Table II. Internalization parameters of the wild-type and mutant receptors in HEK293 cells.

The kinetics parameters of internalization of the wild type and mutant receptors were determined by one exponential curve fitting using the GraphPad program; internalization (%) represents the estimated plateau calculated using the GraphPad program. Mutant receptors could be organized into two groups, when comparing their internalization kinetics to that of the wild-type receptor. The first group comprised receptors showing similar properties as those of DOR, and the second group included receptors showing a substantially slower rate and/or slower extent of internalization.