Homologous IRCM-serine protease 1 from pituitary, heart atrium and ventricle: a common pro-hormone maturation enzyme?

IRCM-Serine Protease 1 (IRCM-SP1) has recently been isolated and characterized from porcine pituitary anterior and neurointermediate lobes (Cromlish et al., 1986a, J. Biol. Chem. 261:10850-10858; Cromlish et al., 1986b, J. Biol. Chem. 261:10859-10870). This pituitary serine protease was shown to selectively cleave human pro-opiomelanocortin (POMC)-derived peptides at both pairs of basic residues and C-terminal to specific Arg residues, all known to be cleaved in vivo. Here, a similar enzyme was isolated from rat heart atria and ventricles. Rat IRCM-SP1 was shown to be highly specific for the same cleavage sites in POMC, as the porcine pituitary homologue. Furthermore, the rat and the porcine enzymes cleave rat pro-Atrial Natriuretic Factor (pro-ANF 1-126) to yield ANF 103-126, 102-126 and 99-126 in that order of preference. This suggests that in vitro the cleavage sites preferred in pro-ANF resemble those found in brain and hypothalamus. The enzyme is nine times more abundant in atria versus ventricles/mg protein. It is concluded that IRCM-SP1, could well represent a common pro-hormone maturation enzyme for POMC and Pro-ANF and possibly many other pro-hormones.