Pancreatic ribonucleases of mammals with ruminant-like digestion. Amino-acid sequences of hippopotamus and sloth ribonucleases.

High levels of pancreatic ribonucleases are found in ruminants, species that have a ruminant-like digestion and several species with coecal digestion. Pancreatic ribonucleases from several independently evolved species with ruminant-like digestion were investigated to test a hypothesis that glycosylation of ribonucleases may have some function in species with coecal digestion and that glycosylation of the enzyme may not be advantageous for ruminants. Ribonucleases from the hippopotamus, two-toed sloth and three-toed sloth were isolated by extraction with sulfuric acid and affinity chromatography. Complete amino acid sequences were determined for the ribonucleases from the hippopotamus and two-toed sloth and a partial sequence for the enzyme from the three-toed sloth. The amino acids 75-78 of hippopotamus ribonuclease were positioned by homology with other artiodactyl ribonucleases. In hippopotamus ribonuclease a heterogeneity was found at position 37, half of the molecules containing glutamine acid the other half lysine. Hippopotamus ribonuclease differs less from pig and bovine ribonuclease than these differ from each other, because more ancestral characteristics have been retained. Although hippopotamus ribonuclease contains all four Asn-X-Ser/Thr sequences previously found to be glycosylation sites in one or more pancreatic ribonucleases, only the sequence Ans-Met-Thr (34-36) is glycosylated in the variant with glutamine at position 37, while the variant with lysine at this position is carbohydrate-free. Both sloth ribonucleases are completely glycosylated at the sequence Ans-Met-Thr (34-36) with a simple type of carbohydrate chain. The amino acid sequence of two-toed sloth ribonuclease shows some interesting coupled replacements.