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.
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