Intestinal absorption of dipeptides and beta-lactam antibiotics. II. Purification of the binding protein for dipeptides and beta-lactam antibiotics from rabbit small intestinal brush border membranes

By photoaffinity labeling of brush border membrane vesicles from rabbit small intestine with photoreactive derivatives of beta-lactam antibiotics and dipeptides, a binding protein for dipeptides and beta-lactam antibiotics with an apparent molecular weight of 127,000 was labeled. The labeled 127 kDa polypeptide could be solubilized with the non-ionic detergents Triton X-100, n-octyl glucoside or CHAPS. If the vesicles were solubilized prior to photoaffinity labeling, no clear incorporation of radioactivity into the 127 kDa polypeptide occurred indicating a loss of binding ability upon solubilization. By affinity chromatography of solubilized brush border membrane proteins on an agarose wheat germ lectin column, the binding protein for dipeptides and beta-lactam antibiotics of Mr 127,000 was retained on the column. With N-acetyl-D-glucosamine the photolabeled binding protein for beta-lactam antibiotics and dipeptides was eluted together with the brush border membrane-bound enzyme aminopeptidase N. Separation from aminopeptidase N and final purification was achieved by anion-exchange chromatography on DEAE-sephacel. Polyclonal antibodies against the purified binding protein were raised in guinea pigs. The photolabeled 127 kDa protein could be precipitated from solubilized brush border membranes with these antibodies. Incubation of brush border membrane vesicles with antiserum prior to photoaffinity labeling significantly reduced the extent of labeling of the 127 kDa protein. Treatment of brush border membrane vesicles with antiserum significantly inhibited the efflux of the alpha-aminocephalosporin cephalexin from the brush border membrane vesicles compared to vesicles treated with preimmune serum. These studies indicate that the binding protein for dipeptides and beta-lactam antibiotics of apparent molecular weight 127,000 in the brush border membrane of rabbit small intestinal enterocytes is directly involved in the uptake process of small peptides and orally active beta-lactam antibiotics across the enterocyte brush border membrane.