Journal of Lipid Research, Vol 16, 39-53, Copyright © 1975 by Lipid Research, Inc.
Comparison of exchange of alpha-tocopherol and free cholesterol between rat plasma lipoproteins and erythrocytes
LK Bjornson, C Gniewkowski and HJ Kayden
The simultaneous exchange of (3h)tocopherol and (14C)cholesterol between
rat plasma, rat plasma lipoproteins, and RBC was studied in vitro to
compare quantitavely (a) the fractional exchange rates and (b) the
half-times for isotope equilibration. In all incubations of RBC with plasma
or with plasma lipoprotein fractions, (14C)cholesterol approached
equilibrium more rapidly than (3H)tocopherol. When the RBC contained the
initial radioactivity, the half-times for equilibration with plasma of
cholesterol and of tocopherol were 1.0 and 2.2 hr, respectively. However,
the fractional exchange rates (KRBC leads to plasma) were 0.097/hr for
cholesterol and 0.188/hr for tocopherol, indicating that the RBC tocopherol
pool is turning over almost twice as rapidly as the RBC cholesterol pool.
The rat plasma lipoproteins were separated into five fractions by
successive ultracentrifugation. Only two fractions, the high density
lipoproteins (d 1.063-1.21) and the very low density lipoproteins (d is
less than 1.006), participated to a significant extent in the exchange of
either tocopherol or cholesterol with RBC. Cholesterol exchange between
individual rat plasma lipoproteins and RBC had the same half-times for
isotope equilibrium for the very low and high density lipoproteins, and the
RBC fractional exchange rates were proportional to the amount of
cholesterol in the lipoproteins. In tocopherol exchange between individual
rat plasma lipoproteins and RBC, the very low density lipoprotein
tocopherol did not equilibrate completely with the RBC. However, the
initial rate of tocopherol exchange appeared to be the same for very low
and high density lipoproteins. The very low density lipoproteins were
disrupted by repeated freezing and thawing or by dehydrating and
rehydrating, and analysis of the resulting lipoproteins indicated that free
cholesterol was associated more closely than tocopherol with the
phospholipid- protein portion of the molecule, which is thought to be on
the surface. This difference in distribution of tocopherol and free
cholesterol within very low density lipoproteins could account for their
different rates of exchange and for the nonequilibrium of tocopherol
between RBC and very low density lipoproteins.
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