Mediates the conversion of 2-amino-4-oxo-6-pyruvoyl-5,6,7,8-tetrahydropteridine (6-PTP; also named 6-pyruvoyltetrahydropterin) to 2-amino-6-acetyl-3,7,8,9-tetrahydro-3H-pyrimido(4,5-b)[1,4]diazepin-4-one (pyrimidodiazepine or PDA), a key intermediate in red eye pigment drosopterin biosynthesis. 2-amino-6-acetyl-3,7,8,9-tetrahydro-3H-pyrimido[4,5-b][1,4]diazepin-4-one + glutathione disulfide + H2O = 6-pyruvoyl-5,6,7,8-tetrahydropterin + 2 glutathione Homodimer. Appears only at the late pupal stage; expressed at high level. Also expressed in the early adult (0-2 days old) at lower level. Expressed exclusively in the adult head. Sepia eye color, due to defects in 2-amino-6-acetyl-3,7,8,9-tetrahydro-3H-pyrimido(4,5-b)[1,4]diazepin-4-one (pyrimidodiazepine or PDA) synthesis, a key intermediate in red eye pigment drosopterin biosynthesis. Belongs to the GST superfamily. Omega family.