The porcine sperm motility inhibitor is identical to β-microseminoprotein and is a competitive inhibitor of Na⁺,K⁺-ATPase

We have isolated a small porcine seminal protein called SMI-1. Our results from peptide sequence, amino acid composition and mass spectral analyses reveal that SMI-1 is identical to porcine β-microseminoprotein, a protein with unknown function. We also report here that this protein inhibits competitively the activity of Na⁺,K⁺-ATPase purified from porcine cerebral cortex in a dose-dependent manner. The inhibitory effect could be reversed by the addition of ATP. The half-maximal inhibition was achieved at an inhibitor concentration of 90 μM.