{{other uses, Meos (disambiguation) The microsomal ethanol oxidizing system (MEOS) is an alternate pathway of

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a h ...

metabolism that occurs in the

smooth endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

in the oxidation of ethanol to

acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the ...

. While playing only a minor role in ethanol metabolism in average individuals, MEOS activity increases after chronic alcohol consumption. The MEOS pathway requires the

CYP2E1 Cytochrome P450 2E1 (abbreviated CYP2E1, ) is a member of the cytochrome P450 mixed-function oxidase system, which is involved in the metabolism of xenobiotics in the body. This class of enzymes is divided up into a number of subcategories, includ ...

enzyme, part of the

cytochrome P450 Cytochromes P450 (CYPs) are a superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various compo ...

family of enzymes, to convert ethanol to

. Ethanol’s affinity for CYP2E1 is lower than its affinity for

alcohol dehydrogenase Alcohol dehydrogenases (ADH) () are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD+) to N ...

. It has delayed activity in non-chronic alcohol consumption states as increase in MEOS activity is correlated with an increase in production of

, seen most conclusively in alcohol dehydrogenase negative deer mice.Chales S. Lieber. 2004. The Discovery of the Microsomal Ethanol Oxidizing System and Its Physiologic and Pathologic Role. ''Drug Metabolism Reviews'' 36:511-529. The MEOS pathway converts ethanol to acetaldehyde by way of a

redox reaction Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a de ...

. In this reaction, ethanol is oxidized (losing two hydrogens) and O₂ is reduced (by accepting hydrogen) to form H₂O. NADPH is used as donor of hydrogen, forming NADP⁺. This process consumes ATP and dissipates heat, thus leading to the hypothesis that long term drinkers see an increase in resting energy expenditure.Francisco Santolaria and Emilio González- Reimers. 2003. Alcohol and Nutrition: an Integrated Perspective in ''Nutrition and Alcohol: Linking Nutrient Interactions and Dietary Intake''. p. 5 Ronald Ross Watson and Victor R. Preedy (eds). Taylor and Francis, CRC Press. The increase in rest energy expenditure has, according to some studies, been explained by indicating that the MEOS "expends" nine calories per gram of ethanol to metabolize versus 7 calories per gram of ethanol ingested. This results in a net loss of 2 calories per gram of ethanol ingested.

References

Metabolic pathways