Alcohol metabolism's damaging effects on the cell: a focus on reactive oxygen generation by the enzyme cytochrome P450 2E1

Abstract

Alcohol metabolism's various processes create harmful compounds that contribute to cell and tissue damage. In particular, the enzyme cytochrome P450 2E1 (CYP2E1) plays a role in creating a harmful condition known as oxidative stress. This condition is related to oxygen's ability to accept electrons and the subsequent highly reactive and harmful byproducts created by these chemical reactions. CYP2E1's use of oxygen in alcohol metabolism generates reactive oxygen species, ultimately leading to oxidative stress and tissue damage.

Figure 1

Sequential reduction of oxygen in four single-electron steps. The addition of one electron to oxygen (i.e., a one-electron reduction of oxygen) produces superoxide. If superoxide is reduced by another electron, the two-electron–reduced form of oxygen, peroxide, is produced. This will accept two hydrogens to produce hydrogen peroxide. Hydrogen peroxide can accept another single electron, and this usually occurs as a result of the transfer from a reduced metal ion (e.g., reduced iron or copper). When it does accept that electron, hydrogen peroxide is converted to a hydroxyl radical (HO^•) and a hydroxide anion (HO⁻) that, when combined with hydrogen, produces water.

Figure 2

Metabolism of ethane to produce alcohol and water. Electrons are transferred from NADPH to cytochrome P450 2E1 (CYP2E1) by cytochrome P450 reductase. A) The CYP2E1 then catalyzes the oxidation of the substrate molecule (ethane) by adding one atom of oxygen to the substrate, producing ethanol; the other atom is reduced to water. B) The CYP2E1catalyzed metabolism of alcohol produces an unstable intermediate (i.e., a gem-diol) that will decompose to produce acetaldehyde.

Figure 3

Generation of reactive oxygen by cytochrome P450 2E1 (CYP2E1). CYP2E1 is represented by the heme iron in the blue ovals. When CYP2E1 uses oxygen to metabolize alcohol, reactive oxygen species (ROS) can be generated by the following chain of events: ethanol binds to the enzyme (step 1). As the first electron is passed to the heme of CYP2E1 and oxygen is bound (step 2), the electron can move and exist on the oxygen, essentially generating superoxide bound to the heme of CYP2E1 (step 3). Occasionally, the superoxide will break down, releasing free superoxide and generating the starting enzyme. If the second electron is added to the enzyme (step 4), then a second form of reduced oxygen is produced that is identical to a heme-bound form of the twoelectron–reduced oxygen (i.e., peroxide) (step 5). When this product breaks down, it picks up two hydrogens to generate hydrogen peroxide. The production of these ROS by CYP2E1 is referred to as an “uncoupled reaction” because the oxygen does not end up in the substrate. If the reduced oxygen species remains bound, then the enzyme will transfer one oxygen atom to the substrate and the other atom becomes water, producing an unstable intermediate (i.e., a gem-diol) product that decomposes to acetaldehyde (step 6).

Figure 4

The variability of cytochrome P450 2E1 (CYP2E1) from 18 human liver samples. Each liver sample is designated with a code number. The level of the of the enzyme is directly proportional to the formation of 6-hydyroxychlorzoxazone. Source: Guengerich 2006.