Inhibition of Sepiapterin Mediated Formation of Dihydrobiopterin and Chemical Redox Cycling by Sulfa Drugs

Abstract

Sepiapterin reductase (SPR) catalyzes the reduction of sepiapterin to dihydrobiopterin (BH2), a precursor for tetrahydrobiopterin (BH4), a cofactor critical for nitric oxide biosynthesis and alkylglycerol and aromatic amino acid metabolism. SPR also mediates chemical redox cycling, catalyzing one electron reduction of redox active chemicals including quinones; rapid reaction of these radicals with molecular oxygen generates reactive oxygen species (ROS). Using recombinant human SPR, sulfonamide and sulfonylurea based sulfa drugs were found to be potent non‐competitive inibitors of both sepiapterin reduction and redox cycling. The most active inhibitors of sepiapterin reduction (IC50's = 31‐180 nM) were sulfasalazine, sulfathiazole, sulfapyridine, sulfamethoxazole and chlorpropamide. Higher concentrations of the sulfa drugs (IC50's = 0.37‐19.4 µM) were required to inhibit redox cycling. In PC12 cells, which generate catecholamine and monoamine neurotransmitters via BH4‐dependent amino acid hydroxylases, sulfa drugs inhibited both BH2/BH4 biosynthesis and redox cycling. This resulted in decreased production of dopamine and dopamine metabolites, 3, 4‐dihydroxyphenylacetic acid and homovanillic acid, and 5‐hydroxytryptamine. This was reversed by BH4. These data suggest that SPR and BH4‐dependent enzymes, are ‘off‐targets' for sulfa drugs, which may underlie their side effects. AR055073, ES005022.