Mechanism of enzymatic conversion of a fatty acid to the corresponding alkane by the loss of the carboxyl carbon was investigated with particulate preparations from Pisum sativum. A heavy particulate preparation (sp. gr., 1.30 g/cm3) isolated by two density-gradient centrifugation steps catalyzed conversion of octadecanal to heptadecane and CO.Experiments with [1-3H,1_14C]octadecanal showed the stoichiometry of the reaction and retention of the aldehydic hydrogen in the alkane during this enzymatic decarbonylation. This decarbonylase showed an optimal pH of 7.0 and a Km of 35 ,uM for the aldehyde. This enzyme was severely inhibited by metal ion chelators and showed no requirement for any cofactors. Microsomal preparations and the particulate fractions from the first density-gradient step catalyzed acyl-CoA reduction to the corresponding aldehyde. Electron microscopic examination showed the presence of fragments of cell wall/cuticle but no vesicles in the decarbonylase preparation. It is concluded that the aldehydes produced by the acyl-CoA reductase located in the endomembranes of the epidermal cells are converted to alkanes by the decarbonylase located in the cell wall/cuticle region.Alkanes are widely distributed in the plant and animal kingdoms (1). Biological hydrocarbons usually have an odd number of carbon atoms, suggesting that they are derived by the loss of one carbon atom from fatty acids with even numbers of carbon atoms. Experiments with higher plant tissue slices strongly suggested that hydrocarbons are formed by chain elongation of fatty acids followed by loss of the carboxyl carbon, presumably by decarboxylation (2, 3). Subsequent work with insects (4) and mammals (5) supported this mechanism for alkane biosynthesis. Experiments with cell-free preparations from pea leaves showed that oxygen and ascorbate were required for the conversion of C32 fatty acid to alkane and that metal ion chelators strongly inhibited alkane synthesis (6). Subsequent studies showed that a major part of this alkane-generating activity was located in a crude microsomal fraction and that C18 to C32 fatty acids could serve as substrates for alkane formation (7). All of these substrates gave rise to mainly alkanes containing two carbon atoms less than the parent acid. Evidence was presented suggesting that in vitro the aldehyde generated from the parent acid by the classical a-oxidation was the immediate precursor of the alkane, whereas in vivo aldehydes generated by acyl-CoA reductase might be the immediate precursor of alkanes. It was suggested that the aldehyde might be decarborlylated to alkane. In the present paper, we describe the isolation of a particulate fraction devoid of a-oxidation activity, and we present direct experimental evidence for enzymatic decarbonylation of an aldehyde to alkane.
MATERIALS AND METHODS
Materials