Isolation of isoxanthopterin from human urine.

The pterins xanthopterin (2-amino-4:6-dihydroxypteridine) (Koschara, 1936) and biopterin (2amino 4 hydroxy-6-[1: 2-dihydroxypropyl (L-erythro)]-pteridine) (Patterson, Saltza & Stokstad, 1956) have been isolated from human urine. It has been suggested that the pterins 2-amino-4-hydroxypteridine-6-carboxylic acid, 2-amino-4-hydroxypteridine and isoxanthopterin (2-amino-4:7-dihydroxypteridine) are products of catabolism of pteroyl-L-glutamic acid (Blair & Graham, 1955). It has also been shown that pteroyl-L-glutamic acid is converted by methylene blue and xanthine oxidase into 2-amino-4-hydroxypteridine-6-carboxylic acid (Blair, 1957). It seemed desirable, therefore, to examine human urine further. When freshly voided adult male urine was analysed, as described below, a small quantity of a purple fluorescent substance could be isolated. On further purification this substance was shown to be i8oxanthopterin, by its similar behaviour to an authentic sample in six solvent systems. No 2-amino-4-hydroxypteridine-6-carboxylic acid could be isolated from freshly voided human urine. If, however, samples were allowed to stand, or if the analysis was unduly protracted, a brightblue fluorescent substance could be isolated. This was not separable from 2-amino-4-hydroxypteridine-6-carboxylic acid in five different solvent systems. It was assumed that this acid was derived from the decomposition of biopterin in the urine. The method used was not suitable for the isolation of 2-amino-4-hydroxypteridine.