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Glimepiride is a novel sulfonylurea drug for treatment of non-insulin-dependent diabetes mellitus with higher blood sugar lowering efficacy in diabetic patients than glibenclamide raising the question whether this characteristics is in line with different binding of glimepiride and glibenclamide to the beta-cell sulfonylurea receptor. Scatchard plot analysis of [3H]sulfonylurea binding to membranes isolated from rat beta-cell tumors and (RINm5F) insulinoma cells and to RINm5F cells demonstrated that glimepiride has a 2.5-3-fold lower affinity than glibenclamide. This corresponded well to the 8-9-fold higher koff and 2.5-3-fold higher kon rates of glimepiride compared to glibenclamide as revealed by the dissociation and association kinetics of [3H]sulfonylurea binding and the Kd values calculated thereof. In agreement, the concentrations required for half-maximal displacement of [3H]sulfonylurea bound to beta-cell membranes were significantly higher for glimepiride compared to glibenclamide. However, the binding affinity of glimepiride measured by both equilibrium binding and kinetic binding studies upon solubilization of beta-cell tumor membranes and RINm5F cell membranes increased up to the value for glibenclamide. This was primarily based on a drastic decrease of the dissociation rate constant of glimepiride whereas the kinetics of glibenclamide binding remained largely unaffected upon solubilization. These data suggest that the Kd value alone is not sufficient for characterization of a sulfonylurea drug, since the kinetic binding parameters may also determine its acute blood sugar lowering efficacy.

Glimepiride is a novel sulfonylurea for the treatment of type II-diabetic patients exhibiting different receptor binding kinetics to beta-cell membranes with 8-9-fold higher koff rate and 2.5-3-fold higher kon rate compared to glibenclamide (see accompanying paper (Müller, G. et al. (1994) Biochim. Biophys. Acta 1191, 267-277)). To elucidate the molecular basis for this differential behaviour of glimepiride and glibenclamide, direct photoaffinity labeling studies using beta-cell tumor membranes were performed. [3H]Glimepiride was specifically incorporated into a membrane polypeptide of M(r) = 65,000 under conditions, which led to predominant labeling of a 140 kDa protein by [3H]glibenclamide (Kramer, W. et al. (1988) FEBS Lett. 229, 355-359). Labeling of the 140 kDa protein by [3H]glibenclamide was inhibited by unlabeled glimepiride and, vice versa, glibenclamide inhibited labeling of the 65 kDa protein by [3H]glimepiride. The 65 kDa protein was also specifically photolabeled by the sulfonylurea [125I]35623, whereas an 4-azidobenzoyl derivative of glibenclamide, N3-[3H]33055, exclusively labeled a 33 kDa protein. Competitive Scatchard analysis of [3H]glimepiride-binding and [3H]glibenclamide-binding to RINm5F cell membranes using glibenclamide and glimepiride, respectively, as heterologous displacing compounds yielded non-linear plots. These findings may be explained by cooperative interactions between the 140 and 65 kDa sulfonylurea-binding proteins. The possibility that sulfonylureas of different structure have different access to the 140 and 65 kDa receptor proteins due to the beta-cell membrane barrier was investigated by photoaffinity labeling of solubilized beta-cell membrane proteins. Interestingly, solubilization of beta-cell tumor membranes led to a shift of specific [3H]glibenclamide binding from the 140 kDa to the 65 kDa binding protein, exclusively, and to an increased labeling of the 65 kDa protein by [3H]glimepiride. The labeling of a unique protein is in agreement with similar Kd values measured for both sulfonylureas upon solubilization of beta-cell tumor and RINm5F cell membranes (see accompanying paper). Furthermore, competitive Scatchard plots of [3H]glimepiride binding to solubilized RINm5F cell membrane proteins in the presence of glibenclamide and vice versa approximate linearity suggesting loss of cooperativity between the 140 kDa glibenclamide-binding and 65 kDa glimepiride-binding proteins upon solubilization. The physiological significance of the differential interaction of glimepiride and glibenclamide with different binding proteins was also substantiated by photoaffinity labeling of RINm5F cells leading to labeling of a 140 kDa protein by [3H]glibenclamide and of a 65 kDa protein by [3H]glimepiride.(ABSTRACT TRUNCATED AT 400 WORDS)