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 < 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.

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