The hybridoma GK5C1, secreting a monoclonal IgG1 antibody, was generated after immunizing a mouse with pig kidney microvillar membranes. An immunoradiometric assay showed that only kidney and intestine contained detectable amounts of the antigen recognized by the antibody, the highest concentration being observed in the ileum. Immunocytochemistry confirmed this observation and revealed that the antigen was associated with renal and intestinal brush borders. By ‘Western’ blotting, the antigen in kidney microvilli was shown to be a 130 kDa polypeptide. Papain treatment of the membrane before blotting converted the antigen to a 125 kDa polypeptide, no longer associated with membrane. Immunoaffinity chromatography of detergent-solubilized kidney membranes yielded a pure 130 kDa protein. When one purification was monitored by the immunoradiometric assay, the yield was 3.5% and the purification factor was 1000-fold. The antigen constituted about 0.8% of the microvillar membrane protein. The protein could be reconstituted into liposomes, where electron microscopy revealed an asymmetric orientation, similar to that of ectoenzymes in this membrane. The stalk length was about 3 nm. In electron micrographs the purified protein appeared to be dimeric. A search for enzymic activity was rewarded when L-leucyl-L-tryptophan was observed to be hydrolysed. Failure to hydrolyse N-blocked peptides and the ability to release the N-terminal residue from extended peptides, including Leu-Trp-Leu and Leu-Trp-Met-Arg, showed that the activity was that of an aminopeptidase. The enzyme was maximally active at pH 7.5 and irreversibly inactivated outside the range pH 6-10. This activity could not be attributed to trace contamination with aminopeptidase N. The best substrates so far identified for the 130 kDa protein were those with tryptophan in the P1′, position. This protein is a new microvillar enzyme and it is proposed that it be called aminopeptidase W.

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