␣-Lipoic acid (LA) is a cofactor for mitochondrial ␣-ketoacid dehydrogenase complexes and is one of the most potent, natural antioxidants. Reduction of oxidative stress by LA supplementation has been demonstrated in patients with diabetic neuropathy and in animal models. To determine how normal development or pathological conditions are affected by genetic alterations in the ability of mammalian cells to synthesize LA and whether dietary LA can circumvent its endogenous absence, we have generated mice deficient in lipoic acid synthase (Lias). Mice heterozygous for disruption of the Lias gene develop normally, and their plasma levels of thiobarbituric acid-reactive substances do not differ from those of wild-type mice. However, the heterozygotes have significantly reduced erythrocyte glutathione levels, indicating that their endogenous antioxidant capacity is lower than those of wild-type mice. Homozygous embryos lacking Lias appear healthy at the blastocyst stage, but their development is retarded globally by 7.5 days postcoitum (dpc), and all the null embryos die before 9.5 dpc. Supplementing the diet of heterozygous mothers with LA (1.65 g/kg of body weight) during pregnancy fails to prevent the prenatal deaths of homozygous embryos. Thus, endogenous LA synthesis is essential for developmental survival and cannot be replaced by LA in maternal tissues and blood.R-␣-Lipoic acid (6,8-thioctic acid) (LA) is a crucial cofactor that is required for the activity of the multienzyme complexes involved in the decarboxylation of ␣-ketoacids, important steps in energy metabolism. These complexes include pyruvate dehydrogenase complex (PDC), ␣-ketoglutarate dehydrogenase complex, branched chain ␣-ketoacid dehydrogenase complex (32), and a glycine cleavage system (11). LA is covalently attached via an amide bond to specific lysine residues of the E2 subunits of these complexes. LA is universally present in prokaryotes and eukaryotes (28), and previous investigations have shown that it is synthesized by lipoic acid synthase from octanoic acid and a sulfur source (23,25). Mouse lipoic acid synthase is encoded by the nuclear gene Lias.LA, one of the most potent natural antioxidants, has received considerable attention as a general dietary supplement for the past 15 years (3). Many lines of evidence have shown that both LA and its reduced form, dihydrolipoic acid (DHLA), have multifunctional antioxidant activities with the following characteristics (29). First, LA/DHLA are amphiphilic and readily cross the blood-brain barrier and cell membranes. Second, LA/DHLA possess metal-chelating activity. Third, LA is reduced to DHLA by several antioxidant enzymes that are expressed constitutively in most types of cells. In addition, because of its strong negative redox potential, DHLA can recycle other antioxidants, such as vitamin C, vitamin E, glutathione, coenzyme Q10, and ubiquinone (14,15,21,29,33).A growing body of evidence implicates oxidative stress as an important pathogenic element in a variety of diseases, including atheroscle...