Lipoprotein Lipase and Lipid Metabolism in Mammary Gland

The lipid content of milk is the most variable of its major components.It ranges from zero in milk of rhinoceros and 2% in that of the horse to as much as 50% in milk of aquatic mammals (35).The lipid content of human and cow's milk is about 4%, rodent milk 12%, and milk of the polar bear 33% (Table 1).Fat provides about 70% of the calories in rat milk (1) and considerably more in milk of the polar bear and seal where fat content is extremely high and lactose content is low (Table 1).The major lipid component of milk of all species is triglyceride, accounting for >95% of the lipid (26).Milk fat droplets, which range from 1 to 7 /~m in diameter (35), are composed of a core of triglyceride with small amounts of diglycerides, free fatty acids and monoglycerides, and traces of cholesterol ester (15,23,36).The core is enveloped by a membrane, approximately 90 A thick, of protein, phospholipid, cholesterol, and other lipids.The fatty acids in milk triglyceride are mostly long-chain, with 14 to 18 carbon atoms per molecule, and are either saturated or unsaturated (7,15).Milk triglyceride also contains medium-chain (C s to C12) and sometimes short-chain ((24 to C6) saturated fatty acids.Table 2 shows the fatty acid composition (tool %) of the milk of five species.The milk fat of carnivores is characterized primarily by C16 and C18 fatty acids.Milk of aquatic mammals contains large amounts of C16:1 and other long-chain unsaturated fatty acids (17).The