Ketoconazole and other imidazole derivatives are potent inhibitors of peroxisomal phytanic acid alpha-oxidation

The imidazole antimycotics like ketoconazole, clotrimazole, bifonazole, miconazole and CO, known as powerful inhibitors of cytochrome P-450, are potent inhibitors of peroxisomal phytanic acid alpha-oxidation to pristanic acid suggesting the possible involvement of the cytochrome P-450 mono-oxygenase system in this oxidation. In contrast to the inhibition of the oxidation of [1-14C]phytanic acid, [1-14C]phytanoyl-CoA and [(2,3)-3H]phytanic acid, these drugs and CO have no effect on the oxidation of [1-14C]alpha-hydroxy phytanic acid indicating that these drugs and CO inhibit only the alpha-hydroxylation of phytanic acid. These studies using purified peroxisomes from liver and cultured human skin fibroblasts and Hep G2 cells clearly demonstrate that alpha-hydroxylation, an intermediate step in the alpha-oxidation of phytanic acid found to be impaired in Refsum Disease, is mediated by cytochrome P-450 containing enzyme.

Phytanic acid must be activated to phytanoyl-CoA prior to its alpha-oxidation in rat liver peroxisomes

alpha-Oxidation of the branched-chain fatty acid, phytanic acid, is defective in patients with Refsum's disease, the disorders of peroxisome biogenesis (e.g., Zellweger syndrome), and in rhizomelic chondrodysplasia punctata. 3H-Release from [2,3-3H]phytanic acid, which is impaired in cultured skin fibroblasts from these patients, was investigated in rat liver peroxisomes. Cofactors necessary for optimal 3H-release, ATP, Mg2+, and coenzyme A, were also necessary for optimal acyl-CoA synthetase activity, suggesting that the substrate for 3H-release might be phytanoyl-CoA. 5,8,11,14-Eicosatetraynoic acid (ETYA), an inhibitor of long-chain acyl-CoA synthetase activity, blocked phytanoyl-CoA synthesis as well as 3H-release from [2,3-3H]phytanic acid in a dose-dependent manner. However, this inhibitor had little effect on 3H-release from [2,3-3H]phytanoyl-CoA. Tetradecylglycidic acid (TDGA) inhibited 3H-release from [2,3-3H]phytanic acid in peroxisomal but not in mitochondrial fractions from rat liver. This agent inhibited 3H-release from [2,3-3H]phytanic acid and [2,3-3H]phytanoyl-CoA equally. In contrast to ETYA, which appeared to decrease 3H-release as a consequence of synthetase inhibition, TDGA appeared to act directly on the enzyme catalyzing 3H-release. This enzyme was partially purified from rat liver. The purified enzyme, which did not possess phytanoyl-CoA synthetase activity, catalyzed tritium release from [2,3-3H]phytanoyl-CoA. This enzyme catalyzed 3H-release from [2,3-3H]phytanic acid only if a source of phytanoyl-CoA synthetase was present. We conclude that in rat liver peroxisomes, phytanic acid must be activated to its coenzyme A derivative prior to subsequent alpha-oxidation.