Expression Analysis of the Aldo-Keto Reductases Involved in the Novel Biosynthetic Pathway of Tetrahydrobiopterin in Human and Mouse Tissues

Tetrahydrobiopterin (BH4) acts as a cofactor of the aromatic amino-acid hydroxylases, and its deficiency may result in hyperphenylalaninemia (HPA) and decreased production of the neurotransmitters. BH4 is synthesized by sepiapterin reductase (SPR) from 6-pyruvoyl-tetrahydropterin (PPH4). A patient with SPR deficiency shows no HPA; however, an SPR knockout mouse exhibits HPA. We have reported on the SPR-unrelated novel biosynthetic pathway from PPH4 to BH4 (salvage pathway II) in which 3 alpha-hydroxysteroid dehydrogenase type 2 and aldose reductase work in concert. In this study, we performed the expression analysis of both proteins in humans and wild-type mice. The results of expression analysis indicated that salvage pathway II worked in human liver; however, it did not act in human brain or in mouse liver and brain. For this reason, a patient with SPR deficiency may show progressive neurological deterioration without HPA, and SPR knockout mice may exhibit HPA and abnormal locomotion activity.