Factor J: isolation and characterization of a new polypeptide inhibitor of complement C1

An Mr 20,000 protein inhibitor of C1, the first component of complement, has been purified from human urine and characterized. This inhibitor, tentatively designated factor J, is apparently distinct from known complement inhibitors. During purification on QAE-Sephadex, Mono Q, and heparin-Sepharose, factor J was detected by its ability to inhibit the complement-mediated lysis of sheep erythrocytes bearing antibody, C1, and activated C4 (EAC14). The purity of factor J was documented by the concordant elution from a hydroxylapatite column of functional activity and the UV absorbance as measured at three different wavelengths (220, 254, and 280 nm). The relative Mr of 20,000 was determined by sodium dodecyl sulfate-slab gel electrophoresis of radioiodinated protein. Amino acid analysis indicated a high cysteine content and allowed calculations of a specific activity of 7 functional units/pmol. The target of factor J inhibitory activity on the lysis of EAC14 was localized to C1 by the following criteria: factor J inhibited C1 in a C1 transfer assay, but had no effect on C42 activity or decay, and had no effect on the efficiency of isolated C2 or C3-C9 as provided in serum-EDTA. Factor J inhibition was rapid and not significantly influenced by temperature. In a second functional assay, factor J inhibited the association of the tetrameric complex C1r2s2 with 125I-C1q, and the results, when analyzed graphically by a reciprocal plot, were consistent with noncompetitive inhibition (Ki = 529-760 pM range). Functional and/or antigenic data indicated that factor J is distinct from the other known inhibitors of C1, namely the C1 inhibitor and the C1q inhibitor. Antihuman serum precipitated radioiodinated factor J, indicating that an antigen identical or cross-reacting with factor J exists in serum. In summary, factor J is a newly described potent inhibitor of C1 function.