Occurrence of a New Tetrahydrobiopterin Synthesizing enzyme in the human being

四氢生物蝶呤在某些神经递质的生物合成中起重要作用。采用DEAE-Sepharose FF柱层析法，将家蚕柠檬突变体中由6-异戊酰-四氢蝶呤合成四氢生物蝶呤的酶（不同于Sepiapterin还原酶（EC 1.1.1.153））分离成两个组分，分别命名为羰基还原酶I（CR I）和羰基还原酶II（CR II）。CR I酶将6-异戊酰-四氢蝶呤转化为6-乳酰-四氢蝶呤，而CR II将6-异戊酰-四氢蝶呤转化为1 ′-羟基-2 ′-氧代丙基-四氢蝶呤，这两个反应都只在NADPH存在下发生，单独的两种羰基还原酶都不能催化6-异戊酰-然而，当CR I与CR II在反应混合物中混合时，6-乙酰基-四氢蝶呤在NADPH存在下被还原为四氢生物蝶呤。CR Ⅰ催化1 ′-羟基-2 ′-氧代丙基四氢蝶呤生成四氢生物蝶呤，CR Ⅱ催化6-乳酰四氢蝶呤生成四氢生物蝶呤，两个反应都只在NADPH存在下发生，我们的结果表明，在柠檬突变家蚕中，6-乳酰四氢蝶呤生成四氢生物蝶呤有两条可能的途径。第一条路线是由6-乙酰基四氢蝶呤经CR Ⅱ还原成1 '-羟基-2'-氧丙基四氢蝶呤，然后再经CR Ⅰ还原成四氢生物蝶呤，这两个反应都是在NADPH存在下进行的;第二条路线是由6-乙酰基四氢蝶呤经CR Ⅰ还原成6-乳酰四氢蝶呤，然后再经CR Ⅱ还原成四氢生物蝶呤，这两个反应都是在NADPH存在下进行的。

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 occurred 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 froml'-hydroxy-2'-oxopropyl-tetrahydropterin, while CR II converted 6-lactoyl-tetrahydropterin to tetrahydrobiopterin, both reactions occurred only in thepresence 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 occurred 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 occurred only in the presence of NADPH.