Increased levels of immunoreactive plasma renin substrate during infectious peritonitis in patients on continuous ambulatory peritoneal dialysis

1. Renin substrate (RS) and certain serine protease inhibitors share structural homology. Previous studies have shown that RS may be an acute-phase protein. 2. This prospective study was carried out to monitor plasma RS and other components of the renin-angiotensin system and acute-phase proteins during infectious peritonitis in patients on continuous ambulatory peritoneal dialysis. 3. RS was measured by both an enzymatic assay and a direct radioimmunoassay. An enzymatic assay measures intact RS only, whereas a direct radioimmunoassay measures both intact RS, capable of releasing angiotensin I and des-(angiotensin I)-RS, the residue of RS after cleavage of angiotensin I. 4. RS determined by direct radioimmunoassay increased (P less than 0.001) during acute inflammation (peritonitis), whereas neither RS measured enzymatically nor plasma renin activity changed significantly. 5. Our observation points to RS being an acute-phase protein in man. 6. Serum angiotensin-converting enzyme was measured by both an enzymatic and an inhibitor-binding assay. Both assays showed decreased levels of angiotensin-converting enzyme during acute inflammation.