O'Keefe M C, Caporale L H, Vogel C W
Department of Biochemistry, Georgetown University, Washington, D.C. 20007.
J Biol Chem. 1988 Sep 5;263(25):12690-7.
The generation of two cleavage products of human C3, termed C3o and C3p, by incubation with a C3-cleaving protease isolated from cobra venom (Naja naja siamensis) is described. The venom protease removes the C3p fragment (Mr approximately 33,000) from the C3dg region of the C3 alpha-chain. The major cleavage fragment C3o (Mr approximately 140,000) contains the unaltered beta-chain of C3 and two alpha-chain-derived polypeptides of Mr approximately 29,000 and Mr approximately 38,000, respectively. Amino-terminal amino acids sequence analysis of C3p and the three chains of C3o allowed the identification of the exact location of the two alpha-chain-derived fragments of C3o and the three cleavage sites of the venom protease. The chain structure of C3o resembles those of C3c and cobra venom factor. In contrast to C3c but like cobra venom factor (and C3b), C3o was found to support the activation of the serine protease Factor B by cleavage in the presence of Factor D and Mg2+ into Bb and Ba, generating an enzymatically active complex that is able to cleave a fluorogenic peptide substrate for C3 convertases. Since the only stretch of amino acid residues of C3o not present in C3c is the carboxyl terminus of the Mr approximately 29,000 chain of C3o, it is suggested that this region is important for the interaction with Factor B and convertase formation.
本文描述了人C3与从眼镜蛇毒（眼镜蛇）中分离出的一种C3裂解蛋白酶一起温育时，产生两种裂解产物C3o和C3p的过程。这种蛇毒蛋白酶从C3α链的C3dg区域去除C3p片段（分子量约33,000）。主要裂解片段C3o（分子量约140,000）包含未改变的C3β链以及两条分别来自α链的多肽，分子量约为29,000和38,000。对C3p和C3o的三条链进行氨基末端氨基酸序列分析，确定了C3o中两条来自α链的片段的确切位置以及蛇毒蛋白酶的三个裂解位点。C3o的链结构类似于C3c和眼镜蛇毒因子。与C3c不同，但与眼镜蛇毒因子（和C3b）一样，发现C3o在因子D和Mg2+存在的情况下，通过裂解支持丝氨酸蛋白酶因子B活化为Bb和Ba，产生一种能够切割C3转化酶荧光底物的酶活性复合物。由于C3o中唯一一段不存在于C3c中的氨基酸残基是C3o中分子量约29,000链的羧基末端，因此推测该区域对于与因子B的相互作用和转化酶形成很重要。
