Medium-chain fatty acid synthesis.

Most plant and mammalian tissue lipids contain fatty acids with a carbon chain longer than 14 atoms. However, milk lipids from several species like ruminants and rodents (Jenness, 1974), seed lipids from certain plants like coconuts (Child, 1974) and Cuphea (Graham & Kleiman, 1985) and lipids from several insects (AddaeMensah & Cameron, 1978) and from some bird glands (Downing, 1976; Jacob, 1976) contain fatty acids with from four to 14 carbon atoms. Studies of the mechanisms by which mediumand short-chain fatty acids are synthesized have been primarily performed with enzymes isolated from animal mammary glands, sebaceous glands of water fowl and from aphids. Rabbit mammary gland fatty acid synthetase produces in vitro predominantly short-(C, + C,) and long(cl4 cI8) chain fatty acids (Carey & Dils, 1970), whereas in vivo the gland produces mostly medium(cg + C,o) chain fatty acids (Dils et al., 1977). The long-chain fatty acids synthesized by the rabbit fatty acid synthetase in vitro are released by hydrolytic cleavage of the acyl-enzyme complex by thioesterase I, which is an inherent part of the synthetase complex. This enzyme is, however, only able to cleave long-chain ( >, C,,) thioester acylknzyme complexes (Grunnet & Knudsen, 1978). The short-chain (C, + C,) product is released as acylCoA esters (Hansen & Knudsen, 1980). The transferase involved in the termination of synthesis at short-chain length is most likely the loading transferase, which also is an inherent part of the synthetase complex (Hansen & Knudsen, 1980). The loading transferase from the rabbit mammary synthetase complex is unable to handle acylchains longer than C, effectively (Knudsen & Grunnet, 1980). The rabbit mammary synthetase is therefore unable to terminate fatty acid synthesis at medium-chain length by either a thioesterase or acyltransferase reaction. The explanation for rabbit mammary tissue producing mostly medium-chain fatty acid in vivo is that this tissue contains an additional thioesterase (thioesterase 11). This enzyme, which has a molecular weight of 29000, is able to interact with and hydrolyse the fatty acid synthetase acylknzyme complex when the chain length has reached 8-10 carbon atoms (Knudsen et al., 1975, 1976). Similar thioesterases have been found in rat mammary gland (Libertini & Smith, 1978) and mice mammary gland (Libertini et al., 1980), which also synthesize medium-chain fatty acids. The enzyme has also been found in the sebaceous gland of mallard ducks (Rogers et al., 1982) and in aphids (Ryan et al., 1982). In contrast, goat mammary gland, which produces short-, mediumand long-chain fatty acids in vivo, does not contain a thioesterase I1 enzyme (Grunnet & Knudsen, 1979~). Purified fatty acid synthetase from goat mammary gland synthesizes only shortand long-chain fatty acids like the rabbit mammary enzyme (Grunnet & Knudsen, 19796). However, in the presence of an acyl-CoA-utilizing or -complexing system like the triacylglycerol-synthesizing system from mammary microsomes (Grunnet & Knudsen, 19796,1981; Knudsen & Grunnet, 1982) or bovine serum albumin (Knudsen & Grunnet, 1982) and methylated a-cyclodextrin (Knudsen & Grunnet, 1982), the purified enzyme synthesizes mainly