Biosynthesis of sphingomyelin in rat liver endoplasmic reticulum

Abstract

Rat liver microsomal sphingomyelin synthetase (CDPcholine: N-acylsphingosine cholinephosphotransferase (EC 2.7.8.3)) has been shown to be markedly stimulated by ATP and pantothenic acid derivatives such as CoA, pantethine, pantetheine and 4′-phosphopantetheine. Dephospho CoA, oxidized CoA and 4′,4″-diphosphopantethine likewise stimulate the enzyme in the presence of ATP, but to a much less significant extent, while pantothenic acid and pantothenyl alcohol were inactive. These pantothenic acid derivatives or ATP when added to the incubation system singly do not have any stimulatory activity. The stimulatory activity of ATP and CoA or ATP and pantethine was found to be approximately 20-fold. The activation of sphingomyelin synthetase is probably not through formation of an acyl-CoA or acyl thioester because palmityl-CoA or acetyl-CoA had very little stimulatory activity. Ceramides having the erythro configuration must have an acetyl group in N-acyl linkage; longer fatty acyl derivatives did not act as acceptors of the phosphorylcholine moiety of CDPcholine in the presence of ATP and CoA or ATP and pantethine. The incorporation of N-acetyl- and N-propionyl-threo-sphingosine compounds into sphingomyelin was also stimulated by ATP and the pantothenic acid derivatives mentioned above. The possible mechanism of action of ATP and CoA is discussed and other properties of the rat liver microsomal enzyme are also described.