Lipid binding of apolipoprotein CII is required for stimulation of lipoprotein lipase activity against apolipoprotein CII-deficient chylomicrons.

Human apolipoprotein CII (apo CII) consists of 79 amino acid residues. The amino-terminal two thirds of the molecule binds to lipid through the formation of amphipathic helixes, while the carboxy-terminal third is engaged in activation of lipoprotein lipase (LPL). On the basis of studies in model systems, it was previously concluded that fragments of apo CII spanning residues 51-79 were sufficient for activation, although they do not bind to lipid. In the present study, we used chylomicrons from an apo CII-deficient patient to reinvestigate this possibility, with a physiologically relevant substrate. Human LPL expressed very low activity against these chylomicrons. Addition of apo CII caused an immediate > 100-fold increase in lipase activity. The apo CII fragment 50-79 caused very little stimulation, though with some synthetic lipid substrates, this fragment was fully effective. LPL bound to the chylomicrons even in the absence of apo CII but apparently in a nonproductive manner. In accord with this finding, the main effect of apo CII was on the VMAX for the reaction, with little or no change in the apparent K(M). We conclude that the lipid-binding part of apo CII is needed for activity of LPL against chylomicrons. This idea is in accord with previous studies with lipid monolayers, which showed that the lipid-binding part is necessary for activation of the enzyme at high surface pressures.