Hydroxyl radicals and biological damage in vitro: what relevance in vivo?

Hydroxyl radicals have been implicated in various forms of tissue injury ranging from radiation-induced cell death to alloxan-induced diabetes in animals. Although hydroxyl radicals can be readily generated in vitro, for example by the action of radiation or from the reaction of ferrous (FeII) ions with peroxides, they have not been observed directly in vivo. Their involvement in tissue damage has generally only been inferred from studies with so-called selective radical scavengers. In this paper the validity of such inferences will be briefly discussed in the light of current knowledge about the mechanisms and rates of reactions of hydroxyl radicals with biological compounds.