Lipoprotein Lipase: Modification of Its Kinetic Properties by Mild Tryptic Digestion

Abstract

Mild tryptic digestion of [porcine pancreatic] lipoprotein lipase cleaved its polypeptide chain in the middle, but the pieces were held together by disulfide bonds. The modified enzyme retained its ability to bind to heparin and to anionic detergents and, on gel filtration, it eluted in a similar position as the native enzyme does. It also retained essentially full activity against soluble substrates. The overall physico-chemical properties of the enzyme were not markedly changed, and its active site remained intact after treatment with trypsin. The activity of the modified enzyme against long-chain acylglycerols and phospholipids was much reduced. With some emulsions, the decreased activity could be ascribed in part to a decreased ability of the modified enzyme to bind to the emulsion droplets. Under these conditions apolipoprotein CII partially restored binding and activity. With a lysophosphatidylcholine-triacylglycerol emulsion, the modified enzyme adsorbed almost completely to the emulsion droplets, but its activity was nonetheless very low. Tryptic cleavage interfered with the ability of the enzyme to become property orientated at the interface. With this emulsion apoliproprotin CII enhanced the activity of the native enzyme 4-fold, but the activity of the trypsin-treated enzyme 30-fold; therefore, the activity of the modified enzyme became almost as high as that of the native enzyme. Apparently, apolipoprotein CII enhances the activity of lipoprotein lipase by stabilizing an effective orientation conformation of the enzyme at the interface. This effect became more marked when the ability of the enzyme itself to attain this form was reduced by tryptic cleavage.