Comparative uptake of alpha- and gamma-tocopherol by human endothelial cells

The intake of gamma-tocopherol by North Americans is generally higher than that of alpha-tocopherol. However, the levels of alpha-tocopherol in human blood have consistently been shown to be higher than those of gamma-tocopherol suggesting differential cellular retention of the two tocopherol forms. We sought to resolve this question by studying tocopherol metabolism by human endothelial cells in culture. The time- and dose-dependent uptake of gamma-tocopherol by endothelial cells was similar to that of alpha-tocopherol. To determine the comparative uptake between alpha- and gamma-tocopherol, we adopted two approaches in which cells were enriched with either increasing concentrations of an equimolar mixture of alpha- and gamma-tocopherol; or cells were enriched with a fixed concentration of tocopherols in which the alpha to gamma ratio was varied. Our results indicated that there was a preferential uptake of gamma-tocopherol by the cells. When cells were enriched with either alpha- or gamma-tocopherol and the disappearance of individual tocopherols was monitored over time, gamma-tocopherol exhibited a faster rate of disappearance. The faster turnover of gamma-tocopherol can explain the discrepancy between high intake and low retention of gamma-tocopherol in man.