Characteristics of Several Minor-Protein Fractions Isolated from Bovine Milk

Abstract

Four minor-protein fractions were isolated from the same source of skim milk by methods similar to those reported by Rowland for proteose-peptone, by Aschaffenberg for [sigma]-proteose, by Jenness for milk component 5, and by Weinstein for the minor-protein fraction. A 5th fraction, the soluble membrane-protein, a constituent of the fat/plasma interfacial layer, was isolated from washed cream. These fractions were characteristically low in N (10-14%), high in ash (3.7%) and P (0.6-1.5%), and contained hexose sugars. Free-boundary electrophoretic. patterns demonstrated variable multicomponent systems containing what appeared to be a common major component. Sedimentation-velocity diagrams of the skim milk fractions revealed 2 apparently common boundaries, whereas the diagrams for soluble membrane-proteins showed more heterogeneity, suggesting that the constituent components differed from those of the skim milk proteose-peptones. The proteose-peptone [sigma] -proteose, and minor-protein fractions were rennin substrates. Heating (90[degree]C-30 min.) 1% solutions of milk component 5 concentrate and the soluble membrane-protein, 2 fractions obtained from raw milk without employing a heat-treatment step in the isolation procedure, produced changes in both the electrophoretic and sedimentation patterns. Further, heated milk component 5 concentrate was more susceptible to the action of rennin than its unheated counterpart. The membrane fraction was unaffected by rennin.