C10H7NO4; This compound a metabolic product resulting from indoleamine 2,3-dioxygenase (IDO) breakdown of tryptophan, may accumulate in human lens cells and play a role in senile-onset lens pathology, leading to lens cell apoptosis. Excreted in the urine of pyridoxine-deficient animals after the ingestion of tryptophan, and of rats fed almost exclusively with fibrin, the sulfur-yellow crystals form a red compound with Millon reagent, or an intensely green one with ferrous sulfate.