Milk fat triglycerides are synthesized in the mammary epithelial cells. However, the fatty acids used to synthesize the milk triglycerides may arise from two sources:
From 40 to 60% of the fatty acids come from the blood. These are primarily derived from very low density lipoproteins (VLDL), which are synthesized in the intestine or liver. VLDL are composed of 90 to 95% lipid (55-60% triglyceride) on the inner core and 5 to 10% protein at the outer surface. Chylomicrons, containing ingested fatty acids from the intestine, also can act as a source of blood-derived fatty acids for the mammary gland.
This linkage gives sucrose certain properties that are quite different from those of maltose and lactose. As long as the sucrose molecule remains intact, neither monosaccharide “uncyclizes” to form an open-chain structure. Thus, sucrose is incapable of mutarotation and exists in only one form both in the solid state and in solution. In addition, sucrose does not undergo reactions that are typical of aldehydes and ketones. Therefore, sucrose is a nonreducing sugar.
In , you learned that monosaccharides can form cyclic structures by the reaction of the carbonyl group with an OH group. These cyclic molecules can in turn react with another alcohol. Disaccharides (C12H22O11) are sugars composed of two monosaccharide units that are joined by a carbon–oxygen-carbon linkage known as a . This linkage is formed from the reaction of the anomeric carbon of one cyclic monosaccharide with the OH group of a second monosaccharide.
Triglycerides in the VLDL are hydrolyzed in the mammary capillaries by an enzyme called lipoprotein lipase (LPL). The LPL can hydrolyze off one, two or all three of the fatty acids from the glycerol backbone, resulting in free fatty acids plus diacylglycerides, monoacylglycerides, or glycerol, respectively. The free fatty acids, monacylglycerides, diacylgycerides and glycerol can all be taken up by the mammary epithelial cell and reused for triglyceride synthesis.