Measurement of renin concenration in human plasma using 125i-labelled sheep renin substrate

Sheep renin substrate was prepared 28% pure, labelled with 125I and then diluted in concentrated unlabelled sheep substrate. Ten percent of the 125I-label was released as a small fragment on incubation with excess human renin. Adequate separation of the labelled fragment from the parent protein was accomplished with charcoal precipitation or gel filtration. Renin concentration in human plasma was assayed by incubating the 125I-labelled substrate with plasma under zero-order kinetic conditions. Renin was assayed in plasma that had been either acidified or untreated. The acidification procedure was employed to activate inactive renin which accounted for more than half of the total plasma renin concentration. The results of this radiochemical assay correlated closely with the results of bioassay. Specificity of the method was established with antirenin. The assay was sufficiently sensitive to measure suppressed plasma renin levels. The iodinated substrate was stable on storage at -10 degrees C over a six-month period. Because of its high affinity for human renin, ease of purification and stability of the iodinated product, sheep substrate provides a convenient means for routine determination of plasma renin concentration by radiochemical assay.