Tryptic activation of plasma renin activity

Trypsinization of human plasma, like acid activation or cryoactivation, increases renin activity, as determined by subsequent enzymatic assay. In three plasma pools, tryptic activation was maximal within 2 min at a trypsin concentration of 500 micrograms/ml, decreasing at higher concentrations. Neither prolongation of trypsin exposure for up to 1 h nor temperature (0 or 37 C) mattered. In plasmas from 26 healthy volunteers ambulatory for 3 h, trypsin increased PRA 3-fold from 2.8 +/- 0.4 to 8.3 +/- 0.7 ng/ml.h (P less than 0.001). Plasma renin reactivity (the amount of angiotensin I generated in plasma by the addition of active renin) and plasma renin substrate were unaltered by trypsin in concentrations up to 500 micrograms/ml. By exclusion, these data support the concept that an inactive precursor form of renin normally exists in human plasma. At concentrations of trypsin of 1 mg/ml or greater, tryptic activation diminished due to degradative effects of trypsin on the renin assay system, indicating the need for careful selection of conditions to maximize tryptic activation.