Specific changes made to the recombinant α1(XI)Npp allowed the antibody to distinguish endogenous bovine α1(XI)Npp from recombinant rat α1(XI)Npp. A, recombinant α1(XI)Npp in 10× increasing incremental concentration curve. Lane 1, 0.01-μg protein; lane 2, 0.1-μg protein; lane 3, 1-μg protein; lane 4, 10-μg protein; lane 5, molecular weight markers, 43 and 28 kDa; lane 6, 0.1-μg protein; lane 7, 1.0-μg protein; lane 8, 10-μg protein. B, nickel affinity and immunoblot of recombinant protein. Lanes 1–8, same samples as in A. Lanes 1–4, nickel affinity blot; lanes 6–8, immunoblot with α1(XI)Npp antibody. C, sequence of antigenic peptide compared with the analogous site in bovine and rat α1(XI)Npp and with the sequence of the recombinant protein. Although the antibody recognized endogenous rat α1(XI) collagen chains, it did not recognize the recombinant protein. The proteolytic processing site is indicated by the arrow.