Characterization of the lipid-binding properties and lipoprotein lipase inhibition of a novel apolipoprotein C-III variant Ala23Thr

We have identified a G-to-A transition in exon 3 of the APOC3 gene resultin g in a novel Ala23Thr apolipoprotein (apo) C-III variant, associated with a poC-III deficiency in three unrelated Yucatan Indians. The Ala23Thr substit ution modifies the hydrophobic/hydrophilic repartition of the helical N-ter minal peptide and hence could disturb the lipid association. In vitro expre ssion in Escherichia coli of wild-type and mutant apoC-III enabled the char acterization of the variant. Compared with wild-type apoC-III-Ala23, the mu tant apoC-III-Thr23 showed reduced affinity for dimyristoylphosphatidylchol ine (DMPC) multilamellar vesicles with higher amounts of free apoC-III, Dis placement of apoE from discoidal apoE:dipalmitoylphosphatidycholine (DPPC) complex by apoC-III-Thr23 was comparable to wild type but the less efficien t binding of the apoC-III-Thr23 to the discoidal complex resulted in a high er apoE/apoC-III (mol/mol) ratio (34%) than with wild-type/apoE:DPPC mixtur es. The inhibition of lipoprotein lipase (LPL) by apoC-III-Thr23 nas compar able to that of wild type, and therefore effects on LPL activity could not explain the lower triglyceride (Tg) levels in Thr-23 carriers. Thus, these in vitro results suggest that in vivo the less efficient Lipid binding of a poC-III-Thr23 might lead to a faster catabolism of free apoC-III, reflected in the reduced plasma apoC-III level identified in Thr-23 carriers, and po orer competition with apoE, which might enhance clearance of Tg-rich Lipopr oteins and lower plasma Tg levels seen in Thr-23 carriers.