Medium chain acyl-thioester hydrolase activity in goat and rabbit mammary gland fatty acid synthetase complexes

Inhibition of FASN reduces the synthesis of medium-chain fatty acids in goat mammary gland

Fatty acid synthase (FASN) is known as a crucial enzyme of cellular de novo fatty acid synthesis in mammary gland which has been proved as the main source of short and medium-chain fatty acids of milk. However, the regulatory role of FASN in goat-specific milk fatty acids composition remains unclear. We cloned and analyzed the full-length of FASN gene from the mammary gland of Capra hircus (Xinong Saanen dairy goat) (DQ 915966). Comparative gene expression analysis suggested that FASN is predominantly expressed in fat, small intestine and mammary gland tissues, and expresses higher level at lactation period. Inhibition of FASN activity by different concentrations (0, 5, 15, 25 and 35 μM) of orlistat, a natural inhibitor of FASN, resulted in decreased expression of acetyl-CoA carboxylase α (ACCα), lipoprotein lipase and heart-type fatty acid binding protein (H-FABP) in a concentration-dependent manner in goat mammary gland epithelial cells (GMEC). Similar results were also obtained by silencing of FASN. Additionally, reduction of FASN expression also led to apparent decline of the relative content of decanoic acid (C10:0) and lauric acid (C12:0) in GMEC. Our study provides a direct evidence for inhibition of FASN reduces cellular medium-chain fatty acids synthesis in GMEC.

Biosynthesis of medium chain fatty acids in Drosophila melanogaster

Adult Drosophila melanogaster synthesizes dodecanoic and tetradecanoic acids in vivo, along with the more common 16- and 18-carbon fatty acids. The radiolabeled C12 and C14 fatty acids synthesized from sodium [1-14C]acetate are found primarily in the diacylglycerol and triacylglycerol fractions. Partially purified fatty acid synthetase (FAS) synthesizes C14, C16, and C18 fatty acids (as the free acids) at 0.2 M ionic strength. Increasing the ionic strength to 2.0 M causes partially purified FAS to synthesize primarily C12 and C14 fatty acids. Addition of aliquots of the microsomal pellet and other soluble protein fractions does not alter the pattern of fatty acids synthesized by FAS. The percentage of C12 and C14 fatty acids synthesized at high ionic strength by individual fractions from the FAS peak (Sepharose 6B column) is constant across the peak. None of the soluble protein fractions is able to relieve the inhibition of FAS by phenylmethylsulfonyl fluoride. These results indicate that the FAS of D. melanogaster has the inherent capability to form C12 and C14 fatty acids and that no other soluble protein appears to be involved in their synthesis.

Medium-chain fatty acid synthesis by goat mammary-gland fatty acid synthetase. The effect of limited proteolysis

Fatty acid synthetase from goat mammary gland was subjected to limited proteolysis by trypsin and elastase. Both proteolytic enzymes selectively cleaved the chain-terminating thioester hydrolase component from the enzyme complex, leaving all other partial activities intact in the core peptides. Trypsin, but not elastase, caused extensive degradation of the released thioester hydrolase. The released thioester hydrolase could be purified to homogeneity by gel filtration. The molecular weight was estimated as 29 000 and the enzyme showed only significant hydrolytic activity toward long-chain acyl-CoA esters. The core peptides retained the ability to synthesize medium-chain acyl-CoA esters in the presence of 2,6-di-O-methyl-alpha-cyclodextrin. The results conclusively show that the terminating thioester hydrolase of goat mammary-gland fatty acid synthetase is not involved in termination of medium-chain-length fatty acid synthesis by this enzyme.

Transacylation as a chain-termination mechanism in fatty acid synthesis by mammalian fatty acid synthetase. Synthesis of medium-chain-length (C8-C12) acyl-CoA esters by goat mammary-gland fatty acid synthetase

1. Ruminant mammary-gland fatty acid synthetases can, in contrast with non-ruminant mammary enzymes, synthesize medium-chain fatty acids. 2. Medium-chain fatty acids are only synthesized in the presence of a fatty acid-removing system such as albumin, beta-lactoglobulin or methylated cyclodextrin. 3. The short- and medium-chain fatty acids synthesized were released as acyl-CoA esters from the fatty acid synthetase.

Transacylation as a chain-termination mechanism in fatty acid synthesis by mammalian fatty acid synthetase. Synthesis of butyrate and hexanoate by lactating cow mammary gland fatty acid synthetase

1. Purified cow mammary gland fatty acid synthetase synthesized long-chain unesterified and short-chain esterified fatty acids. 2. A direct relationship was observed between the amount of short-chain products synthesized and the concentration of acetyl-CoA in the incubation medium. 3. The short-chain products were identified as butyryl-CoA and hexanoyl-CoA. 4. Inhibition of the terminating thioester hydrolase of the fatty acid synthetase complex with phenylmethanesulphonyl fluoride did not inhibit the synthesis of short-chain products. 5. It is suggested that the synthesis of short-chain fatty acids involves the reverse of the 'loading' reaction.

Fatty-acid synthesis in lactating-goat mammary gland. 2. Medium-chain fatty acid synthesis

Addition of goat, rat, rabbit and cow mammary microsomal fraction to fatty acid synthetase of goat mammary gland induced C10 fatty acid synthesis. When the microsomal fractions from rat, rabbit and cow mammary gland were incubated with their respective fatty acid synthetases only the cow enzyme synthesized significant amounts of C10 compared to the purified fatty acid synthetases alone. The goat mammary microsomal fraction was not able to induce C10 synthesis by rat and rabbit mammary-gland fatty acid synthetase, but only by the goat and cow enzyme. Both goat and cow mammary-gland fatty acid synthetase have in contrast to the rabbit and rat enzymes the ability to hydrolyse medium-chain acyl-thioesters. We therefore suggest a novel mechanism for medium-chain fatty acid synthesis in lactating-goat mammary gland. This mechanism involves an activation of the medium-chain acyl-thioester hydrolase within the fatty acid synthetase of goat mammary gland by an unknown microsomal factor.