The effects of protease inhibitors on histidine decarboxylase activities and assay of enzyme in various rat tissues.

Histidine decarboxylase (L-histidine carboxy-lyase, EC 4.1.1.22) of an extract of rat stomach was inactivated by a pancreatic extract. This inactivation was prevented by the protease inhibitors leupeptin, antipan, chymostatin, pepstatin, Trasylol and phenylmethanesulfonyl fluoride. Leupeptin, antipain, chymostatin and pepstatin together and phenylmethanesulfonyl fluoride alone prevented complete inactivation of the enzyme, while Trasylol had a weak protective effect. The inactivation and protection of histidine decarboxylase purified from whole fetal rats were similar to those of the stomach enzyme: both enzymes were strongly inactivated by trypsin and chymotrypsin, but not by elatase or carboxypeptidase Y. The histidine decarboxylase activities of various rat tissues were assayed in the presence of protease inhibitors: activity was highest in mast cells followed by the whole bodies of fetal rats and the stomach, while the activities were lower in decreasing order in the brain, spleen, lung and liver. Heart and kidney had no activity.