BILIARY AMINOPEPTIDASE-N AND THE CHOLESTEROL CRYSTALLIZATION DEFECT IN CHOLELITHIASIS

Several biliary proteins have cholesterol crystallisation promoting ac tivity. One of these glycoproteins is aminopeptidase-N, a canalicular ectoenzyme. This study attempted to localise aminopeptidase-N along th e biliary tree, to assess its concentration in a series of 98 patients subjected to abdominal surgery, 40 of them without gap stones, and to correlate its concentration with cholesterol crystal formation time o f gall bladder bile. Aminopeptidase-N was isolated from purified nativ e biliary vesicles. A specific polyclonal rabbit anti-aminopeptidase-N antibody was prepared for quantitative immunoblotting and for immunol ocalisation. Tissue was obtained from liver biopsy specimens and from gall bladders removed at surgery because of gall stone disease. Aminop eptidase-N was immunolocalised to the apical membranes of hepatocytes and to the apical pole of ductular and gall bladder mucosal cells. The nucleation time of gall bladder bile was mean (SD) 4 (3) days in the gall stone group, compared with 21 (18) days in the control group (p < 0.001). Total absolute biliary protein and aminopeptidase-N concentra tions were similar in both the control and gall stone patients. There was a reciprocal significant correlation, however, between the nucleat ion time and the relative aminopeptidase-N concentration (r = -0.35, p < 0.01) only in the gall stone group of patients, This study shows th at this apical transmembrane ectoenzyme with cholesterol crystallisati on promoting activity is present along the biliary tree and the hepato cyte. These findings support the concept that high concentrations or q ualitative changes of biliary aminopeptidase-N contribute to cholester ol gall stone formation.