MICROSOMAL ACETALDEHYDE OXIDATION IS NEGLIGIBLE IN THE PRESENCE OF ETHANOL

The microsomal ethanol oxidizing system (MEOS), inducible by ethanol a nd acetone, oxidizes ethanol to acetaldehyde, which causes many toxic effects associated with excess ethanol. Recent studies reported that r at liver microsomes also oxidize acetaldehyde, thereby challenging the validity of the assessment of MEOS activity by measuring acetaldehyde production and suggesting that MEOS activity results in the accumulat ion not of acetaldehyde but, rather, of its less toxic metabolite, ace tate. To address these issues, we compared both metabolic rates of eth anol and acetaldehyde and the effect of ethanol on the acetaldehyde me tabolism. Liver microsomes were prepared from Sprague-Dawley rats indu ced either with acetone for 3 days or ethanol for 3 weeks. NADPH-depen dent acetaldehyde (300 mu M) metabolism was measured in two ways: (1) by detection of acetaldehyde disappearance by headspace gas chromatogr aphy, and (2) by assessment of acetaldehyde oxidation by liquid scinti llation counting of acetate formed from 1:1,2-C-14]acetaldehyde. Ethan ol (50 mM) oxidation was measured by gas chromatography. In acetone- a nd ethanol-induced rat liver microsomes, the acetaldehyde disappearanc e (p < 0.0001) and oxidation (p < 0.0001) rates were both significantl y increased. The rates of acetaldehyde oxidation paralleled those of p -nitrophenol hydroxylation (r = 0.974, p < 0.0001), with a K-m of 82 /- 14 mu M and a V-max of 4.8 +/- 0.5 nmol/min/mg protein in acetone-i nduced microsomes. Acetaldehyde disappearance in acetone-induced micro somes and acetaldehyde oxidation in acetone-induced and ethanol-induce d microsomes were significantly lower than the corresponding ethanol o xidation, with rates (nmol/min/mg protein) of 4.6 +/- 0.6 versus 9.0 /- 0.8 Go < 0.005), 4.4 +/- 0.3 versus 9.1 +/- 0.5 Go < 0.0005), and 1 4.0 +/- 0.9 versus 19.5 +/- 1.8 Ip < 0.05), respectively. The presence of 50 mM ethanol decreased this metabolism to 0.9 +/- 0.3 (p < 0.005) , 0.5 +/- 0.1 Go < 0.001), and 1.8 rt 0.3 Go < 0.001), resulting in ra tes of acetaldehyde metabolism of only 9.8 +/- 3.2%, 6.0 +/- 0.5%, and 9.5 +/- 1.2% (respectively) of those of ethanol oxidation. In conclus ion, rat liver microsomes oxidize acetaldehyde at much lower rates tha n ethanol, and this acetaldehyde metabolism is strikingly inhibited by ethanol. Accordingly, acetaldehyde formation provides an accurate ass essment of MEOS activity. Furthermore, because acetaldehyde production vastly exceeds its oxidation, the net result of MEOS activity is the accumulation of this toxic metabolite.