Radical Reactions Occur In Biological Systems

Water-soluble (polar) compounds are readily eliminated by the body. In contrast, water-insoluble (nonpolar) compounds are not readily eliminated but, instead, accumulate in the nonpolar components of cells. For cells to avoid becoming toxic dumps, nonpolar compounds that are ingested (such as drugs, foods, and environmental pollutants) must be converted into polar compounds that can be excreted. 

A radical reaction carried out in the liver converts nonpolar hydrocarbons into less toxic polar alcohols by substituting an H in the hydrocarbon with an OH. The reaction is catalyzed by an iron-containing enzyme called cytochrome P450- 

A radical intermediate is created when Fe O removes a hydrogen atom from the alkane. Then Fe —OH dissociates homol5dically into Fe° and OH, and the OH immediately combines with the radical intermediate to form the alcohol. 

This reaction can also have the opposite toxicological effect. For example, studies found that when animals inhale dichloromethane (CH2CI2), it becomes a carcinogen as a result of an H being substituted by an OH. 

Animal studies revealing that dichloromethane becomes a carcinogen when inhaled immediately led to a study of thousands of workers who inhaled dichloromethane daily. However, no increased risk of cancer was found in this group. (This shows that the results of studies done on humans do not always agree with the results of those done on laboratory animals.) 

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