Distribution and abundance of neutral endopeptidase (EC 3.4.24.11) in the alimentary tract of the rat

The distribution of neutral endopeptidase (NEP; EC 3.4.24.11) was examined in the alimentary tract of the rat. Immunoreactive NEP and NEP mRNA were localized to epithelial cells of the small intestine and to muscle cells in the stomach, small intestine, and colon by immunohistochemistry and in situ hybridization histochemistry. NEP antisera recognized a protein on Western blots of membranes from gastric, jejunal, and colonic mucosa and gastric muscle with an electrophoretic mobility identical to that of recombinant human NEP (~95 kDa). An antisense cRNA probe to NEP hybridized to RNA of ~3.5 kb and ~6.5 kb, corresponding to the primary transcripts of rat NEP, on Northern blots of total RNA from the jejunal mucosa. NEP message was detected in mRNA from jejunal and colonic mucosa and gastric, jejunal, and colonic muscle using a ribonuclease protection assay. NEP enzymatic activity, assessed by DL-thiorphan-inhibitable degradation of glutaryl Ala-Ala-Phe-4- methoxy-2-naphthylamine, was highest in homogenates of jejunal mucosa (868 ± 98 pmol · h^-1 · μg protein^-1) and was between 49- and 413-fold lower in other gastrointestinal tissues. The cellular origin of NEP in the gastric and colonic mucosa could not be determined.