Synthesis of Milk Fat
Milk fat has not only complex composition, but complex synthesis also. Some fatty acids are synthesized in the mammary gland (de novo), whereas others are taken up from the blood and repackaged as milk-specific triglycerides.
de novo synthesis. The short and medium-chain-length fatty acids, constituting up to 50% of milk fat, are synthesized in the mammary gland from acetate and β-hydroxybutyrate, products of the ruminal fermentation. Whereas the predominant synthetic pathway is that described for longer chain fatty acids of other tissues and species B i.e., by successive condensation of 2-carbon units with malonyl-CoA as intermediate, butyrate may arise independently of this pathway. β-hydroxybutyrate may be incorporated directly after reduction; this contributes about one-half of milk butyrate (Palmquist et al., 1969). The remainder arises by direct condensation of two molecules of acetate (as acetyl-CoA intermediates), by a mechanism known as the β-reduction pathway (Lin and Kumar, 1972; Strom and Kumar, 1979). Because these two pathways are independent of malonyl-CoA, butyrate synthesis is not inhibited by conditions that inhibit acetyl-CoA carboxylase and the synthesis of other short and medium-chain fatty acids. The latter inhibitory condition is common when dietary fat is increased, which increases mammary uptake of long chain fatty acids from blood. Long-chain fatty acids are potent inhibitors of acetyl CoA carboxylase, the enzyme that mediates conversion of acetate to malonyl CoA (Goodridge, 1972).