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.
Iino
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Department of General Studies, Nihon University, Sakurajosui 3-25-40, Setagayaku, Tokyo, 156, Japan. iino@iis.chs.nihon-u.ac.jp
Takikawa
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SI
Yamamoto
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Sawada
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D000429:Alcohol Oxidoreductases / Q000302:isolation & purification / Q000378:metabolism*; D000818:Animals; D001708:Biopterins / Q000031:analogs & derivatives* / Q000096:biosynthesis; D012831:Bombyx / Q000201:enzymology*; D019476:Insect Proteins / Q000302:isolation & purification; D015394:Molecular Structure; D011622:Pterins / Q000378:metabolism*
D019476:Insect Proteins; D011622:Pterins; D001708:Biopterins; C045652:dyspropterin; D000429:Alcohol Oxidoreductases; C003402:sapropterin