Abstract
Abstract Two enzymes capable of decarboxylating l-histidine in vitro have been identified in rat and mouse stomach; one, located in the fundic portion, shows maximal activity at a pH value of 5.6, whilst the other, in the pyloric portion, requires a pH of 7.6 for optimal activity. The enzyme in rat fundus is stable when stored at low temperatures and is inhibited only slightly by benzene, α-methyldopa and α-methyl-histidine; the pyloric enzyme, on the other hand, is rapidly destroyed on storage at low temperatures, and is slightly stimulated by benzene but much inhibited by α-methyldopa and α-methylhistidine. Dopa and 5-hydroxytryptophan compete with the substrate for the pyloric histidine decarboxylase but have no effect on the fundic enzyme. Starvation inhibits the activity of the fundic enzyme but has only a slight effect on the pyloric enzyme. It is concluded that the two enzymes in the stomach capable of forming histamine differ from the specific and non-specific histidine decarboxylases found in other tissues.
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