Reduction of ferric haemoproteins by tetrahydropterins: a kinetic study

We previously showed that one-electron transfer from tetrahydropterins to iron porphyrins is a very general reaction, with formation of an intermediate cation radical similar to the one detected in NO synthase. As a model reaction, the rates of reduction of eight haemoproteins by diMePH(4) (6,7-dimethyl-tetrahydropterin) have been studied and correlated with their one-electron reduction potentials, E-m (Fe-III/Fe-II). On the basis of kinetic data analyses, a bimolecular collisional mechanism is proposed for the electron transfer from diMePH(4) to ferrihaemoproteins. Haemoproteins with reduction potentials below -160 mV were shown not to be reduced by diMePH(4) to the corresponding ferrohaemoproteins. For haemoproteins with reduction potentials more positive than -160 mV, such as chloroperoxidase, cytochrome b(5), methaemoglobin and cytochrome c, there was a good correlation between the second-order reduction rate constant and the redox potential, E-m (Fe-III/Fe-II): log [rate (M-1 center dot s(-1))] = -0.49 + (0.014 x E-m) The rate of reduction of cytochrome c by BH4 [(6R)-5,6,7,8 -tetrahydrobiopterin] was determined to be similar to that of the reduction of cytochrome c by diMePH(4). These results confirm the role of tetrahydropterins as one-electron donors to Fe-III porphyrins.