Singlet oxygen: the relevance of extracellular production mechanisms to oxidative stress in vivo

Abstract

Since the physiological relevance of ¹O₂ independent of photosensitization has been controversial, we review proposed reaction mechanisms for its extracellular production in vivo and discuss the relevance of this production to oxidative stress. We conclude that extracellular ¹O₂ production by the spontaneous dismutation of O₂˙⁻does have physiological relevance. Also, extracellular ¹O₂ production by the eosinophil peroxidase–H₂O₂–bromide system could have physiological relevance. As regards the other reactions discussed in this review, the evidence is not sufficient to warrant any conclusions as to the physiological relevance of these to extracellular ¹O₂ production. What is evident is that the microenvironment will have a significant influence on the success or failure of extracellular ¹O₂ production. To date, most demonstrations of ¹O₂ production by physiologically relevant mechanisms have used conditions that minimize competitive reactions. More research demonstrating how physiologically relevant competitive reactions influence extracellular ¹O₂ production is needed.