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

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.