Relation between the HDL apoproteins and AI isoproteins in subjects with the AIMilano abnormality

The apoprotein AIMilano (AIM) is the first described molecular abnormality of human apolipoproteins. In order to achieve a better understanding of the biochemical role of high density lipoproteins (HDL) and of their subcomponents, nine members of the AIM family were studied. Five showed the characteristic biochemical features of the AIM abnormality. All were hypertriglyceridemic; their plasma and very low density lipoprotein (VLDL) triglycerides had a significant negative correlation with the HDL cholesterol (HDL-C) levels (r = −0.961, p < 0.01 and r = −0.939, p < 0.05, respectively). Compositional studies of HDL in these subjects revealed a marked triglyceride (TG) enrichment, with reduced cholesterol content. HDL-C and/or phospholipid (PL) levels correlated significantly with the AIM apoprotein in HDL (r = 0.995, p < 0.001 for HDL-C and r = 0.994, p < 0.001 for HDL-PL) but not with the other HDL apoproteins, suggesting that only apo AIM in monomeric form, can bind lipids. Isoelectric focusing studies on the isolated AIM isoproteins from the five affected subjects revealed an isoprotein distribution markedly different from normal AI. A possible structural role for isoprotein AI1 is suggested by the strong correlation between HDL content of this isoprotein and HDL-C and HDL-PL levels (r = 0.965 and r = 0.990 respectively, both p < 0.001). AI4 in AIM was nearly doubled, as compared to normal AI. The reported results may be of help for a better understanding of the role of the major HDL apoprotein and of its isoproteins in lipid binding and lipoprotein metabolism.