Isolation of apolipoprotein(a) from lipoprotein(a).

An easy method was developed for the rapid and selective isolation of apo(a) from human plasma Lp(a). This procedure was applied to a "low density" Lp(a) subspecies (usually found in the density interval 1.050 to 1.070 g/ml) from a single individual whose apo(a) was of a size smaller than apoB-100. After reduction with 0.01 M dithiothreitol, apo(a) was separated from the Lp(a) particle by rate zonal centrifugation on a 7.5-30% NaBr density gradient. Two completely water-soluble products were recovered: apo(a), which contained less than 1% each of phospholipid and cholesterol, remained at the bottom of the gradient, and a lipid-rich floating LDL-like particle which contained apoB but not apo(a) and which we referred to as Lp(a-). The separation of these two components was also achieved by subjecting reduced Lp(a) to electrophoresis on 2.5-16% polyacrylamide gradient gels. However, dissociation of reduced Lp(a) could not be achieved by gel filtration in either low or high salt solutions. These observations indicate that apo(a) is associated to Lp(a) by non-covalent interactions in addition to its disulfide linkage to apoB. The latter is sensitive to chemical reduction whereas the former are broken through the action of a gravitational or electrical field.