Interaction of apoHDL with HDL and with other lipoproteins

Abstract

Ultracentrifugation at a density of 1.21 of mixtures of apoHDL and HDL in which one component was labeled with 125I showed that interchange of labeled protein took place. Since no evidence of intermediates was obtained it seems more probable that these results are explained by interchange of the relatively small protein subunits (mol.wt. approx. 15,000) between apoHDL and the large HDL complex (mol.wt. approx. 200,000) than that lipids move from HDL to apoHDL, as has been previously suggested. The top fraction obtained by centrifuging a mixture of [ 125I]apoHDL and HDL at d 1.21 lost less than 20% of the labeled protein when it was recentrifuged at the same density. The fraction of [ 125I]apoHDL which floated at d 1.21 in the presence of HDL was larger than the fraction of [ 125I]HDL which sedimented to the bottom in the presence of apoHDL, indicating the occurrence of some complex formation between HDL and [ 125I]apoHDL in addition to the interchange of labeled protein subunits. Similar results were obtained on electrophoresis; [ 125I]HDL after incubation with apoHDL gave zones corresponding to HDL and to apoHDL both of which contained 125I. Complex formation between [ 125I]apoHDL and LDL was demonstrated by ultracentrifugation of a mixture. A small fraction of the labeled protein floated at d 1.063 but on recentrifugation half of the labeled protein sedimented. When trace amounts of [ 125I]apoHDL were mixed with serum and the lipoprotein fractions separated by ultracentrifugation, 60% of the label was recovered in the HDL fraction and only 1.5% in the LDL fraction.