Specificity of mAb BB9 for human ACE. (A) Coomassie blue–stained acrylamide gel showing the 160-kDa band (left) immunopurified by mAb BB9 from BB9⁺ L cells and used for protein sequencing. Shown on the right is an immunoprecipitation done using nonbinding isotype control mAb 1B5. The arrow indicates the position of the BB9-specific band with respect to molecular weight markers (in kilodaltons). The smaller bands present in both lanes correspond to Ig heavy and light chain monomers. (B,C) Murine L cells stably expressing full-length human ACE cDNA and nontransfected L cells were stained with mAb BB9, anti-ACE mAb 5F1, and a nonbinding isotype control, mAb 1B5. Nontransfected cells (B) did not react with any of the antibodies while ACE-transfected L cells (C) were stained with both 5F1 and BB9. (D) Immunoprecipitates of ACE-transfected L cells with BB9 and 1B5 were Western blotted and probed with polyclonal Ab to human ACE. Note the specific reactivity with a 160-kDa protein in the BB9 immunoprecipitation and in whole lysate.