LAUSR

Significance Lipoprotein lipase (LPL) plays a central role in plasma lipid metabolism, hydrolyzing the triglycerides in lipoproteins and releasing fatty acid nutrients for vital tissues. LPL is synthesized by adipocytes and myocytes and secreted into the interstitial spaces, whereupon it is captured by an endothelial cell protein, GPIHBP1, and transported to its site of action within the capillary lumen. For decades, LPL has been assumed to be a homodimer, with dimerization thought to be required for both secretion and catalytic activity. In the present study, we revisited the notion that LPL is a homodimer. Our findings indicate that LPL and GPIHBP1-bound LPL are active in a monomeric state. Lipoprotein lipase (LPL), the enzyme that hydrolyzes triglycerides in plasma lipoproteins, is assumed to be active only as a homodimer. In support of this idea, several groups have reported that the size of LPL, as measured by density gradient ultracentrifugation, is ∼110 kDa, twice the size of LPL monomers (∼55 kDa). Of note, however, in those studies the LPL had been incubated with heparin, a polyanionic substance that binds and stabilizes LPL. Here we revisited the assumption that LPL is active only as a homodimer. When freshly secreted human LPL (or purified preparations of LPL) was subjected to density gradient ultracentrifugation (in the absence of heparin), LPL mass and activity peaks exhibited the size expected of monomers (near the 66-kDa albumin standard). GPIHBP1-bound LPL also exhibited the size expected for a monomer. In the presence of heparin, LPL size increased, overlapping with a 97.2-kDa standard. We also used density gradient ultracentrifugation to characterize the LPL within the high-salt and low-salt peaks from a heparin-Sepharose column. The catalytically active LPL within the high-salt peak exhibited the size of monomers, whereas most of the inactive LPL in the low-salt peak was at the bottom of the tube (in aggregates). Consistent with those findings, the LPL in the low-salt peak, but not that in the high-salt peak, was easily detectable with single mAb sandwich ELISAs, in which LPL is captured and detected with the same antibody. We conclude that catalytically active LPL can exist in a monomeric state.