The third factor to influence activity, after absorption and distribution, is metabolism. Metabolism was defined as conversion into metabolites. The different classification of metabolites was outlined and that specific enzymes are required for the conversion of specific metabolic compounds(drugs). The chemical definition of an enzyme was shown, using the decomposition of hydrogen peroxide into water and oxygen. The importance of the metabolites, is that many are the active chemical which brings about the response. That the action of a drug could be enhanced by the inhibition of a chemical which would degrade was explained by the presence of enzymes which destroy that drug. Some enzyme systems cause more drug be metabolized, resulting in less drug action.