A new protein with a particular thermoprecipitability in bovine milk.

A new protein with a particular thermoprecipitability was isolated from bovine milk and tentatively termed milk pyroglobulin. The protein which was soluble at a relatively cold temperature was precipitated by raising the temperature to a certain degree depending on the concentration of the protein. The precipitate disappeared on recooling. This protein had the electrophoretic mobility of gamma globulin but did not carry either antigenic specificities of immunoglobulins or of free secretory component. The molecular weight was estimated to be approximately 60,000 in thin-layer gel filtration on Sephadex G-200 superfine gel, but the protein appeared to be convertible to molecules with a lower molecular weight of approximately 20,000 in the presence of bovine serum albumin. The presence of the albumin inhibited the thermoprecipitation as did alpha-lactalbumin but not IgG immunoglobulin from bovine colostrum. In SDS-polyacrylamide gel electrophoresis, the protein was separated into two components having a molecular weight of 19,000 and 10,000, respectively. Both components were thermoprecipitable and carried identical antigenic determinants.