Serum RNase (RNase I; ribonucleate 3'-pyrimidino-oligonucleotidohydrolase, EC 3.1.4.22) activity (mean ± SD) with polycytidine as substrate was determined in normal individuals (24.9 ± 3.0 units/ml) and in patients with pancreatic cancer (37.3 + 14.8), pancreatitis (38.5 ± 12.6), nonpancreatic diseases (48.7 ± 14.8), or renal failure (175.8 : 92.8 Blood drawn from patients and controls was allowed to clot and then was centrifuged; serum was removed within 2 hr of collection. The serum was stored at -20'C. Freezing, thawing, and frozen storage for at least 1 month do not affect RNase activity.The assay for serum RNase activity was based on the method for bovine pancreatic RNase first reported by Zimmerman and Sandeen (2) with the synthetic substrate polycytidine. Serum was diluted 1:1000 with 5 mM Tris at pH 7 containing 0.01% bovine serum albumin, and a 50-sl aliquot was added to a Microfuge tube containing 150 Al of 0.1 M Tris at pH 8 and 50 /1l of polycytidine at 1 mg/ml. After incubation for 20 min at 370C, 250-A.l of cold 1.2 M perchloric acid/20 mM La203 was added. After 10 min at 00C, the mixture was centrifuged at 8000 X g for 5 min at room temperature. The supernatant, containing acid-soluble nucleotide, was diluted with water and the A280 was measured in a Beckman spectrophotometer. All serum dilutions and assays were performed in duplicate. Substrate blanks containing all reagents but no enzyme (serum) were analyzed in an identical manner and the values were subtracted from each assay. An enzyme (serum) blank proved unnecessary because serum did not contribute to the absorbance at these low dilutions. Sera assayed previously or bovine pancreatic RNase served as standards and their analysis was repeated with each group of sera of unknown activity. One RNase unit is the amount of enzyme activity that produces 1 ,mol of acid-soluble nucleotide per min at 37°C in 0.06 M Tris at pH 8. Molarity of acid-soluble nucleotide was calculated on the basis of an extinction coefficient of 13.0 X 103 at pH 2.0 (3).In 17 determinations on aliquots of one serum sample, performed consecutively, the mean (±SD) was 22.4 ± 0.7 units/ml. In 6 to 17 determinations on sera of four normal individuals obtained and performed at three different times, the mean was 21.3 ± 0.8 units/ml. Overall, 117 determinations were performed on the three samples of these four individuals; the results demonstrate remarkable precision of the procedure. As would be desired, the methodological variation is significantly less than the biologic variability: the mean of analyses on sera from 20 female and 20 male normal individuals was 24.9 + 3.0 units/ ml.The activity of bovine pancreatic RNase was the same when assayed with the polycytidine substrate used in my laboratory or with that used by Zimmerman and Sandeen (2) (Fig. 1A).When increasing amounts of three different sera were added to substrate, the rate of appearance of reaction products was linear (Fig. 1B).Patients were divided into five groups according to the following criteria.
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