Binding of glucagon to lipid bilayers

At physiological pH and temperature, glucagon binds to liposomes composed of egg phosphatidylcholine and cholesterol (2:1 mol/mol) in a highly specific manner. The chemical reactivities of the functional groups were determined over the concentration range of 1.0 × 10−6 – 3.0 × 10−8 M by the method of competitive labelling with 1-fluoro-2,4-dinitrobenzene as the labelling reagent. At concentrations above 3 × 10−7 M, the amino terminal histidine and the two tyrosine residues showed a marked decrease in reactivity in the presence of liposomes, but the reactivity of the Lys-12 Nε-amino group was unaltered. At lower concentrations the Lys-12 reactivity also decreased markedly, owing to a change in the environment of this group. These results indicated that two different forms of glucagon existed over the concentration range studied. Both in the absence and presence of liposomes the Lys-12 Nε-amino groups showed a transition in reactivity at 1.8 × 10−7 M. In the presence of liposomes the other functional groups also showed a transition in reactivity at 2 × 10−7 M but the change was much smaller. The pattern of reactivities were consistent with the X-ray crystallographic structure of the type 2 glucagon trimer being the predominant species at 10−6 M, with free monomeric glucagon occurring at 3 × 10−8 M. A trimerization constant of 4 × 1013 M−2 at pH 7.5 and 37 °C was determined. It is concluded that trimeric glucagon binds to phosphatidylcholine–cholesterol bilayers primarily with the amino-terminal segment of the polypeptide chain and that additional regions of the molecule are involved in binding of the free monomeric unit.Key words: glucagon, liposomes, binding, reactivity.