Formation of thieno[3,2-g]pterines from the molybdenum cofactor.

A fluorescent oxidation product of the molybdenum cofactor was isolated from Escherichia coli nitrate reductase (EC 1.9.6.1) and bovine milk xanthine oxidase (EC 1.2.3.2), which showed a visible absorption band at 395 nm and was dephosphorylated by alkaline phosphatase but not by phosphodiesterase I. The dephosphorylated species was oxidized by periodate to thieno[3,2-g]pterin-2-carbaldehyde which was quantitatively converted to thieno[3,2-g]pterin-2-carboxylic acid by subsequent treatment with Ag2O in 2 N NaOH. These results indicate that the oxidation product of the molybdenum cofactor is a thieno[3,2-g]pterin derivative with an unidentified side chain in the 2 position.