IDO1 and diabetes mellitus: The KP downstream products, kynurenine and 3-hydroxykynurenine, were found to be increased in the serum of DR individuals.39 In streptozotocin-induced diabetes in rats, the levels of IDO activity and mRNA in the lenses were raised when compared with those of nondiabetic animals, and oxidative stress markers, for example, thiobarbituric acid-reacting substances, were also increased when compared with control lenses.35