Polycytidylate hydrolysing ribonuclease variants of human pancreas

Abstract

Human pancreatic ribonuclease (RNase) comprises three variants with isoelectric points at pH 4.8, pH 9.3 and pH 9.7. These variants are not artifacts of the purification procedure, since they can be demonstrated in crude pancreatic extracts obtained with mild procedures (4). Although these RNase variants differ in their isoelectric points, they are similar in all the other properties so far studied. With polycytidylic acid as a substrate, their activities are optimal in 0.05M sodium phosphate buffer at pH 6.5. Of the several buffers tested, sodium phosphate buffer yielded maximal enzyme activity, but the concentration of phosphate is highly critical. Borate concentrations of 0.05M and 0.1 M respectively brought about 25% and 50% reductions in enzyme activity. Presence of phosphate in the reaction mixture obviates to a great extent the borate inhibition. These RNase variants are highly specific for the secondary phosphate esters of cytidine 3'-phosphate. They have no measurable activity on polyadenylic and polyguanylic acids and their activity on polyuridylic acid is less than 2% of that on polycytidylic acid. These RNase variants could serve as distinctive biochemical markers for the pancreas.