Peroxisomal metabolism of adrenic acid; No Δ4 desaturase detected in rat liver peroxisomes

The existence of a peroxisomal Δ4 desaturation of 22:4n‐6 and 22:5n‐3 to yield, respectively, 22:5n‐6 and 22:6n‐3 has been questioned. An alternative pathway has been formulated to include microsomal chain elongation and Δ6 desaturation and peroxisomal chain shortening. We incubated [1‐ 14 C]adrenic acid (22:4n‐6) in a system for desaturation (i.e., in the presence of NADH) with purified rat liver peroxisomes. The fatty acids were separated as methyl derivatives by high‐performance liquid chromatography. Four ultraviolet‐absorbing product peaks appeared, three of which contained radioactivity, which we investigated as methyl, trimethylsilyl, and oxazoline derivatives on gas chromatography‐mass spectrometry. In addition to adrenic and arachidonic acids, the product peaks were trans ‐enoyl, hydroxy, and keto derivatives of adrenic acid: the three first steps of β‐oxidation cycle. This indicated that the NAD‐dependent dehydrogenase step in the peroxisomal β‐oxidation cycle of adrenic acid was inhibited due to a high concentration of added NADH. Incubation in the presence of NAD instead of NADH reduced radioactivity in the peaks that corresponded to intermediates, while radioactivity in the acid‐soluble fraction increased considerably, consistent with a complete β‐oxidation cycle of adrenic to arachidonic acid. There were no indications of Δ4 desaturation in purified peroxisomes incubated in a standard desaturation system. Instead, adrenic acid as substrate underwent β‐oxidation.