Effects of chronic ethanol exposure on composition and metabolism of Tetrahymena membrane lipids

Abstract

Plasma membranes, mitochondria and microsomes were isolated from Tetrahymena pyrifonnis WH-14-strain cells exposed chronically to ethanol, and examined for Iipid composition by thin-layer and gas-liquid chromatography and membrane fluidity by electron spin resonance. Appreciable differences were observed in phospholipid polar head group composition between membranes from the control and ethanol-exposed cells. There was a dramatic increase in phosphatidylethanolamine, with a compensating decrease in 2-aminoethylphosphonolipid in all membranes from ethanol-treated cells. Little or no significant changes were seen in phosphatidylcholine. The fatty acyl chain composition of membrane phospholipids was also considerably modified: a markedly elevated level of palmitic (16:0) and linoleic (18:2) acids, with a concomitant decline of oleic (18:1) and γ-linolenic (18:3) acids. Such acyl chain changes leading to the reduced degree of unsaturation would account in part for an adaptive response against ethanol-induced disordering of membrane Iipid bilayers. The profound increase of phosphatidylethanolamine could not be explained by a key enzyme for de novo synthesis of this phospholipid, CDPethanolamine: 1,2-diacylglycerol ethanolaminephosphotransferase. But the depressed activity of conversion from phosphatidylethanolamine to phosphatidylcholine would cause an accumulation of the former Iipid. The lowered unsaturation of phospholipid acyl chains was due to the inhibition of Δ 9 desaturases in microsomes. Microsomal membrane lipids of ethanol-exposed cells were slightly less fluid than those of control cells, whereas both plasma and mitochondrial membranes became rather more fluid by ethanol exposure, indicating that the latter membranes do not conform to the concept of homeoviscous adaptation. Moreover, the ethanol added in vitro did not demonstrate any difference in its membrane fluidizing effect between membranes from control and ethanol-exposed cells, indicating that ethanol tolerance was not induced.