Pancreatic Ribonucleases of Mammals with Ruminant‐Like Digestion

Abstract

High levels of pancreatic RNases are found in ruminants, species that have a ruminant-like digestion and several species with cecal digestion. Pancreatic RNases from several independently evolved species with ruminant-like digestion were investigated to test a hypothesis that glycosylation of RNases may have some function in species with cecal digestion and that glycosylation of the enzyme may not be advantageous for ruminants. RNases from the hippopotamus (Hippopotamus amphibius) 2-toed sloth (Choloepus hoffmani), and 3-toed sloth (Bradypus infuscatus) were isolated by extraction with sulfuric acid and affinity chromatography. Complete amino acid sequences were determined for the RNases from the hippopotamus and 2-toed sloth and a partial sequence for the enzyme from the 3-toed sloth. The amino acids 75-78 of hippopotamus RNase were positioned by homology with other artiodactyl RNases. In hippopotamus RNase a heterogeneity was found at position 37, half of the molecules containing glutamine and the other half lysine. Hippopotamus RNase differs less from pig and bovine RNase than these differ from each other, because more ancestral characteristics have been retained. Although hippopotamus RNase contains all 4 Asn-X-Ser/Thr sequences previously found to be glycosylation sites in one or more pancreatic RNases, only the sequence Asn-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 RNases are completely glycosylated at the sequence Asn-Met-Thr (34-36) with a simple type of carbohydrate chain. The amino acid sequence of 2-toed sloth RNase shows some interesting coupled replacements.