High density lipoprotein-3 heterogeneity in subjects with the apo-AIMilano variant.

The structure of high density lipoproteins (HDL) isolated from subjects with the AIMilano (AIM) apoprotein variant was studied by the use of the cross-linking reagent dimethylsuberimidate. The HDL2 subfraction is markedly reduced, as compared to control subjects; the HDL3 subfraction, on the other hand, shows a marked heterogeneity, being characterized by at least three particle subpopulations, identified as HDL3-I, HDL3-II, and HDL3-III. The HDL3 fraction purified from the AI Milano subjects eluted as a symmetrical peak from a 6% agarose column, corresponding to a unimodal particle size distribution. The content of the different HDL3 particles, detected by cross-linking, varied widely along the elution profile, the tail of the peak being enriched in anomalous particles of very small size (HDL3-III). Apoprotein compositional studies indicated that these small HDL3-III may be enriched in the AIM monomer, the larger particles containing more AIM-AII complexes and AIM dimers. All the anomalous HDL3 particles are triglyceride enriched, with a decreased cholesterol ester content. They may be an intermediate product in the cholesterol transfer chain between HDL and very low density lipoproteins, or be generated during interconversion of HDL. These particles may have a functional role in tissue cholesterol homeostasis; their unusual compositional changes may help explain the protection of the studied subjects from tissue cholesterol deposition, in spite of the marked decrease of the total HDL fraction.