Why do mitochondria synthesize fatty acids? Evidence for involvement in lipoic acid production

<jats:p> The function of acyl carrier protein (ACP) in mitochondria isolated from pea leaves has been investigated. When pea leaf mitochondria were labeled with [2- <jats:sup>14</jats:sup> C]malonic acid <jats:italic>in vitro</jats:italic> , radioactivity was incorporated into fatty acids, and, simultaneously, ACP was acylated. [1- <jats:sup>14</jats:sup> C]Acetate was much less effective as a precursor for fatty acid synthesis, suggesting that mitochondria do not possess acetyl-CoA carboxylase. The incorporation of radioactivity from [2- <jats:sup>14</jats:sup> C]malonate into fatty acids and the labeling of ACP were inhibited by cerulenin and required ATP and Mg <jats:sup>2+</jats:sup> . These findings indicate that plant mitochondria contain not only ACP, but all enzymes required for <jats:italic>de novo</jats:italic> fatty acid synthesis. Over 30% of the radioactive products from pea mitochondria labeled with [2- <jats:sup>14</jats:sup> C]malonate were recovered in H protein, which is a subunit of glycine decarboxylase and contains lipoic acid as an essential constituent. In similar experiments, the H protein of <jats:italic>Neurospora</jats:italic> mitochondria was also labeled by [2- <jats:sup>14</jats:sup> C]malonate. The labeling of pea H protein was inhibited by addition of cerulenin into the assay medium. Together, these findings indicate that ACP is involved in the <jats:italic>de novo</jats:italic> synthesis of fatty acids in plant mitochondria and that a major function of this pathway is production of lipoic acid precursors. </jats:p>