On the role of long-chain aldehydes in mammalian plasmalogen biosynthesis

[1-3H, 1-14C]Palmitaldehyde (3H:14C = 15) was injected intracerebrally to 18-day-old rats and incorporation of radioactivity into brain lipids was followed over a 24-h period. The substrate was metabolized primarily by oxidation to palmitic acid with loss of tritium and, to a lesser extent, by reduction to hexadecanol. The alkyl moieties of the ethanolamine phospholipids showed considerably lower 3H:14C ratios than the substrate, indicating a substantial participation in either lipid synthesis by tritium-free alcohols derived from 14C-labeled fatty acids. Virtually no 3H radioactivity was found in alkenyl moieties, indicating stereospecificity of both reduction of aldehyde and dehydrogenation of alkyl to alkenyl glycerolipid. The data are consistent with the general concept that plasmalogen biosynthesis proceeds exclusively through fatty alcohols and alkyl glycerolipids and that fatty aldehydes cannot be utilized directly.