PEPSIN-INHIBITORY ACTIVITY OF THE UTERINE SERPINS

Among the major products secreted by the uteri of cattle, sheep, and p igs during pregnancy are glycoproteins with amino acid sequences that place them in the serpin (serine proteinase inhibitor) superfamily of proteins. The inferred amino acid sequences for bovine uterine serpin (boUS-1) and ovine uterine serpin (ovUS-1) exhibit about 72% sequence identity to each other but only about 50% and 56% identity, respective ly, to two distinct porcine uterine serpins (poUS-1 and poUS-2). Despi te these differences in primary structure, the uterine serpins possess well-conserved reactive center loop regions that contain several moti fs present in the propeptide regions of pepsinogens. One such motif, V VVK, aligns with the first 4 amino acids of the aspartic proteinase in hibitor pepstatin. Although no inhibitory activity toward any serine p roteinase has been found, at least one of the uterine serpins, ovUS-1, can bind specifically to immobilized pepsin A and can weakly inhibit the proteolytic activities of pepsin A and C (but not cathepsins D and E). OvUS-1 is the first specific inhibitor of aspartic proteinases to be identified in vertebrates and provides another example of a serpin with ''crossover'' activity. The pregnancy-associated glycoproteins ( PAGs), which are secreted by the trophoblast layer of the placentas of ungulate species and are inactive members of the aspartic proteinase family, can also bind ovUS-1 and may be the natural target partners fo r the uterine serpins.