Proteolytic inactivation of substance P in the epithelial layer of the intestine

Metabolites of substance P, produced by incubation with isolated epithelial cells and with purified brush border and basolateral membrane from pig small intestine, were isolated by high performance liquid chromatography and identified by amino acid analysis. Rapid cleavages between Gln⁶-Phe⁷, Phe⁷-Phe⁸ and Gly⁹-Leu¹⁰ and oxidation of the methionine residue at position 11 were observed with cells and with both membrane fractions. Formation of substance P_3-11 indicative of the action of dipeptidylaminopeptidase IV (EC 3.4.14.5), was observed only at high substrate concentration. Proteolytic degradation was inhibited by phosphoramidon and by EDTA but was insensitive to chloride ion concentration and to captopril. These observations suggest that inactivation of substance P in the epithelial layer of the gut is mediated through endopeptidase-24.11 (EC 3.4.24.11) in the cell-surface membrane and that degradation by angiotensin-converting enzyme (EC3.4.15.1), although present in high concentration in the mucosa, is unimportant.