THE MOLECULAR INTERACTION OF SULFONYLUREAS WITH BETA-CELL ATP-SENSITIVE K-CHANNELS()

The molecular interaction of glimepiride and glibenclamide with the be ta-cell sulfonylurea receptor was investigated by kinetic and steady s tate binding as well as photoaffinity labeling. The novel sulfonylurea , glimepiride, exhibits a significantly higher exchange rate with the sulfonylurea receptor but a 2.5-3 fold lower binding affinity compared to glibenclamide. [H-3]Glimepiride was specifically incorporated into a 65-kDa polypeptide under conditions which led to predominant labeli ng of a 140-kDa protein by [H-3]glibenclamide. Labeling of the 140-kDa protein by [H-3]glibenclamide was inhibited by unlabeled glimepiride and, vice versa, glibenclamide inhibited labeling of the 65-kDa protei n by [H-3]glimepiride. The 65-kDa protein was also specifically photol abeled by the sulfonylurea [I-125]35623, whereas an 4-azidobenzoyl der ivative of glibenclamide, N-3-[H-3]33055, exclusively labeled a 33-kDa protein. Solubilization of beta-cell tumor membranes led to a shift o f specific [H-3]glibenclamide-binding from the 140-kDa to the 65-kDa p rotein, exclusively and to an increased labeling of the 65-kDa protein by [H-3]glimepiride. The labeling of a unique protein is in agreement with similar K-d-values for binding to the sulfonylurea receptor meas ured for both sulfonylureas upon solubilization of beta-cell membranes . Photoaffinity labeling of intact cultured beta-cells led also to lab eling of a 140-kDa protein by [H-3]glibenclamide and of a 65-kDa prote in by [H-3]glimepiride. These studies suggest that the beta-cell sulfo nylurea receptor consists of at least two protein subunits of M(r) 140 000 and 65000 which bind sulfonylureas of different structure with di fferent binding affinities and kinetic parameters. Furthermore, the ex change rate of a sulfonylurea determines the insulin releasing activit y in vitro more closely than the binding affinity.