Synthesis of fatty acids in milk fat

In all species, the principal precursor for fatty acid synthesis is acetyl CoA, derived in non-ruminants from glucose and in ruminants from acetate or oxidation of /5-hydroxybutyrate. Acetyl CoA is first converted, in the cytoplasm, to malonyl CoA  

Reduced bicarbonate supply (source of CO2) depresses fatty acid synthesis. 

Some / -hydroxybutyrate is reduced to butyrate and incorporated directly into milk fat hence, the high level of this acid in ruminant milk fat. 

The large supply of NADPH required for the above reactions is obtained through the metabolism of glucose-6-phosphate via the pentose pathway. 

In ruminants, -hydroxybutyrate is the preferred chain initiator (labelled /8-hydroxybutyrate appears as the terminal four carbons of short- to medium-chain acids), i.e. the first cycle in fatty acid synthesis commences at j3-hydroxybutyryl-S-ACP. 

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