Sepiapterin reductase mediates chemical redox cycling in lung epithelial cells

TitleSepiapterin reductase mediates chemical redox cycling in lung epithelial cells
Publication TypeJournal Article
Year of Publication2013
AuthorsYang S., Jan Y.H, Gray J.P, Mishin V., Heck D.E, Laskin D.L, Laskin J.D
JournalJ Biol ChemJ Biol Chem
Volume288
Pagination19221-37
Date PublishedJun 28
ISBN Number1083-351X (Electronic)<br/>0021-9258 (Linking)
Accession Number23640889
KeywordsAlcohol Oxidoreductases/*chemistry, Animals, Biopterin/analogs & derivatives/chemistry, Cell Line, Tumor, Dose-Response Relationship, Drug, Enzyme Inhibitors/pharmacology, Epithelial Cells/*cytology, Genetic Vectors, Humans, Lung/*cytology, Mice, Models, Chemical, Models, Molecular, Mutagenesis, Site-Directed, Oxidation-Reduction, Oxidative Stress, Quinones/chemistry, Reactive Oxygen Species, Recombinant Proteins/chemistry
Abstract

In the lung, chemical redox cycling generates highly toxic reactive oxygen species that can cause alveolar inflammation and damage to the epithelium, as well as fibrosis. In this study, we identified a cytosolic NADPH-dependent redox cycling activity in mouse lung epithelial cells as sepiapterin reductase (SPR), an enzyme important for the biosynthesis of tetrahydrobiopterin. Human SPR was cloned and characterized. In addition to reducing sepiapterin, SPR mediated chemical redox cycling of bipyridinium herbicides and various quinones; this activity was greatest for 1,2-naphthoquinone followed by 9,10-phenanthrenequinone, 1,4-naphthoquinone, menadione, and 2,3-dimethyl-1,4-naphthoquinone. Whereas redox cycling chemicals inhibited sepiapterin reduction, sepiapterin had no effect on redox cycling. Additionally, inhibitors such as dicoumarol, N-acetylserotonin, and indomethacin blocked sepiapterin reduction, with no effect on redox cycling. Non-redox cycling quinones, including benzoquinone and phenylquinone, were competitive inhibitors of sepiapterin reduction but noncompetitive redox cycling inhibitors. Site-directed mutagenesis of the SPR C-terminal substrate-binding site (D257H) completely inhibited sepiapterin reduction but had minimal effects on redox cycling. These data indicate that SPR-mediated reduction of sepiapterin and redox cycling occur by distinct mechanisms. The identification of SPR as a key enzyme mediating chemical redox cycling suggests that it may be important in generating cytotoxic reactive oxygen species in the lung. This activity, together with inhibition of sepiapterin reduction by redox-active chemicals and consequent deficiencies in tetrahydrobiopterin, may contribute to tissue injury.