Indoleamine 2,3-dioxygenase: antioxidant enzyme in the human eye.
Indoleamine 2,3-dioxygenase (IDO) is the first enzyme of the tryptophan degradation pathway. IDO is an antioxidant enzyme because it is a direct scavenger of superoxide radicals. In this study, we measured the activity of IDO in the human eye. IDO was detected in the protein extract of human retina, iris/ciliary body, and lens. The products formed were measured using HPLC with electrochemical detection. Enzyme activity was expressed as the quantity of kynurenine and 3-hydroxykynurenine formed. IDO activity in the retina extract was 51.5 (+/-10) nmol/g tissue/h, and kynurenine formation was detected. In the iris/ ciliary body, IDO activity was 191.8 (+/49) nmol/g tissue/h, and both kynurenine and 3-hydroxy-kynurenine were formed from tryptophan. In the extract of lens cortex only 3-hydroxykynurenine was formed from tryptophan. IDO activity was 351 (+/-67.3) nmol/g tissue/h. Free tryptophan is degradated in the human eye by IDO, an antioxidative enzyme. IDO may be an antioxidant mechanism in the eye.