Novel peptide inhibitor (SPAI) of Na+, K+-ATPase from porcine intestine.

Three unique inhibitors (SPAI-1, -+2, and -3) were first purified from porcine duodenal extract based on the Na+, K+-ATPase inhibitory activity. These peptide inhibitors had four disulfide bridges in common. The sequencing results of their S-carboxymethyl derivatives, lysilendopeptidase fragments, and chymotryptic peptides disclosed their entire primary structures. Both SPAI-2 and -3 consisted of 61 amino acids, respectively, and had almost the same sequences except for two amino acid substitutions, while SPAI-1 was found to lack the N-terminal twelve amino acid sequence of SPAI-2. The kinetics study revealed that SPAIs inhibited Na+, K+-ATPase by the competitive mode against Na+ and were uncompetitive with K+.