Independence of Glucagon Receptors and Glucagon Inactivation in Liver Cell Membranes

THERE is increasing evidence that receptors for polypeptide hormones are localized on the cell membrane. Hormone-receptor interactions have been studied primarily by measuring the bmding of ¹²⁵I-labelled hormones to intact¹ or broken-cell preparations^2–6. Peptide hormones, however, are often inactivated after exposure to the cell extract and numerous enzymes reported as specific hormone-degrading have been described. With some hormones, such as insulin^1,6,7, biologically significant receptor interactions have been demonstrated in the absence of hormone degradation, but with other hormones, such as glucagon, it has not been possible to dissociate the processes of specific receptor binding and of hormone inactivation³, which suggests that these two processes may be functionally or structurally related. Until this question is resolved, it will not be possible to characterize properly the kinetics of the hormone-receptor interaction or to isolate and purify the receptor.