Tetrahydrobiopterin rapidly reduces the SOD mimic Mn(III) ortho-tetrakis(N-ethylpyridinium-2-yl)porphyrin

Mn(III) ortho-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnIIITE-2-PyP5+) effectively scavenges reactive oxygen and nitrogen species in vitro, and protects in vivo, in different rodent models of oxidative stress injuries. Further, MnIIITE-2-PyP5+ was shown to be readily reduced by cellular reductants such as ascorbic acid and glutathione. We now show that tetrahydrobiopterin (BH4) is also able to reduce the metal center. Under anaerobic conditions, in phosphate-buffered saline (pH 7.4) at 25 ± 0.1°C, reduction of MnIIITE-2-PyP5+ occurs through two reaction steps with rate constants k1 = 1.0 × 104 M−1 s−1 and k2 = 1.5 × 103 M−1 s−1. We ascribe these steps to the formation of tetrahydrobiopterin radical (BH4 +) (k1) that then undergoes oxidation to 6,7-dihydro-8H-biopterin (k2), which upon rearrangement gives rise to 7,8-dihydrobiopterin (7,8-BH2). Under aerobic conditions, MnIIITE-2-PyP5+ catalytically oxidizes BH4. This is also true for its longer chain alkyl analog, Mn(III) ortho-tetrakis(N-n-octylpyridinium-2-yl)porphyrin. The reduced Mn(II) porphyrin cannot be oxidized by 7,8-BH2 or by l-sepiapterin. The data are discussed with regard to the possible impact of the interaction of MnIIITE-2-PyP5+ with BH4 on endothelial cell proliferation and hence on tumor antiangiogenesis via inhibition of nitric oxide synthase.