Na+,K+-ATPase inhibition by an endogenous peptide, SPAI-1, isolated from porcine duodenum

Abstract SPAI-1, a peptide isolated from porcine duodenum, has been shown to inhibit Na + ,K + -ATPase in vitro (Araki et al. (1989) Biochem. Biophys. Res. Commun. 164, 496–502). The characteristics of ATPase inhibition by this novel peptide were examined. SPAI-1 inhibited Na + ,K + -ATPase preparations isolated from various organs of dog or rat or from sheep kidney with similar potency. Three isoforms of rat Na + ,K + -ATPase had similarsensitivity to inhibition by SPAI-1 although these isoforms had remarkable differences in their sensitivity to the inhibitory effect of ouabain. Ca 2+ -ATPase isolated from the sarcoplasmic reticulum of rabbit skeletal muscle was insensitive to inhibition by SPAI-1. Ouabain-insensitive Mg 2+ -ATPase activity was unaffected by low concentrations of SPAI-1, but was stimulated at high concentrations. SPAI-1 inhibited H + ,K + -ATPase from hog stomach in concentrations similar to that required for Na + ,K + -ATPase inhibition. These results indicate that SPAI-1 is a specific inhibitor for monovalent cation transporting ATPases.