CoQ0 hysteresis and inhibition of the cytochrome b5 reductase activity of human Cb5R by CoQ0 and CoQ10.

BACKGROUND

In this study, the effect of ubiquinone (CoQ10) and its soluble analogue (CoQ0) were measured on the NADH:Cb5 reductase activity of recombinant human membrane and soluble Cb5R, respectively using soluble Cb5 as a substrate. The aim of this study was to determine whether these quinones are inhibitors and can induce a hysteretic modulation of the activity, similar to that described for the reduction of Cb5 in microsomal systems.

RESULTS

CoQ0 and CoQ10 inhibit the NADH:cytochrome b5 reductase activity of soluble and membrane Cb5R respectively, using soluble Cb5 as an electron acceptor. However, only CoQ0 induced hysteresis of the soluble Cb5R isoform. CoQ0 induced the appearance of a lag phase, whose duration increased with the concentration of CoQ0 and decreased by increasing the concentration of soluble Cb5 or Cb5R. Additionally, a concentration-dependent decrease in the maximum rate of Cb5 reduction and the appearance of positive cooperativity was observed in the presence of CoQ0 which resulted in lower KM values for Cb5. This suggests the formation of a CoQ0:Cb5R complex which increases the affinity for Cb5. Cyclic voltammetry data support the formation of CoQ0/protein complex that could be responsible for the hysteretic behavior.

CONCLUSIONS

Both quinones inhibit the NADH: cytochrome b5 reductase activity of human Cb5R isoforms, but only CoQ0 triggers hysteresis in the soluble Cb5R isoform. Our data support the use of fully soluble components for studying hysteresis in this system, as they recapitulate key features of the behavior observed in biological membranes.