Activation of branched and other long-chain fatty acids by rat liver microsomes

Acyl coenzyme A synthetase (EC 6.2.1.3) of rat liver microsomes activates iso- and anteiso-branched long-chain fatty acids containing 12 to 20 carbon atoms. Fatty acid chain length appears to be the major determinant of the maximum rate of acyl CoA biosynthesis of branched, or saturated, or cis monounsaturated long-chain fatty acids. Based on activation studies conducted at 22-45 degrees C, it is concluded that the rate of activation is a function of long-chain fatty acid solubility. The shape of the in vitro activation curve with respect to fatty acid concentration appears to be determined by fatty acid melting point as well as by the presence and position of double bonds. Differently shaped activation curves were observed for cis or trans Delta(6) to Delta(12) central positional isomers of octadecenoic acid and for Delta(3), Delta(4), Delta(13) to Delta(15) terminal isomers of octadecenoic acid. The relationships between fatty acid structure, melting point, solubility, and shape of the activation curve observed during in vitro measurement of acyl CoA formation are discussed.