Horse Pancreatic Ribonuclease

Horse ribonuclease was purified after acid extraction of pancreas by ammonium sulfate fractionation and chromatography on CM‐cellulose. It is a glycoprotein with an average molecular weight of about 18000. The pure enzyme was chromatographically and electrophoretically heterogeneous due to the heterogeneity of the carbohydrate moiety.The amino‐acid sequence was determined from four series of peptides obtained by different cleavage methods. All but four peptide bonds were overlapped by one or more peptides. The polypeptide chain contains 125 amino‐acid residues and carries oligosaccharide side‐chains at positions 21, 34, and 62; part of Asn‐21 occurs in the carbohydrate‐free form. There are two additional amino acids at the C‐terminus and a deletion at position 39. Including these, horse ribonuclease differs in 35 positions from the bovine enzyme. Horse ribonuclease is more closely related to the artiodactyl ribonucleases than is the rat enzyme.Except for the deletion, all differences can be accommodated in the three‐dimensional model of bovine ribonuclease‐S without altering the folding of the backbone. Model building of the loop containing the deletion offered an explanation for t