Purification of a novel apolipoprotein H-like milk protein with ribonucleolytic and cell-free translation inhibitory activities

A new whey protein designated apolipoprotein H-like whey protein, with a mo lecular weight of 62 kDa and an N-terminal amino acid sequence similar to t hat of apolipoprotein H, was isolated from bovine milk. The isolation proce dure involved removal of globulin from acid whey by precipitation with 1.8M (NH4)(2)SO4, followed by addition of (NH4)(2)SO4 to attain a concentration of 3.6M. Subsequent steps included chromatography on CM-Sepharose and Mono S and elution of the adsorbed protein of interest with a linear NaCl gradi ent. The new whey protein displayed some ribonuclease (RNase) activity. It was most active at pH 7.5 with yeast transfer RNA (tRNA) as substrate and s howed potent specific ribonuclcolytic activity toward poly C, It inhibited cell-free translation in a rabbit reticulocyte lysate system with an IC50 o f approximately 63 nM. (C) 2000 Elsevier Science Inc. All rights reserved.