APOLIPOPROTEIN-(A) - A COMPARISON OF ISOFORMS IDENTIFIED BY SODIUM DODECYL SULFATE-POLYACRYLAMIDE GEL-ELECTROPHORESIS OR BY SODIUM DODECYL SULFATE-AGAROSE GEL-ELECTROPHORESIS

Lipoprotein(a) resembles low density lipoprotein in structure, except that a unique apolipoprotein (apo), apo(a), is linked to apo B-100. Variations in the number of sequence repeats in the apo(a) gene give rise to a range of isoforms. Depending on the method used, 6-30 apo(a) isoforms have been observed; however, the correspondence of these different isoforms has not been reported, making between-study comparisons difficult. In the present study we address this question by characterizing the apo(a) phenotypes of 48 sera using two previously reported separation methods, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE, 3-12% gels) and SDS-agarose gel electrophoresis. In addition, the molecular weight of each isoform was estimated using haptoglobin 2-2 polymers as molecular weight standards. Among the 48 sera, 15 distinct apo(a) isoforms were separated by SDS-PAGE and 28 by SDS-agarose gel electrophoresis. There was excellent correlation between the two nomenclature systems (r = -0.97, p < 0.001, by rank correlation), and the ranges were totally overlapping, with the same two isoforms being identified as the largest and smallest by either method. The apparent molecular mass range for the isoforms was 294-624 kDa, which is in close agreement with the theoretical molecular mass range of 238-643 kDa, calculated from the sequence and carbohydrate content of recombinant apo(a). The disparity in number of isoforms between methods was expected, due to the poorer separation of apo(a) by SDS-PAGE; 3.1 +/- 1.7 (median, 2.0) SDS-agarose isoforms were combined for each SDS-PAGE isoform. The present study demonstrates that the nomenclature systems for apo(a) isoforms separated by SDS-PAGE or by SDS-agarose gel electrophoresis are well correlated mathematically and encompass the same size range; however, the better resolution of SDS-agarose electrophoresis suggests that it is the method of choice for apo(a) phenotyping. As further apo(a) isoforms are identified, it will be important to address the question of a standardized nomenclature, in order to facilitate between-study comparisons.