The role of nonspecific hydrophobic interactions in the biological activity of N epsilon-acyl derivatives of glucagon. Studies of conformation, receptor binding, and adenylate cyclase activation.
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Glucagon was acylated at position 12 using conditions favoring reaction with the epsilon-amino group of lysine. The N epsilon-acetyl, N epsilon-hexanoyl, and N epsilon-decanoyl derivatives were prepared and purified. Secondary structure as measured by circular dichroism was lower in all derivatives than in glucagon, both in 95% methanol and in 25 mM sodium dodecyl sulfate at pH 2 and pH 12. N epsilon-Acetyl glucagon was less active than the native hormone in a radioreceptor assay and higher concentrations of this derivative were required to stimulate the adenylate cyclase activity of rat liver plasma membranes. The maximal extent of cyclase activation by this derivative was less than that found with the native hormone. N epsilon-Hexanoyl glucagon and N epsilon-decanoyl glucagon had greater activity than N epsilon-acetyl glucagon in receptor binding as well as in adenylate cyclase activation, although these two derivatives were not as active as the native hormone. N epsilon-hexanoyl glucagon and N epsilon-decanoyl glucagon were more potent in receptor binding than in adenylate cyclase activation. From these results it appears that the positive charge of the epsilon-amino groups may have a specific role in obtaining maximal biological activity, while the acyl groups contribute to the nonspecific hydrophobic interactions between the hormone and its receptor. In addition, a possible relationship between stabilization of the amphipathic helix in solution and the activity of these and other N epsilon-derivatives of glucagon is discussed.
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