The enzymatic acylation and hydrolysis of lysolecithin

Abstract

The present experiments show that lysolecithin is converted to lecithin by ratliver preparations which are free from cell particles. In this process lysolecithin is esterfied directly rather than being degraded to simpler compounds which are subsequently incorporated into lecithin. Two separate reactions are postulated whereby lysolecithin is converted to lecithin. These reactions are as follows: 2 lysolecithin → lecithin + glycerophosphorylcholine (1) lysolecithin + acyl-CoA → lecithin (2) Reaction 1 is from a quantitative standpoint the more important reaction although Reaction 2 has a faster rate. The Michaelis-Menten plot of lecithin synthesis vs. lysolecithin concentration gave support for Reactions 1 and 2 since the rate curve was anomalous, being composed of two separate parts which gave K m values of 0.08 mM (Reaction 2) and 3.3. mM (Reaction 1). The value of K m obtained for lysolecithin hydrolysis was 1.03 mM. When [ 32P]lysolecithin was used as substrate, the synthesis of lecithin as measured by either 32P-incorporation or by chemical phosphorus was the same and either method could be used to quantitate lysolecithin esterification or lysolecithin hydrolysis. The studies given here show that the rate of metabolism of lysolecithin was essentially constant at pH values between 6.2 and 6.9 but at values greater than 6.9 there was suppression of both lecithin synthesis and lysolecithin hydrolysis. Iodoacetic acid failed to inhibit either lecithin synthesis or lysolecithin hydrolysis whereas HgCl 2 inhibited both. CN- inhibited lecithin synthesis but not lysolecithin hydrolysis. Stearylglycollecithin was found to be inactive as a substrate for lysolecithinase in the rat-liver supernatant fluid but inhibited lysolecithin hydrolysis and lecithin synthesis 73 and 80% respectively. Transesterification has been shown to occur by the use of doubly-labeled lysolecithin.