Some Properties of Secretory IgA Purified from Bovine Colostrum

The major component of a purified sample of secretory IgA (SIgA) in colostrum was revealed as a single peak on gel filtration with Sepharose 6B, having an estimated molecular weight of 540,000. The existence of a higher molecular weight component was suggested by a small shoulder on the ascending limb of the peak, but another component of IgA reported as IgA lacking the secretory component (SC) could not be found. When the purified SIgA was concentrated by dialysis against polyethylene glycol, its molecular size was apparently significantly decreased.

Analysis by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE) showed that all SC in SIgA binds covalently. The band corresponding to the J chain was easily detected when a reduced and alkylated sample was analysed. Estimation of the molecular weight by SDS-PAGE gave the following values for each of the constituent polypeptide chains of bovine colostral SIgA: SC, 76,000; H chain, 62,000; L chain, 23,000; and J chain, 18,000. The molecular weight of the whole molecule was calculated to be 434,000.

Analysis of carbohydrates by gas-liquid chromatography showed 6.8% neutral and amino hexoses, consisting of 0.4% fucose, 1.8% mannose, 1.1% galactose and 3.5% glucosamine. Galactosamine, which has been found in bovine free secretory component from milk, could not be detected.