Indoleamine 2,3-dioxygenase: Antioxidant enzyme in the human eye

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• Background: 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. • Methods: 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. • Results: 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. • Conclusion: Free tryptophan is degradated in the human eye by IDO, an antioxidative enzyme. IDO may be an antioxidant mechanism in the eye.