Abstract
Lactophorin (LP) is a glycoprotein that reacts with the antiserum to the soluble glycoprotein of bovine milk fat globule membrane, and is present in component-3 of proteose-peptone. The present study was undertaken to confirm that auto-proteolytic degradation of LP in skim milk depends on plasmin, which is the major milk proteinase. LP was incubated with plasmin or trypsin, and analyzed by polyacrylamide gel electrophoresis in the presence or absence of sodium dodecyl sulfate, by immunoelectrophoresis, and by a single radial immunodiffusion.
Lactophorin was degraded by incubating for 1min at 37°C at the ratio of 44:1 for LP and plasmin, and at 50:1 for LP and trypsin. Furthermore, at the ratios of 1, 100:1 for LP and plasmin and 1, 000:1 for LP and trypsin, it was observed by SDS-PAGE that LP was degraded slowly via transient intermediates with prolonged incubation times of over 14 days at 37°C. By immunoelectrophoresis, the precipitation lines of LP were transformed by becoming longer, with a decreasing migration rate, and then becoming shorter. The antigenicity of LP retained about 80% of its original value after 14 days of incubation, and LP was more susceptible to plasmin than to trypsin. These results are indicative of proteolytic degradation of LP in skim milk being mostly dependent on the plasmin. It is suggested that one of the two major polypeptides of LP is an intermediate of the other by proteolysis.
