Tetrahydrobiopterin is synthesized from 6-pyruvoyl-tetrahydropterin by the human aldo-keto reductase AKR1 family members

Tetrahydrobiopterin (BH4) is a cofactor for aromatic amino acid hydroxylases and nitric oxide synthase. The biosynthesis includes two reduction steps catalyzed by sepiapterin reductase. An intermediate, 6-pyruvoyltetrahydropterin (PPH4) is reduced to 1′-oxo-2′-hydroxypropyl-tetrahydropterin (1′-OXPH4) or 1′-hydroxy-2′-oxopropyl-tetrahydropterin (2′-OXPH4), which is further converted to BH4. However, patients with sepiapterin reductase deficiency show normal urinary excretion of pterins without hyperphenylalaninemia, suggesting that other enzymes catalyze the two reduction steps. In this study, the reductase activities for the tetrahydropterin intermediates were examined using several human recombinant enzymes belonging to the aldo-keto reductase (AKR) family and short-chain dehydrogenase/reductase (SDR) family. In the reduction of PPH4 by AKR family enzymes, 2′-OXPH4 was formed by 3α-hydroxysteroid dehydrogenase type 2, whereas 1′-OXPH4 was produced by aldose reductase, aldehyde reductase, and 20α-hydroxysteroid dehydrogenase, and both 1′-OXPH4 and 2′-OXPH4 were detected as the major and minor products by 3α-hydroxysteroid dehydrogenases (types 1 and 3). The activities of aldose reductase and 3α-hydroxysteroid dehydrogenase type 2 (106 and 35 nmol/mg/min, respectively) were higher than those of the other enzymes (0.2–4.0 nmol/mg/min). Among the SDR family enzymes, monomeric carbonyl reductase exhibited low 1′-OXPH4-forming activity of 5.0 nmol/mg/min, but l-xylulose reductase and peroxisomal tetrameric carbonyl reductase did not form any reduced product from PPH4. Aldose reductase reduced 2′-OXPH4 to BH4, but the other enzymes were inactive towards both 2′-OXPH4 and 1′-OXPH4. These results indicate that the tetrahydropterin intermediates are natural substrates of the human AKR family enzymes and suggest a novel alternative pathway from PPH4 to BH4, in which 3α-hydroxysteroid dehydrogenase type 2 and aldose reductase work in concert.