COMPLEMENTARY-DNA CLONING AND SEQUENCING OF RAT ENTEROPEPTIDASE AND TISSUE DISTRIBUTION OF ITS MESSENGER-RNA

A cDNA clone encoding enteropeptidase (EC 3.4.21.9), a key enzyme for the conversion of trypsinogen to trypsin, was isolated from a rat duod enal mucosa cDNA library. Sequence of the 3585 base pair clone predict ed that enteropeptidase is synthesized as a single-chain precursor for m, proenteropeptidase, consisting of 1058 amino acid residues with an internal signal sequence (51 residues) and is then processed into the mature enzyme consisting of three different peptide chains, i.e., mini , light and heavy chains, not the previously reported two-chain enzyme . The structure of enteropeptidase is relatively conserved among diffe rent species and the rat enteropeptidase is 24 and 39 amino acids long er than the porcine and human ones, respectively. Northern blot analys is of RNAs from normal rat tissues revealed that the enteropeptidase m RNA of around 4.4 kb in size was expressed only in the duodenal mucosa , and high proteolytic activity of the enzyme was detected in the prox imal small intestine. Additional analysis of the RNAs by RT-PCR reveal ed that a low level of the mRNA was also expressed in the other parts of the small intestine, i.e., jejunum and ileum. These results indicat e that the biosynthesis of enteropeptidase takes place mainly in the p roximal small intestine, the duodenum, and the importance of the regio n in the physiology of intestinal protein digestion regulated by the e nzyme is suggested. Furthermore a faint signal of the mRNA was also de tected in the stomach, colon and brain in which the existence of tryps in-like serine proteases were reported. The significance of the low le vel expression of the gene is unclear, but the potential peptide-proce ssing function of the enzyme in these tissues is also suggested. (C) 1 996 Academic Press, Inc.