The Fat-Globule Membrane of Normal Cow's Milk. I. The Isolation and Characteristics of Two Membrane-Protein Fractions

Abstract

The fat-globule protein (s) consist of 2 fractions of approximately equal distribution based upon their solubility in 0.02 [image] NaCl solution. The "soluble" fraction was readily soluble in aqueous solvents, whereas the "insoluble" fraction required strong reducing agents, such as those commonly used to attack disulfide linkages, or detergents to effect solution. Successful solubilization of the insoluble protein fraction makes it possible to explore further the physical and chemical nature of the fat-globule membrane. The soluble protein fraction contained 11.10% N, 0.7% S, 0.46% P and 7.06% ash and was white in color. Aqueous solutions of this protein gave a strongly positive Molisch test, reduced Fehling''s solution slightly and was nitroprusside negative after heating to 75C. The insoluble fraction contained 13.8% N, 0.94% S, 0.23% P, 2.08% ash and was reddish-brown in color. Aqueous suspensions of this fraction were Molisch negative and nitroprusside positive upon momentary heating to 75C. The insoluble fraction was tentatively classified as a pseudokeratin on the basis of insolubility in 0.02[image] NaCl solution, reactivity to agents commonly employed to disrupt disulfide linkages, affinity for alkaline solution, and a 1:1 ratio of lysine to arginine.