Ribonuclease inhibitor from human placenta: rapid purification and assay

The ribonuclease inhibitor from human placenta may be isolated in 65% yield (2.5 mg per placenta) in 2 days. The performance of the affinity chromatography on Sepharose-RNase A has been expedited through adaption of the spectrophotometric assay of ribonuclease toward 2',3'-cyclic cytidine monophosphate to determination of the inhibitor activity. The result of these improvements in procedure is increased availability of the ribonuclease inhibitor for study of its chemical properties (Blackburn, P., and Jailkhani, B.L. (1979) J. Biol. Chem. 254, 12488-12493) and for its inclusion into in vitro systems in which inhibition of mammalian neutral ribonucleases is desired.