Proteolysis of Lactophorin in Skim Milk by Bovine Milk Indigenous Proteinases

Lactophorin (LP), which reacts with the antiserum to milk fat globule membrane (MFGM), has been designated as the major glycoprotein in component-3 (C-3) of proteose-peptone (PP). We investigated whether LP could be derived from any skim milk (SKM) proteins by indigenous milk proteinases during the incubation of SKM at 37°C. After incubatine, the total PP isolated from SKM was fractionated into component-5, -3 (rich in LP) and-8 fractions by salting out. A marked difference was found between heated SKM (95°C, 30min) and unheated SKM. The amounts of the total PP fractions isolated from unheated SKM increased with incubation time, while those of heated SKM (control) did not. The LP content in C-3 from heated SKM, which was determined by a single radial immunodiffusion, did not change during the incubation period. In contrast, the LP content in C-3 from unheated SKM decreased with incubation time, and was 20% of the 0-day amount after 14 days. Electrophoretic analysis showed that major band of LP from unheated SKM disappeared after 6 days of incubation. An immunoelectrophoretic assay showed that the precipitation lines of C-3 from unheated SKM had shifted to the more-negative electrode side after 2 and 3 days, changed to one line after 4 days, and become shorter after 6 days, suggesting that LP was degraded via the intermediates formed during proteolysis. The results indicate that the increase of PP and degradation of LP was caused by proteolysis with indigenous milk proteinases. It is concluded that, far from being newly formed, LP was degraded by proteolysis.