Purification and characterization of a bovine colostrum low molecular weight cysteine proteinase inhibitor

Large amounts of cysteine proteinase inhibitors were found in bovine colostrum. One had a molecular weight of 90,000, and the other a molecular weight of 10,500. The concentrations of both these inhibitors were highest the day after parturition, and were about one-tenth as much on day 7. The lower molecular weight inhibitor was purified by acid treatment, ammonium sulfate fractionation, gel filtration on Sephadex G-50, CM-Sephadex chromatography and rechromatography on Sephadex G-50. The purified preparation gave a single band on SDS-polyacrylamide gel electrophoresis. This inhibitor contained one tryptophanyl residue and one cystinyl residue, and did not contain a free thiol group. Values obtained for its isoelectric point (pI) were 10.0 and 10.3. This material strongly inhibited cathepsin B, cathepsin H, and papain. the higher molecular weight inhibitor was partially purified. It had a pI of 4.2 and inhibited papain, cathepsin H, and cathepsin B.