The human brain contains three classes of alcohol dehydrogenase (ADH) isoenzymes: classes I, III, and IV. These isoenzymes of ADH participate in the metabolism of many biological substances, such as retinol or serotonin. ADH is responsible for the metabolism of ethanol, but it has not been definitively demonstrated to play a significant role in the brain. Catalase and cytochrome P450 2E1 are distributed throughout the brain and these systems can oxidize ethanol to acetaldehyde. After incidental abuse of alcohol, it is metabolized mainly by ADH and catalase, but chronic alcohol consumption results in an increased the role of cytochrome P450 2E1.