Induction of alcohol dehydrogenase by ethanol in Drosophila melanogaster.

When Drosophila melanogaster larvae were fed a defined fat-free, low sucrose medium, alcohol dehydrogenase (ADH) was increased to a higher activity with a moderate, nontoxic level of ethanol (2.5% vol/vol) within 5 h. Ethanol-stimulated increases in ADH activity and cross-reacting material in late third-instar larvae were paralleled by increases in the larval ADH mRNA as indicated by dot blot analysis. Northern blot observations indicated that both adult and larval ADH messages were increased by dietary ethanol. The increased levels of the ADH mRNA transcribed from the proximal and distal promoters of ethanol-fed larvae argue that the induction is a consequence of elevated levels of mRNA, not a result of changes in enzyme stability or synthesis. To determine whether the induction is of nutritional significance to larvae, the rate of flux from ethanol to lipid was estimated in control larvae and larvae that were pre-fed ethanol. Flux changes occurred; the rate of incorporation of [14C]ethanol into body lipid showed a strong association with larval ADH activity. Because the induced increase in larval ADH activity did not extend into the adult stage and attempts to stimulate ADH activity by exposing adults to ethanol were unsuccessful, the modulation of ADH activity by dietary ethanol may be a mechanism by which larvae utilize environmental ethanol as a resource, especially when free sugar levels are low. In addition, ADH in larvae is postulated to perform a second, nonethanol function that expedites the conversion of sugars to lipid when habitats are low in fats, low in ethanol and high in sugars.

The effect of dietary ethanol on the composition of lipids of Drosophila melanogaster larvae.

At a moderate concentration (2.5%, v/v) dietary ethanol reduced the chain length of total fatty acids (FA) and increased the desaturation of short-chain FA in Drosophila melanogaster larvae with a functional alcohol dehydrogenase (ADH). The changes in length in total FA were postulated to be due to the modulation of the termination specificity of fatty acid synthetase. Because the ethanol-stimulated reduction in the length of unsaturated FA was blocked by linoleic acid, it was thought to reflect the properties of FA 9-desaturase. Although the ethanol-stimulated reduction in chain length of unsaturated FA was also observed in ADH-null larvae, ethanol promoted an increase in the length of total FA of the mutant larvae. Thus, the ethanol-stimulated change in FA length was ADH dependent but the ethanol effect on FA desaturation was not. Ethanol also stimulated a decrease in the relative amount of phosphatidylcholine and an increase in phosphatidylethanolamine. Because similar ethanol-induced changes have been found in membrane lipids of other animals, ethanol may alter the properties of membranes in larvae. It is proposed that ethanol tolerance in D. melanogaster may be dependent on genes that specify lipids that are resistant to the detrimental effects of ethanol.

Dietary ethanol and lipid synthesis in Drosophila melanogaster.

When cultured on a defined diet, ethanol was an efficient substrate for lipid synthesis in wild-type Drosophila melanogaster larvae. At certain dietary levels both ethanol and sucrose could displace the other as a lipid substrate. In wild-type larvae more than 90% of the flux from ethanol to lipid was metabolized via the alcohol dehydrogenase (ADH) system. The ADH and aldehyde dehydrogenase activities of ADH were modulated in tandem by dietary ethanol, suggesting that ADH provided substrate for lipogenesis by degrading ethanol to acetaldehyde and then to acetic acid. The tissue activity of catalase was suppressed by dietary ethanol, implying that catalase was not a major factor in ethanol metabolism in larvae. The activities of lipogenic enzymes, sn-glycerol-3-phosphate dehydrogenase, fatty acid synthetase (FAS), and ADH, together with the triacylglycerol (TG) content of wild-type larvae increased in proportion to the dietary ethanol concentration to 4.5% (v/v). Dietary ethanol inhibited FAS and repressed the accumulation of TG in ADH-deficient larvae, suggesting that the levels of these factors may be subject to a complex feedback control.

Regulation of sn-glycerol-3-phosphate dehydrogenase in Drosophila melanogaster larvae by dietary ethanol and sucrose.

Dietary sucrose and ethanol are potent modulators of sn-glycerol-3-phosphate dehydrogenase (GPDH) in the third instar larvae of Drosophila melanogaster. When added to modified Sang's medium C, 428 mM ethanol and 146 mM sucrose each increased the GPDH tissue activity more than 90% and GPDH cross-reacting material (CRM) more than 50% over the levels found in larvae fed the 14.6 mM sucrose control diet. When fed together, ethanol and sucrose exerted synergetic effects on GPDH activity and CRM. The activity of glycerol-3-phosphate oxidase was also stimulated by dietary ethanol and sucrose, indicating that the glycerol-3-phosphate cycle was operating in the larvae. Dietary ethanol caused similar shifts in the NADH:NAD+ ratio in wild-type and Gpdh null larvae, suggesting that the maintenance of the cofactor equilibrium is not the primary function of GPDH in larvae. Increases in triacylglycerol content associated with the administration of ethanol and sucrose to larvae suggested that the formation of glycerol-3-phosphate for use in lipid synthesis is an important function of GPDH in larvae. Because ethanol is a constituent of the natural diet of D. melanogaster, nutritional modulation of GPDH is postulated to be an important aspect of the adaptation of the species to its environment.