histamine may mediate gastric acid secretion in primates.