In vitro studies of the interaction of calcium ions and other divalent cations with the Lp(a) lipoprotein and other isolated serum lipoproteins

The interaction of isolated Lp(a) lipoprotein with different divalent cations was studied and compared to that of other isolated lipoprotein classes. Purified Lp(a) lipoprotein was found to be most sensitive to the metal ions tested, and the Lp(a) lipoprotein was the only lipoprotein which was precipitated by calcium ions alone. The precipitation apparently depends on the ionic radii of the cations used as well as on the lipoprotein class tested. The precipitation reaction between calcium ions and the Lp(a) lipoprotein, and the interaction between calcium ions and LDL (without precipitation) seem to follow the known rules for small ion-macromolecule interaction reasonably well. The calcium ion - Lp(a) lipoprotein interaction results in a small aggregate. The binding is of ionic type and the precipitation reaction is initially reversible. It was estimated that LDL particles have a mean of 290 equivalent and non-interacting binding sites for calcium ions. The above observations concerning the Lp(a) lipoprotein may be of interest in view of the significantly higher frequency of early coronary heart disease in Lp(a+) than in Lp(a-) individuals, and in view of the previously reported biochemical differences between individuals of different Lp phenotype.