The kynurenine metabolic pathway in the eye: studies on 3-hydroxykynurenine, a putative cataractogenic compound.

The rabbit lens has an elevated content of 3-hydroxykynurenine (3OHKYN) in spite of a very low activity of the enzymes leading to its synthesis. The iris/ciliary body, on the contrary, has very high activity of 3OHKYN synthesizing enzymes but a content of 3OHKYN lower than that of the lens. These observations suggest that 3OHKYN is formed in the iris/ciliary body, released into the aqueous humor and then taken up into the lens where it may be used for the synthesis of UV filtering products. An excessive accumulation of 3OHKYN in the lens has been associated with cataract formation. We found that available selective inhibitors of kynurenine hydroxylase reduced 3OHKYN synthesis in both the lens and the iris/ciliary body.