PHYTANIC ACID ALPHA-OXIDATION - DIFFERENTIAL SUBCELLULAR-LOCALIZATIONIN RAT AND HUMAN TISSUES AND ITS INHIBITION BY NYCODENZ

The subcellular site of oxidation of [1-C-14]phytanic acid to pristani c acid and CO2 was examined by measurement of the release of (CO2)-C-1 4 in different organelles from human and rat tissues prepared by isopy cnic density gradient centrifugation in Nycodenz. The activity of phyt anic acid oxidation in human tissues (liver and cultured skin fibrobla sts) paralleled that of the peroxisomal marker catalase. We also obser ved that Nycodenz (commonly used gradient material for isolation of su bcellular organelles) has a strong inhibitory effect on the alpha-oxid ation of phytanic acid. This inhibition is reversible and can be decre ased or eliminated by dialysis of isolated organelles against isotonic solution. The dialysis of endoplasmic reticulum, mitochondrial, and p eroxisomal fractions from human liver and cultured skin fibroblasts fo r 2 h against isotonic solution increased the specific activity of phy tanic acid oxidation by 1.3-, 1.3-, and 5-21-fold, respectively, after removal of Nycodenz as compared with nondialyzed samples. After dialy sis, 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 prist anic acid in peroxisomes. On the other hand, the oxidation of phytanic acid in rat liver paralleled the distribution of the mitochondrial ma rker cytochrome-c oxidase. The 18-fold higher rate of oxidation in dia lyzed mitochondria (198.6 +/- 4.20 pmol/h/mg of protein) than in perox isomes (11.0 +/- 0.5 pmol/h/mg of protein) demonstrates that, in roden ts, phytanic acid is oxidized in mitochondria. 2-[5-(4-Chlorophenyl)pe ntyl]oxiran-2-carboxylic acid, an inhibitor of carnitine palmitoyltran sferase I and mitochondrial fatty acid oxidation, inhibits the oxidati on of phytanic acid in rat tissues (liver and cultured skin fibroblast s), whereas it has no effect on the oxidation of phytanic acid in huma n tissues (liver and cultured skin fibroblasts). The higher specific a ctivity of phytanic acid oxidation in peroxisomes compared with that i n mitochondria and the endoplasmic reticulum from human tissues and th e inhibition of phytanic acid oxidation by 2-[5-(4-chlorophenyl)pentyl ]oxiran-2-carboxylic acid in rat tissues (but not human tissues) demon strate clearly that, in human tissues, phytanic acid is predominantly oxidized in peroxisomes.