A modified Schayer procedure for the estimation of histidine decarboxylase activity: its application on tissue extracts from gastric mucosa of various mammals.

The question of whether the gastric mucosa of mammals other than rats, mice and hamsters contains an acid (specific) histidine decarboxylase was re-investigated by using a modification of the classical isotope dilution method of Schayer. Its reliability was tested regarding sensitivity, specificity, precision and accuracy, applying criteria recommended by the International Federation of Clinical Chemistry. The assay is now suitable for measuring rather large series of samples. The suitability of several blanks for detecting histidine decarboxylase activity was investigated as a special problem of accuracy. The semicarbazide blank was found to be as reliable as the heating blank, the blank with tissue extract pretreated with perchloric acid or an incubation mixture without radio-labelled histidine. Reaction kinetics of histamine formation were linear over an incubation period of at least 3 h. Histidine decarboxylase activity as a rather low pH and substrate concentration, which is characteristic for the acid (specific) histidine decarboxylase, was demonstrated inthe gastric mucosa of human subjects, dogs, rabbits and guinea-pigs, always using the same incubation conditions for all species investigated. The highest enzymic activity was present in the oxyntic mucosa, but could be measured also in other parts of the stomach.