MOLECULAR-WEIGHT DETERMINATION OF LIPOPROTEIN(A) [LP(A)] IN SOLUTIONSCONTAINING EITHER NABR OR D2O - RELEVANCE TO THE NUMBER OF APOLIPOPROTEIN(A) SUBUNITS IN LP(A)

Molecular weight determination of low-density lipoprotein (LDL) is usu ally performed in solutions containing high concentrations of salt (up to 13.4 M NaBr) by sedimentation velocity and diffusion experiments, because it does not preferentially bind salt or water. Considering tha t lipoprotein(a) [Lp(a)] is structurally similar to LDL, differing onl y by the presence of Apo(a), the molecular weight, M, of Lp(a) has als o been measured in solutions containing high concentrations of NaBr. W e questioned the suitability of this practice by comparing the apparen t molecular weight, M(app), and partial volume, phi', of Lp(a) determi ned by sedimentation and flotation equilibrium in a three-component sy stem containing NaBr with the analogous parameters, M and partial spec ific volume, <(nu)over bar>, determined in a two-component system cont aining D2O. LDL served as a control, In agreement with previous findin gs obtained with different methods, our results indicate no significan t differences in M and <(nu)over bar> of four different LDL samples an d apparently no significant preferential binding of solvent components . In contrast, values of M(app) and phi' of Lp(a) evaluated in NaBr ar e significantly greater than M and <(nu)over bar>. Preferential bindin g of solvent components appeared to be a function of Apo(a) mass or th e number of kringle IV domains, as expressed by increasing percentage differences between the two sets of parameters, ranging from 4 to 13% in M and 0.2 to 0.5% in <(nu)over bar> of Lp(a) species having Apo(a) with 15-27 kringle IV domains. Furthermore, our results indicate that the variable Apo(a) kringle IV domains are more involved in this proce ss than the constant domain of Apo(a). These findings indicate that th e Lp(a) molecular weight should be determined in D2O and that high con centrations of NaBr should be avoided as their use would lead to overe stimated molecular weights and partial specific volumes. Application o f this method to the question of how much Apo(a) is released upon the reduction of Lp(a) led to the conclusion that Lp(a) contains only one Apo(a) molecule.