Action of Lipoprotein Lipase on Phospholipid Monolayers. Activation by Apolipoprotein C-II

Action of lipoprotein lipase and its activation by apolipoprotein C-II (apoC-II) were studied with monomolecular films of 1,2-didodecanoyl-sn-glycero-3-phosphoglycerol as a substrate. The enzyme velocity and the specific activity of the interface-bound enzyme show a bell-shaped curve as a function of lipid packing, both in the presence and absence of apoC-II. Above critical surface pressure of 20 dyn cm^-1, lipoprotein lipase alone is no longer able to hydrolyze a monolayer of 1,2-didodecanoyl-sn-glycero-3-phosphoglycerol. However, lipoprotein lipase readily penetrates into the phospholipid interface up to surface pressures exceeding 40 dyn cm^-1, without any effect by apoC-II. Activation of lipoprotein lipase by apoC-II can be assigned to be due to two specific effects. Below the critical surface pressure of 20 dyn cm^-1, apoC-II merely increases the turnover number of lipoprotein lipase 4-fold. The minimal sequence region required to produce this effect is contained in the carboxyl-terminal residues 55-78 of the activator, as determined with synthetic peptide fragments apoC-II(43-78), -(50-78), -(55-78), -(60-78), and -(66-78). Above the surface pressure of 20 dyn cm^-1, apoC-II activates in an all-or-none manner a noncatalytic enzyme already bound to the substrate interface, causing the critical surface pressure to increase from 20 to 25 dyn cm^-1. The presence of the phospholipid-associating residues 43-50 in the carboxyl-terminal synthetic activator peptide is mandatory for the latter effect. The main conclusions are that the expression of the catalytic activity of lipoprotein lipase can be regulated by the physical state of the substrate interface and that apoC-II can affect this regulation.