The Enzyme That Synthesizes Tetrahydrobiopterin from 6-Pyruvoyl-tetrahydropterin in the lemon Mutant Silkworm Consists of Two Carbonyl Reductases

Tetrahydrobiopterin plays an important role in the biosynthesis of certain neurotransmitters. Using DEAE–Sepharose FF column chromatography, we separated the enzyme that synthesizes tetrahydrobiopterin from 6-pyruvoyl-tetrahydropterin [which is different from sepiapterin reductase (EC 1.1.1.153)] in the lemon mutant of the silkworm Bombyx mori into two fractions, which were named carbonyl reductase I (CR I) and carbonyl reductase II (CR II). The CR I enzyme converted 6-pyruvoyl-tetrahydropterin to 6-lactoyl-tetrahydropterin, while CR II converted 6-pyruvoyl-tetrahydropterin to 1′-hydroxy-2′-oxopropyl-tetrahydropterin, both reactions occurring only in the presence of NADPH. Neither of the two carbonyl reductases alone was able to catalyze the conversion of 6-pyruvoyl-tetrahydropterin to tetrahydrobiopterin in the presence of NADPH. However, when CR I was mixed with CR II in the reaction mixture, 6-pyruvoyl-tetrahydropterin was reduced to tetrahydrobiopterin in the presence of NADPH. Moreover, CR I catalyzed the formation of tetrahydrobiopterin from 1′-hydroxy-2′-oxopropyl-tetrahydropterin, while CR II converted 6-lactoyl-tetrahydropterin to tetrahydrobiopterin, both reactions occurring only in the presence of NADPH. Our results suggest that there are two potential routes for formation of tetrahydrobiopterin from 6-pyruvoyl-tetrahydropterin in the lemon mutant silkworm. In the first route, 1′-hydroxy-2′-oxopropyl-tetrahydropterin is formed from 6-pyruvoyl-tetrahydropterin by CR II and then reduced to tetrahydrobiopterin by CR I, both reactions occurring only in the presence of NADPH. In the other route, 6-pyruvoyl-tetrahydropterin is reduced to 6-lactoyl-tetrahydropterin by CR I and then converted to tetrahydrobiopterin by CR II, both reactions occurring only in the presence of NADPH.