Mikoryak C A, Elliott B W, Kimball M E, Steiner L A
J Immunol. 1986 Jan;136(1):217-23.
The immunoglobulins of the bullfrog Rana catesbeiana are unusual in that, in all classes, the light chains are not disulfide bonded to heavy chains or to other light chains. Moreover, the light chains contain six, rather than the usual five, residues of half-cystine. As none of these half-cystines is in the sulfhydryl form or is alkylated after mild reduction, we suggested that the light chains probably contain three intrachain disulfide bridges. We have now carried out experiments to confirm the existence of an extra intrachain disulfide bridge in Rana catesbeiana light chains and to determine its location. Disulfide bridge assignments were based on 1) isolation and sequence analysis of S-(carboxymethyl)cysteine-containing peptides and 2) isolation, from unreduced light chains, of peptides containing a disulfide bridge. Half-cystine residues were found at positions 134 and 194, and these were shown to be joined in the conserved intradomain disulfide bridge. In addition, we found that a residue of half-cystine, located at the third position from the carboxy-terminus, forms a disulfide bridge with a half-cystine at position 119, near the amino-terminus of the domain, the latter residue replacing a proline that has been found at this position in all other light chains. An intrachain disulfide bridge has not been found at this location in any other light chain.
牛蛙（Rana catesbeiana）的免疫球蛋白不同寻常之处在于，在所有类别中，轻链都不通过二硫键与重链或其他轻链相连。此外，轻链含有六个而非通常的五个半胱氨酸残基。由于这些半胱氨酸在轻度还原后均不以巯基形式存在或未被烷基化，我们推测轻链可能含有三个链内二硫键。我们现在进行了实验，以证实牛蛙轻链中存在一个额外的链内二硫键并确定其位置。二硫键的归属基于：1）含S-（羧甲基）半胱氨酸肽段的分离和序列分析，以及2）从未经还原的轻链中分离出含二硫键的肽段。在第134位和第194位发现了半胱氨酸残基，并且显示它们在保守的结构域内二硫键中相连。此外，我们发现位于羧基末端第三个位置的一个半胱氨酸残基与结构域氨基末端附近第119位的一个半胱氨酸形成二硫键，后一个残基取代了在所有其他轻链该位置发现的脯氨酸。在任何其他轻链的这个位置都未发现链内二硫键。
