In vivo conversion of human HDL3 to HDL2 and apoE-rich HDL1 in the rat: effects of lipid transfer protein.

In this study we determined in vivo conversions of human 3H-labeled cholesteryl ester-labeled HDL3 [( 3H]CE-HDL3) in male rats and the effects of partially purified lipid transfer protein on the conversion processes. Zonal centrifugation techniques were used to prepare the [3H]CE-HDL3 and to follow the conversion processes. One hour after the injection, a complete conversion of HDL3 to the HDL2-density species was found. With time, [3H]CE separated with apoE-rich HDL1 and, by 18 hr, 35.9% of plasma radioactivity was associated with the apoE-rich HDL1 lipoprotein fraction. In vitro incubation of [3H]CE-HDL3 in rat plasma reproduced in part the HDL3----HDL2 conversion, but no movement of radioactivity to HDL1 was observed. Injection of the rats with partially purified lipid transfer proteins induced [3H]CE exchange between lipoproteins. The conversion of HDL3 to HDL2, however, was minimally affected. Formation of [3H]CE-HDL1, in contrast, was reduced to about one-half of that found in control animals. It is concluded that in vivo conditions are necessary for conversions of HDL3 (and HDL2) to HDL1, and that lipid transfer reactions delay this process.