Redox hysteresis controls the NADH-dependent reduction of cytochrome b5 in rat microsomes

In enzymology, hysteresis is manifested as a time-dependent shift in the kinetic behavior of an enzyme. Through hysteresis, the activation or inhibition of a biological pathway can be regulated by a molecule or metabolite that acts as a hysteretic modulator of the enzyme within that metabolic route. This mechanism of regulation contrasts with those that act on gene expression leading to modulation of enzyme protein levels. Through hysteresis, the amplitude of natural oscillations in metabolic pathways can be adjusted according to the levels of a metabolite that might be beneficial for cells. At physiological level, the slow response of hysteretic enzymes to changes, in the cellular levels of substrates, allows a time-dependent buffering effect on certain metabolites. Understanding the mechanisms and properties of hysteretic enzymes has been important for developing new therapies and improving our understanding of these enzymes in biological systems. However, due to their complex kinetics, the study of hysteretic enzymes has remained a challenge over time. In this study, we characterized the reduction of cytochrome b5 by NADH-dependent microsomal enzymes from rat liver using recombinant purified cytochrome b5, coenzyme Q10 and coenzyme Q0, as substrates, to mimic the conditions found in biological membranes, where competition between cytochrome b5 and other substrates might influence their reduction. We found a lag-time-dependent behavior in the cytochrome b5 reduction compatible with the existence of hysteretic modulation induced by endogenous molecules present in these membranes. Our data suggest that at least for the case of coenzyme Q10, fluctuations in its levels may impact metabolic pathways in which reduced cytochrome b5 levels play a key for the function of the cytochrome b5-dependent route.