Apolipoprotein C-II39-62 activates lipoprotein lipase by direct lipid-independent binding

Apolipoprotein C-II (apoC-II) is an exchangeable plasma apolipoprotein and an endogenous activator of lipoprotein lipase (LpL). Genetic deficiencies of apoC-II and overexpression of apoC-II in transgenic mice are both associated with severe hyperlipidemia, indicating a complex role for apoC-II in the regulation of blood lipid levels. ApoC-II exerts no effect on the activity of LpL for soluble substrates, suggesting that activation occurs via the formation of a lipid-bound complex. We have synthesized a peptide corresponding to amino acid residues 39-62 of mature human apoC-II. This peptide does not bind to model lipid surfaces but retains the ability to activate LpL, Conjugation of the fluorophore 7-nitrobenz-2-oxa-1,3-diazole (NBD) to the N-terminal alpha-amino group of apoC-II39-62 facilitated determination of the affinity of the peptide for LpL using fluorescence anisotropy measurements. The dissociation constant describing this interaction was 0.23 mu M, and was unchanged when LpL was lipid-bound. Competitive binding studies showed that apoC-II39-62 and full-length apoC-II exhibited the same affinity for LpL in aqueous solution, whereas the affinity for full-length apoC-II was increased at least 1 order of magnitude in the presence of lipid. We suggest that while the binding of apoC-II to the lipid surface promotes the formation of a high affinity complex of apoC-II and LpL, activation occurs via direct helix-helix interactions between apoC-II39-62 and the loop covering the active site of LpL.