Lipoprotein lipase: molecular interactions of the enzyme.

Lipoprotein lipase (EC 3.1.1.34), as the extrahepatic enzyme responsible for the hydrolysis of plasma lipoprotein triacylglycerol, plays a pivotal role in the metabolism of circulating lipids. The Occurrence and action of the enzyme in lipoprotein metabolism has been the subject of a number of recent reviews (Cryer, 1981 ; Quinn et al., 1982; Hamash & Hamash, 1983) and this aspect will not be considered here. Rather, the molecular interactions that occur in the exercise of the function of the enzyme will be considered, particularly as these are reflected by studies in uitro. From studies of lipoprotein lipase action in vitro it is clear that the enzyme may interact with a number of different molecular species and that the relationships which exist between these various binding events contribute to the control of functional activity. Taking the: events separately, the first is associated with the catalytic or active site of the enzyme. The active site of lipoprotein lipase is relatively nonspecific in that it hydrolyses tri-, diand mono-acylglycerols as well as phospholipids. However, the positional specificity of hydrolysis is strict for all substrates with a preferential cleavage of the sn-1(3) ester bonds with subsequent isomerization of the 2-monoacylglyce:rol for complete breakdown (Scow & Olivecrona, 1977) The enzyme will also hydrolyse water-soluble model substrates, e.g. p-nitrophenyl-butyrate. A study of the effects of pH on the kinetic parameters exhibited by the enzyme in s'olution suggest the involvement of the imidazole side chain of histidine in the catalytic mechanism. However, corroboration by covalent modification of the active site and the isolation of a labelled histidine is awaited. Although little direct evidence exists for the involvement of an active-site sei-ine residue in the action of the enzyme, various inhibitors cf serine proteinase and serine esterase activity can also inhibit lipoprotein lipase under appropriate conditions (e.g. di-isopropylfluorophosphate, phenylmethanesulphonyl fluoride, diethyl-pnitrophenylphosphate). Quinn et ul. (1982) have indicated that a 'serine esterase-like' mechanism appears best able to accommodate observations from both t t e pH rate and the covalent modification experiments on lipoprotein lipase. The catalytic efficiency of the enzyme is high, and with long-chain acylglycerols as substrate each lipoprotein lipase can hydrolyse 1000 ester bondsis. In order to accommodate the observed rate at which, for example, a chylomicron of 0 . 1 6 ~ ~ diameter containing 1.3 x lob triacylglycerol molecules is degraded, a number of enzyme molecules must act together on each lipoprotein particle. It can be shown that, in general, the rate of triacylglycerol hydrolysis is proportional to the number of enzyme molecules bound to each lipoprotein particle and for a chylomicron of the size indicated above, approx. 30-40 enzyme molecules would be required to act in concert to bring about the maximal rate of hydrolysis observed in ritro (see below also). The second discernible site on the lipoprotein lipase molecule is that which may be described as the lipid (interfacial)-binding site. This site is distinct from the active site and has been proposed because of the observed ability of the enzyme to bind to lipoproteins, lipid emulsions and liposomes in the absence of the activator protein or hydrolysis. The binding through this site is reversible and accounts for the observed rapid movement of the enzyme between lipoprotein particles in vitro.