Serum cysteine proteinase inhibitors with special reference to kidney failure

Serum levels of proteins reactive in radioimmunoassay with an antiserum prepared in rabbits against purified human spleen neutral cysteine proteinase inhibitor (NCPI) was determined in 70 healthy controls and from 80 patients suffering from suspected or proven kidney failure. The values varied from less than 0.2 mg/l in normal sera to levels over 2 mg/l in some patient sera. Serum level of NCPI was found to roughly correlate with serum creatinine values. However, there were sera with high NCPI levels which did not have increased serum creatinine values. In sera with high NCPI levels subjected to double radial immunodiffusion two precipitin lines, one completely and the other partially identical to NCPI were registered. After fractionating of serum proteins with gel chromatography on Sephadex G 100, two protein peaks of immunological similarity to purified NCPI were found: one low molecular weight (MW around 12,000) and one high molecular weight (MW around 100,000). The low molecular weight NCPI-like material appeared to inhibit human cathepsin B and papain and is thus free serum NCPI. alpha-Cysteine proteinase inhibitor did not increase with serum creatinine as did NCPI.