Identification of a 130-kilodalton human biliary concanavalin a binding protein as aminopeptidase N

Background/Aims: Human gallbladder bile contains a group of nonmucin glycoproteins that binds to the lectin concanavalin A (con A) and has been reported to promote cholesterol monohydrate crystal nucleation, an event preceding the formation of gallstones. Several of these proteins, including a 130-kilodalton protein, have been isolated and shown to promote nucleation in vitro. The aim of this study was to identify this and other major biliary con A binding glycoproteins. Methods: Gallbladder bile was chromatographed on con A agarose, and the eluted proteins were electrophoresed, blotted, and subjected to amino-terminal sequence analysis. Results: The major con A binding proteins were identified as aminopeptidase N (a 130-kilodalton protein), α2 macroglobulin, hemopexin, immunoglobulin heavy chains, and the β chain of haptoglobin. After further purification, aminopeptidase N was found to be enzymatically active and to promote cholesterol crystallization at its approximate physiological concentration in bile. Conclusions: It is likely that aminopeptidase N is the previously characterized 130-kilodalton biliary crystallization promoting protein. Aminopeptidase N is probably released from the biliary canalicular membrane by the detergent activity of bile salts and may be one factor that promotes cholesterol crystallization in the gallbladder.