Phytanic Acid α‐oxidation in Rat Liver Peroxisomes: Production of α‐hydroxyphytanoyl‐CoA and Formate is Enhanced by Dioxygenase

Patients with generalized peroxisomal disorders, rhizomelic chondrodysplasia punctata, and Refsum disease are all unable to α‐oxidize 3,7,11,15‐tetramethylhexadecanoic (phytanic) acid. The exact cause of the oxidation defect in these patients is not well characterized, in part because there is only limited knowledge of the biochemical pathway. In 1969, the α‐oxidation of phytanic acid was reported [Tsai, S.‐C., Avigan, J. & Steinberg, D. (1969) Studies on the α‐oxidation of phytanic acid by rat liver mitochondria, J. Biol. Chem. 244, 2682–2692] to involve the formation of an α‐hydroxyphytanic acid intermediate prior to removal of the α carbon. Subsequently, most researchers have had difficulty detecting this intermediate. In the present study, cofactors known to form hydroxy intermediates by both monooxygen‐ase and dioxygenase reaction mechanisms were incubated with purified rat liver peroxisomes and either [2,3‐³H]phytanic acid or [1‐¹⁴C]phytanic acid. Reaction products were separated by reverse‐phase HPLC. A single reaction product, identified as α‐hydroxyphytanoyl‐CoA rather than the free fatty acid, was detected when 2‐oxoglutarate/Fe⁺²/ascorbate, cofactors associated with a dioxygenase reaction mechanism, were present. Concomitant with α‐hydroxyphytanoyl‐CoA production, there was an increased accumulation of formate and CO₂. This increase in α‐oxidation products is evidence that α‐hydroxyphytanoyl‐CoA is a true pathway intermediate and that the entire pathway functions in peroxisomes. In contrast, α‐hydroxyphytanoyl‐CoA was not formed in any quantity in mitochondria. These studies suggest that the α‐hydroxylation step of phytanic acid oxidation, which has been shown to be defective in Refsum disease, is located in peroxisomes.