The effect of the chain length of heparin on its interaction with lipoprotein lipase

Abstract

The interactions of bovine milk lipoprotein lipase (triacylglycero-protein acylhydrolase, EC 3.1.1.34) with the glycosaminoglycans heparin and heparan sulphate were investigated using the technique of fluorescence polarization spectroscopy. The type of complex formed with the enzyme depends on the chain length of the heparin. In 0.05 M NaCl and when the heparin was in molar excess, one heparin chain of Mr 10000-18400 formed a very stable complex with the dimeric protein molecule (the 1:1 complex). With excess protein, weaker interactions produced complexes with higher molecular weights. These two classes of complex were also detected with shorter heparins (Mr 6600-8000), although in these circumstances the more stable complex possessed a heparin:protein dimer ratio of 2:1. In higher salt (0.2 M NaCl) and lower heparin concentrations (less than 6 . 10(-8) M) the weaker class of compound was undetectable and Kd values of 4 . 10(-8) M and 6 . 10(-9) M were assigned to the 2:1 and 1:1 complexes, respectively. Heparan sulphate of Mr 17000 could only form one class of complex. This had a 1:1 stoichiometry and with Kd values of 3 . 10(-8) M and 1.6 . 10(-7) M at 0.05 and 0.2 M NaCl, respectively. The results could be explained if there is a distinct binding region for glycosaminoglycans on each subunit of the dimeric enzyme and a single heparin chain of Mr greater than 10000 can satisfy both sites to form a 1:1 complex. Smaller heparin chains are unable to span the sites and, in order to occupy them, two chains must interact with each enzyme molecule.