The subcellular site of oxidation of [1-14C]phytanic acid to pristanic acid and CO2 was examined by measurement of the release of 14CO2 in different organelles from human and rat tissues prepared by isopycnic density gradient centrifugation in Nycodenz. The activity of phytanic acid oxidation in human tissues (liver and cultured skin fibroblasts) paralleled that of the peroxisomal marker catalase. We also observed that Nycodenz (commonly used gradient material for isolation of subcellular organelles) has a strong inhibitory effect on the alpha-oxidation of phytanic acid. This inhibition is reversible and can be decreased or eliminated by dialysis of isolated organelles against isotonic solution. The dialysis of endoplasmic reticulum, mitochondrial, and peroxisomal fractions from human liver and cultured skin fibroblasts for 2 h against isotonic solution increased the specific activity of phytanic acid oxidation by 1.3-, 1.3-, and 5-21-fold, respectively, after removal of Nycodenz as compared with nondialyzed samples. After dialysis, the rate of oxidation of phytanic acid in peroxisomes from human liver and cultured skin fibroblasts was 4-26 times higher than that in mitochondria and 43-130 times than that in the endoplasmic reticulum, suggesting that, in human tissues, phytanic acid is oxidized to pristanic acid in peroxisomes. On the other hand, the oxidation of phytanic acid in rat liver paralleled the distribution of the mitochondrial marker cytochrome-c oxidase. The 18-fold higher rate of oxidation in dialyzed mitochondria (198.6 +/- 4.20 pmol/h/mg of protein) than in peroxisomes (11.0 +/- 0.5 pmol/h/mg of protein) demonstrates that, in rodents, phytanic acid is oxidized in mitochondria. 2-[5-(4-Chlorophenyl)pentyl]oxiran-2-carboxylic acid, an inhibitor of carnitine palmitoyltransferase I and mitochondrial fatty acid oxidation, inhibits the oxidation of phytanic acid in rat tissues (liver and cultured skin fibroblasts), whereas it has no effect on the oxidation of phytanic acid in human tissues (liver and cultured skin fibroblasts). The higher specific activity of phytanic acid oxidation in peroxisomes compared with that in mitochondria and the endoplasmic reticulum from human tissues and the inhibition of phytanic acid oxidation by 2-[5-(4-chlorophenyl)pentyl]oxiran-2-carboxylic acid in rat tissues (but not human tissues) demonstrate clearly that, in human tissues, phytanic acid is predominantly oxidized in peroxisomes.