Davis A E, Harrison R A, Lachmann P J
J Immunol. 1984 Apr;132(4):1960-6.
The fragments that result from the inactivation of C3b have not been completely characterized. Initial inactivation is catalyzed by the protease factor I, which, in the presence of its cofactor (factor H), cleaves two peptide bonds in the alpha'-chain of C3b. This results in the release of a small peptide (C3f, Mr 3000) from iC3b, which consists of the C3 beta chain covalently bonded to two alpha'-chain-derived peptides (Mr 68,000 and Mr 43,000). Surface-bound iC3b is cleaved at a third site by factor I to produce C3c and C3d,g (or alpha 2D). The factor I cofactor for this cleavage is the C3b receptor that is present on erythrocyte and leukocyte membranes. This report describes the isolation and initial structural characterization of C3c and C3d,g generated in whole blood after complement activation with cobra venom factor. These fragments were compared with the C3 fragments isolated from the serum and plasma of a patient with complement activation in vivo. The fragments were isolated with two solid phase monoclonal antibodies, one of which recognizes a determinant on C3g (clone 9) and one of which recognizes a determinant on C3c (clone 4). C3c isolated from normal blood showed three polypeptides that had apparent m.w. of 75,000, 43,000, and 27,000. The C3d,g consisted of a single polypeptide chain with a m.w. of 40,000. Amino terminal sequence analysis showed that the Mr 27,000 peptide from C3c is derived from the amino terminal portion of the alpha'-chain of C3b, whereas the Mr 43,000 peptide is derived from the carboxy terminus of the same chain. Amino terminal sequence analysis showed also that C3g is derived from the amino terminus of C3d,g. The C3 fragments isolated from a patient with partial lipodystrophy, nephritic factor activity, low serum C3 levels, and circulating C3 cleavage products showed a more complicated pattern on SDS-PAGE. The fragment isolated with clone 9 had an apparent m.w. of 40,000, identical to C3d,g generated in vitro, and it had the same amino terminal sequence as C3d,g generated in vitro. The eluate from insolubilized clone 4, however, showed prominent bands with Mr of 75,000, 56,000, 43,000, and 27,000, together with a triple-banded pattern at 68,000 and a minor band at 80,000. This eluate thus appears to contain C3c, and iC3b or an iC3b-like product. The origin of the Mr 56,000 and Mr 80,000 peptides have not yet been determined. These studies, with previous data, definitively order the C3c and C3d,g peptides in the alpha-chain of C3.(ABSTRACT TRUNCATED AT 400 WORDS)
C3b 失活产生的片段尚未完全被鉴定。初始失活由蛋白酶 I 因子催化，在其辅因子（H 因子）存在的情况下，I 因子在 C3b 的α'-链上切割两个肽键。这导致从 iC3b 释放出一个小肽（C3f，分子量 3000），iC3b 由共价连接到两个α'-链衍生肽（分子量 68,000 和 43,000）的 C3β链组成。表面结合的 iC3b 在第三个位点被 I 因子切割产生 C3c 和 C3d,g（或α2D）。该切割的 I 因子辅因子是存在于红细胞和白细胞膜上的 C3b 受体。本报告描述了用眼镜蛇毒因子激活补体后全血中产生的 C3c 和 C3d,g 的分离及初步结构鉴定。将这些片段与体内补体激活患者的血清和血浆中分离的 C3 片段进行比较。用两种固相单克隆抗体分离片段，其中一种识别 C3g 上的一个决定簇（克隆 9），另一种识别 C3c 上的一个决定簇（克隆 4）。从正常血液中分离的 C3c 显示出三条多肽，表观分子量分别为 75,000、43,000 和 27,000。C3d,g 由一条分子量为 40,000 的单多肽链组成。氨基末端序列分析表明，来自 C3c 的 27,000 分子量肽来自 C3b 的α'-链氨基末端部分，而 43,000 分子量肽来自同一条链的羧基末端。氨基末端序列分析还表明 C3g 来自 C3d,g 的氨基末端。从一名患有部分脂肪营养不良、肾炎因子活性、低血清 C3 水平和循环 C3 裂解产物的患者分离的 C3 片段在 SDS - PAGE 上显示出更复杂的模式。用克隆 9 分离的片段表观分子量为 40,000，与体外产生的 C3d,g 相同，并且其氨基末端序列与体外产生的 C3d,g 相同。然而，来自固定化克隆 4 的洗脱液显示出明显的条带，分子量分别为 75,000、56,000、43,000 和 27,000，以及 68,000 处的三带模式和 80,000 处的一条小带。因此，该洗脱液似乎含有 C3c 和 iC3b 或类似 iC3b 的产物。56,000 分子量和 80,000 分子量肽的来源尚未确定。这些研究与先前的数据一起，明确了 C3α链中 C3c 和 C3d,g 肽的顺序。（摘要截断于 400 字）
