Evidence for the stabilization of NADPH relative to NADP+ on the dIII components of proton-translocating transhydrogenases from Homo sapiens and from Rhodospirillum rubrum by measurement of tryptophan fluorescence

A unique Trp residue in the recombinant dIII component of transhydrogenase from human heart mitochondria (hsdIII), and an equivalent Trp engineered into the dIII component of Rhodospirillum rubrum transhydrogenase (rrdIII.D155W), are more fluorescent when NADP(+) is bound to the proteins, than when NADPH is bound. We have used this to determine the occupancy of the binding site during transhydrogenation reactions catalysed by mixtures of recombinant dI from the R rubrum enzyme and either hsdIII or rrdIII.D155W. The standard redox potential of NADP(+)/NADPH bound to the dIII proteins is some 60-70 mV higher than that in free solution. This results in favoured reduction of NADP(+) by NADH at the catalytic site, and supports the view that changes in affinity at the nucleotide-binding site of dIII are central to the mechanism by which transhydrogenase is coupled to proton translocation across the membrane. (C) 1999 Elsevier Science B.V. All rights reserved.