Inhibits Na(+),K(+) ATPase by the competitive mode against Na(+). Small intestine > large intestine. The plasma contains the pro-SPAI form circulating. The repetitive domain of pro-SPAI serves as a substrate for transglutaminase. The short form (AA 127-187) may be an artifact due to the strongly acidic conditions of the duodenum. The pro-SPAI form may be the native form.