In vitro studies on the aggregation of isolated lipoprotein(a) upon addition of lysolecithin.

A precipitating effect of lysolecithin on isolated and purified lipoprotein (a) [Lp (a)] was detected and investigated in vitro. After addition of external lysolecithin at an initial concentration of 100 mu mol/l (which is about the concentration of lysolecithin produced per hour in human serum), the lipoprotein solution started to get turbid; turbidity was monitored at 450 nm. This effect was studied in dependence on the ionic strength of the buffer, revealing maximum precipitation in isotonic solutions. Limited trypsinisation or treatment of Lp(a) with neuraminidase changed the surface of that lipoprotein so that no precipitation could be caused by lysolecithin, indicating the role of the protein-carbohydrate part for the aggregation. A difference between L-alpha and DL-alpha lysolecithin could also be established. A weaker precipitating reaction was found for the latter. The presence of serum albumin at physiological serum concentrations weakened the precipitating power of lysolecithin to a certain degree, but could not stop the process. The aggregates were investigated by means of the electron microscope using negative staining for fixation. The individual Lp(a)-particles could still be distinguished but they stuck together in a bulky conformation. Low density lipoprotein (LDL) used as a reference did not show any precipitation by lysolecithin.