The occurrence of immunoglobulin fragments, two types of lactoferrin and a lactoferrin-IgG2 complex in bovine colostral and milk whey

Abstract

When the fraction of bovine colostral or normal whey soluble in 40% saturated (NH 4) 2SO 4 is applied to a column of DEAE-Sephadex A-50 equilibrated with 0.05 M NaCl, 0.1 M Tris, pH 8.3, the unadsorbed protein appears as three peaks (I, II, III). Each of these peaks can be subsequently resolved into 5–8 fractions by gel filtration. This fractionation procedure, combined with immunodiffusion and immunoelectrophoretic analyses, has allowed for characterization and identification of the proteins comprising the original three peaks obtained by DEAE chromatography. Peak I is predominately lactoferrin but IgG2-lactoferrin complexes and ribonuclease can also be detected. Peak II is predominately 7-S IgG2 while Peak III contains Fab, IgG2-Fc and an immunoelectrophoretically distinct form of lactoferrin. The secondary form of lactoferrin, present in purified lactoferrin preparations, appears to be of lower molecular weight and like lactoferrin, is antigenically distinct from the bovine milk protein lactollin. F(ab′) 2 fragments are also present in this complex mixture while glycoprotein-a (free secretory component) is absent. The immunoelectrophoretic arcs of the IgG immunoglobulins described could explain the minor IgG-related precipitin arcs often observed in bovine serum and secretions without postulating a new subclass of IgG. The kinds of immunoglobulin fragments detected are similar to those observed in stored human serum and certain human exocrine secretions (except urine) and may result from proteolysis of intact IgG rather than de novo synthesis.