Abstract
Pteridine biosynthesis has been examined in extracts of the heads of Drosophila melanogaster by measuring the conversion of dihydroneopterin triphosphate to sepiapterin and the "drosopterins" (six eye pigments that are dipterin derivatives). These two products share a common first step in the production of an intermediate that is a branch point from which both products are formed. This first step can be catalyzed by sepiapterin synthase or by an enzyme found in particles that sediment at 600g. A substance named "quench spot" was found earlier to be at low levels in the purple mutants that were defective in drosopterin synthesis and to be 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. In this report we propose that quench spot is a precursor of drosopterins, but not sepiapterin, and that it is formed from the sepiapterin synthase intermediate mentioned above. An additional precursor that is formed independently of the sepiapterin synthase pathway is also proposed that would react 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 (prbwcn) contain only 30% of the quench spot biosynthetic activity as compared to heads from wild type (Oregon-R); (4) quench spot has been purified from heads of wild-type Drosophila; (5) addition of quench spot stimulates the biosynthesis of drosopterins in an enzyme preparation from Oregon-R.
