Kai S, Fujita T, Gigli I, Nussenzweig V
J Immunol. 1980 Dec;125(6):2409-15.
Mouse C3b/C4b inactivator (C3b/C4bINA) was purified approximately 400 times from mouse serum. It is a beta-globulin and consists of 2 disulfide bonded chains of m.w. 60,000 and 35,000. Under nonreducing conditions, its m.w. is 95,000. It cleaves the alpha'-chain of cell-bound C4b into 3 fragments: alpha 2, alpha 3, alpha 4. The alpha 2 fragments remain bound to the cell surface (C4d), and the rest of the molecule (C4c) is released into the fluid phase. In fluid phase, C3b/C4bINA cleaves the alpha'-chain of C4b in a similar manner but only in the presence of mouse or human C4-binding protein (C4-bp). Mouse C4-bp and human C3b/C4bINA do not cleave human C4b, although mouse C4-bp binds to human C4b. This incompatibility suggests that C4-bp and C3b/C4bINA must interact to cleave fluid phase C4b. Mouse C3b/C4bINA also cleaves the alpha'-chain of human C3b in solution into 2 fragments in the presence of human beta 1H. Therefore, it is likely that mouse and human C3b/C4bINA are homologous proteins. A monospecific antiserum to mouse C3b/C4bINA has been prepared in rabbits. By crossed immunoelectrophoresis, this antiserum detects, in addition to the protein described above, a fast beta-globulin with a m.w. of approximately 200,000 and antigenically identical to C3b/C4bINA but enzymatically inactive. This protein could represent a precursor of C3b/C4bINA.
小鼠C3b/C4b灭活因子（C3b/C4bINA）从小鼠血清中纯化出来后纯度提高了约400倍。它是一种β球蛋白，由两条通过二硫键相连的链组成，分子量分别为60,000和35,000。在非还原条件下，其分子量为95,000。它将细胞结合的C4b的α'链切割成3个片段：α2、α3、α4。α2片段仍结合在细胞表面（C4d），分子的其余部分（C4c）释放到液相中。在液相中，C3b/C4bINA以类似方式切割C4b的α'链，但仅在存在小鼠或人C4结合蛋白（C4-bp）时才会发生。小鼠C4-bp与人C3b/C4bINA不会切割人C4b，尽管小鼠C4-bp与人C4b结合。这种不兼容性表明C4-bp和C3b/C4bINA必须相互作用才能切割液相中的C4b。小鼠C3b/C4bINA在人β1H存在的情况下，也会将溶液中的人C3b的α'链切割成2个片段。因此，小鼠和人C3b/C4bINA很可能是同源蛋白。已在兔中制备了针对小鼠C3b/C4bINA的单特异性抗血清。通过交叉免疫电泳，该抗血清除了能检测到上述蛋白质外，还能检测到一种分子量约为200,000的快速β球蛋白，其抗原性与C3b/C4bINA相同，但无酶活性。这种蛋白质可能代表C3b/C4bINA的前体。
