Modulation of biosynthesis of phosphatidylcholine via CDP-choline in rat liver: Influence of ethanol on the microsomal cholinephosphotransferase activity

We have studied in vitro the effects of ethanol on the different enzymes in volved in the biosynthesis of phosphatidylcholine (PC) via CDP-choline. Eth anol alters neither choline kinase (CK) nor CTP:phosphocholine cytidylyltra nsferase (CT) activities but, at levels higher than 50 mM, it does signific antly inhibit microsomal cholinephosphotransferase (CPT) activity concomita ntly with an increase in the ethanol concentration. A study of the kinetics of the reaction catalysed by CPT shows that ethanol decreases V-max withou t altering K-m, indicating a non-competitive inhibitory effect. An analysis of the thermodependence of CPT activity in the absence of ethanol reveals a break in the Arrhenius plot and thus a straight relationship between enzy me activity and the physico-chemical state of the microsomal membrane. Incu bation of microsomes in the presence of ethanol increased the transition te mperature from 25.8-28.2 degreesC. Microsomes were also incubated with n-al kanols with chain-lengths of fewer than five carbon atoms at concentrations which, according to their partition coefficients, produce equimolar levels in the membrane. Under these conditions all the alkanols caused the same i nhibitory effect. All these results demonstrate that ethanol modulate the P C biosynthesis at the level of CPT activity and does not affect the CT enzy me. The inhibition found on CPT is clearly dependent on the alteration prod uced by ethanol on the hepatic microsomal membrane.