Free and Apo B-associated Lpa-specific protein in human serum.

The distribution of Lpa-specific protein (Apo A) among various fractions of human serum was investigated. The following results were obtained. In very low density lipoproteins (VLDL) of fasting human serum Apo A cannot be detected. Most of Apo A is complexed to LDL forming lipoprotein a (Lpa) present in the density fraction of 1.006-1.125 g/ml. Only approximately 5% of Apo A are found in the d greater than 1.125 bottom fraction after ultracentrifugation. Phosphotungstate-Mg (PTA), heparin-Mn (Hep) and dextran sulfate-Mg (DS) precipitate approximately 95% of serum Lpa together with the Apo B containing lipoproteins. Also monospecific antibodies against Apo B do not precipitate all Lpa and leave approximately 3% of Apo A in the supernatant. With polyethylene glycol (PEG) on the other hand, virtually all Lpa is removed since this reagent also precipitates Lpa and Apo A. Apo A of the bottom fraction after ultracentrifugation and of the supernatant after selective precipitation is not stainable with Sudan black and probably represents a protein which is not associated with lipids.