PROTEOLYTIC ACTIVATION OF A BIOACTIVE CARDIAC PEPTIDE BY INVITRO TRYPSIN CLEAVAGE

Mammalian cardiac atria possess several unidentified biologically active peptides. Fractionation of rat atrial extracts by gel filtration chromatography revealed 2 major fractions [apparent MW of 20,000-30,000 (peak I) and < 10,000 (peak II)], both of which were potent natriuretic agents (eliciting a 25-fold increase in Na excretion) and smooth muscle relaxants. Vigorous treatment with trypsin (100 U/ml at 37.degree. C for 15 min) of both fractions abolished all biological activity. Further purification of the lower MW fraction (peak II) by ion-exchange chromatography indicated 2 subfractions that possessed potent natriuretic activity and that preferentially relaxed either intestinal (designated peak IIA) or vascular (peak IIB) smooth muscle assay tissues. The similarity of the biological effect of the high (peak I) and low (peak II) MW peptides led us to test the possibility of precursor-product relationship. Mild proteolytic treatment of the high MW peptide with trypsin (1 U/ml at room temperature) markedly enhanced the smooth muscle relaxant activity. Subsequent analysis of the trypsin (1 U/ml)-treated high MW peptide (peak I) by gel filtration and ion-exchange chromatography revealed that the peptide now resembled the low MW peptides (peaks IIA and IIB) present in the original atrial extract. These data suggest that the cardiac atria contain a relatively inactive (smooth muscle relaxant) high MW peptide and suggest that biologically active low MW peptides can subsequently be generated by proteolytic cleavage.