Purification and biosynthesis of quench spot, a drosopterin precursor in Drosophila melanogaster

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Abstract
Pteridine biosynthesis was examined in extracts of the heads of D. melanogaster by measuring the conversion of dihydroneopterin triphosphate to sepiapterin and the drosopterins (6 eye pigments that are dipterin derivatives). These 2 products share a common 1st step in the production of an intermediate that is a branch point from which both products are formed. This 1st step can be catalyzed by sepiapterin synthase or by an enzyme found in particles that sediment at 600 g. A substance named quench spot was found at low levels in the purple mutants that were defective in drosopterin synthesis and restored to normal when a suppressor mutant, su(s)2, restored drosopterins in purple to normal levels. The sepia mutant is also deficient in the levels of both quench spot and drosopterins. Quench spot is probably a precursor of drosopterins, but not sepiaterin and it is formed from the sepiaterin synthase intermediate mentioned above. An additional precursor that is formed independently of the sepiaterin synthase pathway is also proposed that reacts with quench spot to form drosopterins. These proposals are based on the following: (1) quench spot biosynthesis is observed in extracts of Drosophila heads in which [U-14C]dihydroneopterin triphosphate is the substrate; (2) Mg2+ is required for the synthesis of quench spot but either NADH or NADPH causes diminished incorporation of the label; (3) extracts from heads of a purple mutant (prbw cn) contain only 30% of the quench spot biosynthetic activity as compared to heads from wild type (Oregon-R); (4) quench spot is purified from heads of wild-type Drosophila; (5) addition of quench spot stimulates the biosynthesis of drosopterins in an enzyme preparation from Oregon-R.