Biosynthesis of milk

Dairy Cattle Production 342-450A Milk Biosynthesis

Department of Animal Science

BIOSYNTHESIS OF MILK COMPONENTS

Dairy Cattle Production 342-450A

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Dairy Cattle Production 342-450A Milk Biosynthesis

Introduction: Precursors of milk come from the bloodstream. It is estimated that the production of 1 liter of milk requires 500 liter of blood moving through the mammarygland to provide the milk precursors. Some materials in the milk come unchanged from blood. These include minerals, some hormones and some proteins (e.g. immunoglobulins). Only precursors of milk protein and carbohydrates are present in blood. The primary substrates extracted from blood by the lactating mammary gland include glucose, amino acids, fatty acids, β-hydroxybutyrate, and salt.Biosynthesis of Milk Fat Background Cow's milk contains 3.5 to 5% fat. About 97 to 98 of the fat is triglycerides (also known as triacylglycerols or triacylglycerides) and Phospholipids constitute about 1% (Table 1). Palmitic (C16:0) and oleic (C18:1) acid are the main fatty acid in milk fat (Table 2). Milk fat contains low levels short chain fatty acid (C12 and less).
Table 1. Milk lipid composition ofdairy cows Lipid class % of total lipids Triglycerides 95.8 1,2 diglycerides 2.3 Phospholipids 1.1 Cholesterol 0.5 Free fatty acids 0.3 Table 2. Fatty acid composition of milk. Fatty acid % weight C4:0 3.6 C6:0 2.2 C8:0 1.2 C10:0 2.5 C12:0 2.8 C14:0 10.1 C15:0 1.1 C16:0 25.0 C16:1 2.6 C17:0 0.9 C18:0 12.1 C18:1 27.1 C18:2 2.4 C18:3 2.1 Other 2.4

Biosynthesis of Milk Lipids (Triglycerides)Page 2 of 9

Dairy Cattle Production 342-450A Milk Biosynthesis

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Milk fat triglycerides are synthesized in the cytoplasm surface of the smooth endoplasmic reticulum of mammary epithelial cells. Milk lipids (triglycerides) are synthesized from fatty acids and glycerol through the α-glycerol phosphate pathway (Figure). Acetyl CoA carboxylase is the key milk biosynthesis enzyme and itsactivity increases considerably during lactogenesis (copious milk secretion). Two acyl CoA molecules react with α-glycerol-3-phosphate to form phosphatidic acid, which upon removal of the phosphate, leaves a 1,2 diacylglycerol. An additional long chain acyl CoA adds the final fatty acid, with the formation of triacylglycerol and CoA.

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B i o s y n t h e s i s o f M ilk T r i g l y c e r i d e s
Blood A cetate β -hydroxybutyrate L ipoprotein lipase M onoglycerides Free FA G lycerol

B lood triglycerides

C a p i l l a r y w a ll B a s a l m em b r a n e M a m m a ry cell A cetate β -h y d r o x y b u t y r a t e

F a tty A cyl C o A

α -g lycerol-P

C 4-C 1 2

M ilk triglycerides

Sources of Milk Fatty Acids • The fatty acids used to synthesize milk fat (triglycerides) comefrom two sources: 1- Blood lipids 2- De novo synthesis within the mammary epithelial cells (synthesis of new molecules of fatty acids from precursors absorbed from the blood) 1- Blood lipids: Derived from digestion and absorption of dietary fat and from mobilization of fatty acids from adipose tissue. Most of the fatty acids derived from blood plasma are of dietary origin (> 80%). This amountcould differ according to stage of lactation and milk yield. Blood lipids are the source of all of the C18 and most of the C16 fatty acids in milk. One third of the C16 and most of the C18 fatty acids in the milk are of dietary origin. In total, about 1/2 of the milk fatty acids are derived from the blood plasma lipids.

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Dairy Cattle Production 342-450A Milk Biosynthesis

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Indairy cow diets, dietary fats consist mainly of long chain fatty acids (palmitic, C16; stearic, C18:0; oleic, C18:1, linoleic, C18:2; linolenic, C18:3). Dietary fatty acids are biohydrogenated (saturated) in the rumen by ruminal microbes. Therefore fatty acids in adipose tissue and in milk of dairy cows are more saturated in nature than those of the diet. Intestinal and mammary epithelia of...