An Optimized Micromethod for Determining the Catalytic Activity of Serum Ribonuclease

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Abstract
An optimized assay is described for the catalytic activity determination of serum ribonuclease, using polycytidylic acid as substrate and measuring the released acid-soluble ultra-violet absorbing products. Recommended final reaction concentrations are 0.3 mmol/l polycytidylic acid, 200 mmol/l imidazole/HCl buffer, pH 7.0, and 50 mmol/l NaCl. Optimal concentrations for the precipitation procedure, guaranteeing sufficient precipitation and minimal decomposition of unreacted substrate, are 160 mmol/l perchloric acid and 4 mmol/l lanthanum nitrate. Coefficients of variation for the method (within series and between days) ranged from 2.2 to 7.9%. No sex-related differences of catalytic activity were observed. In 63 blood donors with normal values of serum creatinine, the upper limit of the reference intervals (99th percentile) was 33.7 kU/l.