Ethanol Metabolism

Every chemical reaction that occurs has its own speed at which product is formed from reactants. This speed is called the rate of reaction for a specific chemical process, and knowledge of the rate can provide scientists with valuable information about the microscopic nature of a reaction. The rate of reaction is defined as the change in concentration of product per unit of time: 

\[\text{Rate = }\dfrac{-\Delta c_{R}}{\Delta t} = \dfrac{\Delta c_{P}}{\Delta t}\]

The rate of reaction can be affected by multiple factors, including temperature and the presence of a catalyst. For any reaction, a mathematical equation can be determined that gives the rate of reaction based on the concentrations of reactants. This equation is called the rate law.

Metabolism of Ethanol in Humans

Alcohol consumption is an integral part of many societies and has been throughout history. Most notable is its integration in social events, whether these be weddings, holiday celebrations, or a simple get-together with some friends. In fact, the ubiquity of alcohol in its many forms often causes one to forget that alcohol is not a nutrient. Rather, it is a drug that affects the central nervous system in a negative manner. The human body does have a method by which alcohol can be metabolized, but the rate of metabolism is often far surpassed by the rate of absorption in the bloodstream. In this fashion, the consumption and processing of alcohol can be analyzed from the perspective of chemical kinetics in order to discover some interesting information about this common pastime.

Almost all of ethanol that is ingested is metabolized in the liver and the kidneys. In these two organs exist relatively high concentrations of specific forms of the alcohol dehydrogenase enzymes, or ADH. From a biological standpoint, these enzymes were likely incorporated into animal systems so that when ethanol-containing foods were eaten, such as fermented fruit, the toxic ethanol ingested could be broken down. When the ADH enzymes metabolize the alcohol, acetaldehyde is produced. This compound is then converted to acetate by different enzymes and, after a few more reactions, water and carbon dioxide are the final products.^[1]

Oxidization of ethanol (1st jmol) produces acetaldehyde (2nd jmol)

Rate Law of Ethanol Metabolism

The ADH reaction pathway is the largest contributor to the oxidation of alcohol, and thus the rate law is largely based on this ADH pathway. In the derivation it is assumed that the concentration of the co-substrate NAD+ reaches the rate-limiting state rapidly and remains constant thereafter.

In the above equations, C_Al is the ethanol concentration, C_Ac is the acetaldehyde concentration, V_maxADH is the maximum enzymatic oxidation rate of ethanol, V_revADH is the maximum rate of the reverse reaction of acetaldehyde to ethanol, and K_mADH and K_revADH are reaction constants for the rate law.^[2] It quickly becomes apparent that enzymes are key to the metabolism of ethanol. In general, with more enzymes existing in the processing organs comes a faster rate of reaction. The human body seems to pick up on this concept with the implementations of another ethanol metabolizing enzyme, cytochrome P450IIE1. This enzyme actually increases in concentration as a result of chronic drinking and is therefore a factor in what is considered an increase in tolerance for drinking.^[3]

Macroscopically, the above equations equate to a few important truths about alcohol metabolism.

1. For most people, a good rate estimation is one drink per hour (using the 1 drink = 1 beer = 1 shot = 1 glass of wine conversion factor if necessary)

2. The peak in the blood alcohol level caused by one drink will not be reached until 30 to 45 minutes have passed.

3. The rate of ethanol absorption will always surpass the rate of ethanol metabolism regardless of how much alcohol is consumed. Thus, the best way to prevent dangerous levels of intoxication is through moderation of consumption.^[4]

From ChemPRIME: 18.1: The Rate of Reaction

References

1. ↑ en.Wikipedia.org/wiki/Ethanol_metabolism
2. ↑ www.engin.umich.edu/~cre/cour...en/Alcohol.pdf
3. ↑ http://www.google.com/url?sa=t&source=web&ct=res&cd=1&ved=0CAcQFjAA&url=http%3A%2F%2Fwww2.hurlburt.af.mil%2Fshared%2Fmedia%2Fdocument%2FAFD-060110-019.doc&ei=yLQYS4yvKpLWNvPnnPQC&usg=AFQjCNG24EBhCtZ6VDtm1jNIAl1_rdK96w&sig2=FOFUKXh4viugPKT5aY1AVA
4. ↑ http://www.intox.com/about_alcohol.asp

Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.