COMMON SEQUENCE VARIANTS OF LIPOPROTEIN-LIPASE - STANDARDIZED STUDIESOF IN-VITRO EXPRESSION AND CATALYTIC FUNCTION

We have assessed the functional activity of three common sequence vari ants of human lipoprotein lipase (LPL). Two of these, Asn291Ser and As p9Asn arise from missense mutations while the third, Ser447Ter, derive s from a nonsense mutation, truncating LPL by two residues. As previou s in vitro studies have produced conflicting results, we have re-analy zed the catalytic function of these variants using the COS cell transf ection system, under optimized and standardized experimental protocols . We found the Asn291Ser variant to manifest with a decrease in cataly tic activity (57% of normal) due to a reduction in secretion and stabi lity of the active homodimeric form. The Asp9Asn variant also showed a significant decrease in catalytic activity (85% of normal), but this was found to be due to a decreased rate of secretion only, as the homo dimeric form was stable. The findings for these mutants contrasted wit h those of the Ser447Ter truncation variant which proved to be catalyt ically normal; this variant also manifested normal homodimer stability . The truncated variant did however, present with a higher total secre ted mass level (131%) than control LPL, This was most likely due to en hanced secretion of the monomeric form. None of these mutations exhibi ted defects in binding affinity to cell surface proteoglycans. Each of these variants deviated significantly from normal as regards to their secreted activity or mass levels in the COS cell transfection system.