INACTIVATION OF RENIN SUBSTRATE BY SOYBEAN TRYPSIN-INHIBITORS - IMPLICATIONS FOR MEASUREMENT OF CIRCULATING INACTIVE RENIN

Semipurified soybean trypsin inhibitor added to rat and human plasma l eads to a concentration dependent decrease in the rate of angiotensin I generation. This inhibition is due to binding of renin substrate to the inhibitor. Renin substrate present in nephrectomized rat plasma wa s more susceptible to binding than substrate of the normal rat suggest ing structural differences in the substrate generated following nephre ctomy. Because trypsin inhibition is necessary for measurement of acti ve and inactive renin, we examined several alternate trypsin inhibitor s. The Bowman-Birk inhibitor from soybean had similar actions as purif ied soybean trypsin inhibitor while trypsin inhibitors from lima bean and chicken did not depress renin substrate, but did have variable eff ects on the measured levels of active and total plasma renin.- Surpris ingly, crude soybean trypsin inhibitor did not suppress renin substrat e and actually increased angiotensin I generation during PRA and PRC m easurements. Since the crude preparation did not suppress renin substr ate, changes in the specificity of the inhibitor may occur during its purification. The augmentation of PRA and PFC may be related to angiot ensinase inhibitory actions.