Identification of a soluble protein stimulator of plasmalogen biosynthesis and stearoyl-coenzyme a desaturase

Stimulation of the desaturation of 1-alkyl-2-acyl-sn-glycero-3-phosphoethanolamine (GPE), which forms ethanolamine plasmalogens, by a component of the 105,000g supernatant has been previously reported. We have isolated the stimulatory protein and identified it as catalase. Purified rat liver catalase or commercial bovine liver catalase is as effective in stimulating microsomal 1-alkyl-2-acyl-GPE desaturation as the soluble proteins. The stimulatory effect of these proteins is eliminated by catalase inhibitors. It appears that catalase stimulates the desaturation of 1-alkyl-2-acyl-GPE by preventing inactivation of the enzyme system by H₂O₂ or a decomposition product of H₂O₂. The cytochrome b₅ content and NADH oxidation are depressed in Fischer R-3259 sarcoma microsomes by H₂O₂; this effect is eliminated by catalase. However, since measurable inhibition of 1-alkyl-2-acyl-GPE desaturase by H₂O₂ still occurred in the presence of catalase, the inhibition by H₂O₂ cannot be explained solely on the basis of cytochrome b₅ inactivation. The desaturation of stearoyl-coenzyme A, a reaction analogous in many respects to 1-alkyl-2-acyl-GPE desaturation, was also found to be stimulated by catalase.