Quantitation of bovine beta 2-microglobulin: occurrence in body fluids, on milk fat globules and origin in milk.

The concns of bovine beta 2-microglobulin (beta 2M) and selected control proteins were measured using a competitive immunoassay to determine the origin of beta 2M in cows' milk. Using milk samples collected at various times, separated into different fractions and treated with protease inhibitors, it was established that beta 2M appears in cows' milk by protease-dependent degradation of the cellular fraction of milk, probably mononuclear cells, but is not derived from milk fat globules (MFG) or from polymorphonuclear leukocytes despite positive immunofluorescence of the former. The latter source could be eliminated by the induction of neutrophilia which produced no changes in beta 2M levels. Our data also indicate a minimal contribution by MFG to levels of secretory component despite its detection by indirect immunofluorescence on MFG but are consistent with the view that the milk fat globule membrane protein, bovine-associated microprotein, is derived from MFG by proteolysis. Protease-dependent degradation of milk components is an in vivo storage effect which occurs as early as the first 3 hr of in vivo storage. Compared to three other secretions tested, beta 2M was most concentrated in lacteal secretions and codistributed with bovine lymphocyte antigen in milk.