Intermediates in the enzymic synthesis of tetrahydrobiopterin in Drosophila melanogaster

A partially purified enzyme (Enzyme A) from Drosophila melanogaster catalyzes the conversion of 7,8-dihydroneopterin triphosphate to a compound that, from its ultraviolet absorption spectrum and other characteristics, appears to be 6-pyruvoyl-tetrahydropterin. This product can be converted to 6-lactoyl-tetrahydropterin in the presence of another partially purified enzyme (Enzyme B) and NADPH, and to 5,6,7,8-tetrahydrobiopterin in the presence of a third enzyme preparation (biopterin synthase) and NADPH. The enzymically-produced 6-lactoyl-tetrahydropterin, when exposed to air, is oxidized nonenzymically to sepiapterin (6-lactoyl-7,8-dihydropterin). The results indicate that although 6-lactoyl-tetrahydropterin can be converted enzymically to tetrahydrobiopterin, neither it nor sepiapterin is an obligate intermediate in the conversion of 7,8-dihydroneopterin triphosphate to tetrahydrobiopterin.