Biosynthesis of "drosopterins" by an enzyme system from Drosophila melanogaster

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Abstract
The red eye pigment of D. melanogaster consists of 6 complex pteridines known as neodrosopterin, drosopterin, isodrosopterin, fraction e and aurodrosopterins (2); these pigments are greatly reduced in the purple (pr) mutant. Conditions for biosynthesis of these drosopterins are described and compared with those for the synthesis of sepiapterin. The enzymes are contained in a soluble, pteridine-free extract obtained between 40-60% saturated ammonium sulfate. Sepiapterin synthase consists of 2 enzymes, the 1st of which provides a precursor for drosopterin biosynthesis. The pr mutant, low in accumulated sepiapterin and drosopterins, about 10% of the sepiapterin synthase activity of wild type. The unlabeled sepiapterin does not cause isotope dilution of drosopterin synthesis. The 600 g pellet prepared from a wild-type head homogenate contains drosopterin synthesizing activity and no sepiapterin synthase, yet a heat-labile factor in this fraction stimulates sepiapterin synthesis in the 100,000 g supernatant of wild-type or pr flies. Sepiapterin and drosopterin syntheses require Mg2+. Sepiapterin synthesis is stimulated by NADPH [drosopterin synthesis responds to either NADPH or NADH]. Although drosopterins are complex pteridine-type pigments, biosynthesis by soluble enzymes was demonstrated. The mechanism by which the amounts of these pigments are regulated can now be studied.