The compound 2-amino-4-oxo-6-acetyl-7,8-dihydro-3H,9H-pyrimido[4,5-b]-[1,4]diazepine (pyrimidodiazepine or PDA, for short) is a precursor of the red eye pigments called the drosopterins in Drosophila melanogaster. The precursor of PDA is 2-amino-4-oxo-6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydrop teridine triphosphate (dihydroneopterin triphosphate or H2-NTP). The synthesis of of PDA from H2-NTP requires reduced glutathione, another thiol such as 2-mercaptoethanol, Mg2+, and at least three enzymes: one that is missing in the eye color mutant, sepia; one that is present only in limited quantities in the mutant, clot; and a third one that has been described as sepiapterin synthase A. The last enzyme is present only in relatively small quantities in the mutant, purple. Because PDA is two electrons more reduced than H2-NTP, it would appear that the reducing power needed for this transformation is probably supplied by glutathione. Oxidized glutathione cannot replace reduced glutathione in the system. The yield of PDA produced enzymatically from H2-NTP can be as high as 40% under optimal conditions.