Identification and Measurement of Acid (Specific) Histidine Decarboxylase Activity in Rabbit Gastric Mucosa: Ending an Old Controversy?

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Abstract
One of the main obstacles in assigning any distinct function to histamine in health and disease was the longlasting controversy on the existence of any physiological, endogenous histamine formation in man and most of the other mammals except the rat. Using a modification of Schayer''s isotope dilution method, a renewed attempt was made to identify the very low activities of an acid (specific) histidine decarboxylase in rabbit gastric mucosa capable of producing endogenous histamine in physiological conditions, to develop tests for its identification in crude enzyme extracts and to demonstrate the specificity of the enzymatic assay by excluding any relevant dopa decarboxylase activity, and also nonenzymatic decarboxylation interfering with the determination of acid (specific) histidine decarboxylase. To achieve this aim, 5 tests were developed. In the pH profile (test 1), a pH optimum was found at 7.0 in the presence of a low substrate concentration (1.6 .times. 10-6 M L-[ring-2-14C]-histidine). The apparent Michaelis concentration at the pH optimum (test 2) was 1.8 .times. 10-4 M, the maximum rate 12.5 pmol [14C]histamine formed .times. min-1. To increase the specificity of inhibition experiments with .alpha.-methylhistidine and .alpha.-methyl-L-dopa, a pH profile was determined in the presence of these 2 enzymatic inhibitors (tests 3 and 4). .alpha.-Methylhistidine was used for a reliable diagnostic confirmation test, .alpha.-methyl-L-dopa for a reliable exclusion test. Benzene showed no influence on either blanks or recovery rates, but inhibited the enzymic activity at pH 7.0, not however that of unspecific histidine decarboxylase and hence was very valuable as an additional diagnostic exclusion test (test 5). Although these new tests identifying acid (specific) histidine decarboxylase and demonstrating the specificity of its determination were tedious, despite the use of the modified isotope dilution method, they excluded the presence of any dopa decarboxylase activity in mixtures with crude enzyme preparations as well as of any kind of nonspecific and nonenzymatic histidine decarboxylation. An endogenous histidine decarboxylase in rabbit gastric mucosa is postulated, capable of forming histamine in vivo.

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