Oxygen‐Dependent Cleavage of the Vinyl‐Ether Linkage of Plasmalogens

The low‐molecular‐weight component of rat brain supernatant, which catalyzed the oxygen‐dependent cleavage of the vinyl‐ether linkage of plasmalogen, was purified by chromatography on SE‐Sephadex and DEAE‐Sephadex. A compound was obtained with an absorption peak at 266 nm. It catalyzed the commensurate vinyl‐ether cleavage and oxygen reduction. Evidence is presented that this purified active component is identical with, or very similar to ascorbic acid. Commercial ascorbic acid mimicked the vinyl‐ether cleaving properties of the active component of rat brain supernatant. Contrary to the rat brain supernatant, the purified active component was unstable and lost activity on heating or storage. Furthermore, vinyl‐ether cleavage and oxygen uptake ceased after 2–3 min. Addition of a protein of rat brain, which by itself was inactive, extended the reaction time for 15–20 min. In the course of the reaction, 3.5–4 mol oxygen were consumed and 0.3 mol of ascorbate disappeared for each mole of vinyl‐ether‐linkage cleaved. Small quantities of compounds which reacted with thiobarbituric acid were also formed in the course of the reaction. The products of the reaction were lysolecithin and several split products produced from the aldehydogenic moiety of the plasmalogen molecule; these reacted with p‐nitrophenylhydrazine. Using silicic acid chromatography, about 10–20% of the total carbonyl‐containing compounds were eluted with chloroform—methanol (95:5, by vol.), these were identified as pentadecanal in the presence of smaller quantities of heptadecanal. The p‐nitrophenylhydrazones of the products were prepared and separated into 3–4 spots on thin‐layer silica gel plates. The fastest moving spot was identified by infrared and mass spectrometry as a hydroxy‐aldehyde, probably α‐hydroxyhexadecanal. A reaction mechanism is proposed as follows: A ferrous‐ascorbate complex reduces and forms an “active” oxygen. This attacks the vinyl‐ether‐linkage of the plasmalogen forming a diol. Hydrolysis of this diol yields α‐hydroxy‐hexadecanal; oxidative cleavage of this bond yields pentadecanal.