Influence of dietary vitamin E on the intermembrane transfer of alpha-tocopherol as mediated by an alpha-tocopherol binding protein.

The effect of dietary vitamin E on the intermembrane transfer of (3R)-alpha-tocopherol, a spontaneous process accelerated in the presence of an alpha-tocopherol binding protein (alpha TBP), was examined. The transfer activity of this cytosolic liver protein was assayed via in vitro transfer of (3R)-alpha-[3H]tocopherol (alpha[3H]T) from egg lecithin liposomes to human erythrocyte ghosts (EG). Male Fisher 344 rats (1 and 20 months old) were fed diets containing 0, 30, and 500 mg/kg vitamin E (dl-alpha-tocopheryl acetate) for 15 weeks. Liver cytosol fractions were assayed for alpha[3H]T transfer activity (alpha TTA). Among young rats, those fed vitamin E-deficient diets had the highest alpha TTA, 5.02 +/- 3.10 pmole alpha[3H]T/min (mean +/- SD), which was different (P less than 0.05) from the spontaneous transfer rate of 2.10 pmole/min. Neither young rats fed 30 and 500 mg/kg vitamin E diets nor any of the aged rats showed alpha TTA which differed significantly from the spontaneous transfer rate. To examine the relationship between hepatic alpha-tocopherol levels and alpha TTA, alpha-tocopherol concentration per gram of wet liver was assayed by HPLC. A steep positive slope (6.39 +/- 1.46 pmole min-1 nmole g-1) and strong correlation (r = 0.873) between hepatic alpha-tocopherol and alpha TTA were observed (P less than 0.005) among young vitamin E-deficient rats. The data indicates that alpha TTA varies directly with hepatic alpha-tocopherol concentration when total liver vitamin E stores are very low. Thus, alpha TBP-mediated transfer of alpha-tocopherol may be manifest only when vitamin E status is compromised.