A role for aminopeptidase N in Na(+)-dependent amino acid transport in bovine renal brush-border membranes.

A monoclonal antibody FD19 which removes reconstitutable Na(+)-dependent amino acid transport activity from solubilized bovine renal brush-border membrane vesicles was found to react specifically with the enzyme aminopeptidase N. Cleavage of aminopeptidase N from the membranes with papain inhibited Na(+)-dependent amino acid transport activity without affecting that of alpha-methyl D-glucoside. Removal of aminopeptidase substantially increased the Km values for the Na(+)-dependent transport of alanine, glutamine, leucine and phenylalanine without affecting the Vmax. Both Na(+)-dependent amino acid transport and aminopeptidase activity in intact vesicles were competitively inhibited by amino acids with very similar specificity. These results suggest that the amino acid-binding sites of aminopeptidase N and the transporter interact in some way to increase the Km of the transport process for its substrates. However, independent direct inactivation of the transport system by papain cannot be ruled out.