Extracellular coenzyme Q10 (CoQ10) is reduced to ubiquinol-10 by intact Hep G2 cells independent of intracellular CoQ10 reduction.

Coenzyme Q10 (CoQ10) is an essential factor in the mitochondrial respiratory chain and is closely associated with ATP production in humans. It is known that orally administered CoQ10 in humans is rapidly reduced, and most is detected as a reduced form, ubiquinol-10 (CoQ10H2), in serum. However, the mechanism of exogenous CoQ10 reduction in vivo is unclear. Therefore, in order to clarify how CoQ10 is reduced to CoQ10H2, we conducted a study using human liver cancer cell line Hep G2 cells, which show strong intracellular CoQ10-reducing activity. When intact cells were incubated with CoQ10, the exogenously added CoQ10 was incorporated into the cells, time-, concentration-, and temperature-dependently, and 50-80% of that was detected as CoQ10H2. On the other hand, a part of the extracellular CoQ10 was also detected as CoQ10H2, and the amount was greater than that of the intracellular CoQ10H2. Furthermore, the CoQ10-loaded cells did not leak the intracellular CoQ10H2 (or CoQ10) to the outside of the cells, and modulation of the extracellular CoQ10H2 amount had little effect on the intracellular CoQ10 or CoQ10H2 contents, suggesting the existence of an individual mechanism of CoQ10 reduction inside and outside the cells. Moreover, intact cells could reduce CoQ10 in low-density lipoprotein to CoQ10H2. Therefore, we concluded that a novel CoQ10-reducing mechanism may exist in the plasma membrane, probably the outer surface, of Hep G2 cells, and it may work to reduce extracellular CoQ10 and/or maintain extracellular CoQ10H2.