CHARACTERIZATION OF INACTIVE RENIN (PRORENIN) FROM RENIN-SECRETING TUMORS OF NONRENAL ORIGIN - SIMILARITY TO INACTIVE RENIN FROM KIDNEY AND NORMAL PLASMA

Inactive renin comprises over half the total renin in normal human plasma. There is a direct relationship between active and inactive renin levels in normal and hypertensive populations but the proportion of inactive renin varies inversely with the active renin level; as much as 98% of plasma renin is inactive in patients with low renin while the proportion is consistently lower (usually 20-60%) in high-renin states. Two hypertensive patients with proven renin-secreting carcinomas of non-renal origin (pancreas and ovary) had high plasma active renin (119 and 138 ng/h per ml) and the highest inactive renin levels observed (5200 and 14,300 ng/h per ml; normal range 3-50). The proportion of inactive renin (98-99%) far exceeded that found in other patients with high active renin levels. A 3rd hypertensive patient with a probable renin-secreting ovarian carcinoma exhibited a similar pattern. Inactive renins isolated from plasma and tumors of these patients were biochemically similar to semipurified inactive renins from normal plasma or cadaver kidney. All were bound by Cibacron Blue-agarose, were not retained by pepstatin-Sepharose and had greater apparent MW (in daltons) than the corresponding active forms. Plasma and tumor inactive renins from the 3 patients wre similar in size (MW 52,000-54,000); normal plasma inactive renin had a slightly larger MW than that from kidney (56,000 vs. 50,000). Inactive renin from each source was activated irreversibly by trypsin and reversibly by dialysis to pH 3.3 at 47% C; the reversal process followed the kinetics of a 1st-order reaction in each instance. The trypsin-activated inactive renins were all identical to semipurified active renal renin in terms of pH optimum (pH 5.5-6.0) and kinetics with homologous angiotensinogen (Km 0.8-1.3 .mu.M) and inhibition by pepstatin or by serial dilutions of renin-specific antibody. A markedly elevated plasma inactive renin level distinguishes patients with ectopic renin production from other high-renin hypertensive states. The co-production of inactive and active renin by extrarenal neoplasma provides strong presumptive evidence that inactive reinin is a biosynthetic precursor of active renin. The unusually high proportion of inactive renin in plasma and tumor extracts from such patients is consistent with ineffective precursor processing by neoplastic tissue, suggesting that if activation of prorenin is involved in the normal regulation of active renin levels it more likely occurs in the tissue of origin (e.g., kidney) than in the circulation.