Beale D, Hopley J G
Biochem J. 1985 Mar 15;226(3):661-7. doi: 10.1042/bj2260661.
A tryptic fragment (A) of Mr 25000 was prepared from bovine secretory component. The fragment binds polymeric immunoglobulin, although 9 times less effectively than secretory component on a molar basis. The fragment has four buried half-cystine residues and two exposed half-cystine residues. It gives rise to two fragments of Mr 11000-13000 on prolonged digestion with trypsin, and these do not bind polymeric immunoglobulin. It is proposed that fragment A consists of two immunoglobulin-like domains. Bovine secretory component was found to have 9-11 buried half-cystine residues and four exposed half-cystine residues. Reduction and alkylation of the exposed residues decreases the binding of polymeric immunoglobulin by 3-fold. Initial tryptic cleavage of bovine secretory component gives a fragment (Q) disulphide-bridged to a further fragment (T). Fragment Q is similar in size to a three-domain immunoglobulin fragment, and fragment T is similar in size to a two-domain immunoglobulin fragment. The two-domain fragment A is derived from fragment Q by further tryptic cleavage. The results are compatible with the proposal by Mostov, Friedlander & Blobel [(1984) Nature (London) 308, 37-43] that secretory component consists of multiple immunoglobulin-like domains. The results also indicate that optimal binding of polymeric immunoglobulin involves several domains stabilized by an exposed disulphide bridge.
从牛分泌成分中制备出一个分子量为25000的胰蛋白酶消化片段（片段A）。该片段能结合多聚免疫球蛋白，尽管在摩尔基础上其结合效率仅为分泌成分的1/9。该片段有4个埋藏的半胱氨酸残基和2个暴露的半胱氨酸残基。用胰蛋白酶长时间消化后，它会产生两个分子量为11000 - 13000的片段，且这些片段不结合多聚免疫球蛋白。有人提出片段A由两个免疫球蛋白样结构域组成。发现牛分泌成分有9 - 11个埋藏的半胱氨酸残基和4个暴露的半胱氨酸残基。对暴露残基进行还原和烷基化处理会使多聚免疫球蛋白的结合能力降低3倍。牛分泌成分最初经胰蛋白酶切割产生一个片段（片段Q），它通过二硫键与另一个片段（片段T）相连。片段Q的大小与一个三结构域免疫球蛋白片段相似，片段T的大小与一个二结构域免疫球蛋白片段相似。二结构域片段A是由片段Q经进一步胰蛋白酶切割产生的。这些结果与莫斯托夫、弗里德兰德和布洛贝尔[（1984年）《自然》（伦敦）308, 37 - 43]提出的分泌成分由多个免疫球蛋白样结构域组成的观点相符。结果还表明，多聚免疫球蛋白的最佳结合涉及由一个暴露的二硫键稳定的几个结构域。
