Primary structure of human alpha 2-macroglobulin. Complete disulfide bridge assignment and localization of two interchain bridges in the dimeric proteinase binding unit.

The disulfide bridge pattern of human alpha 2-macroglobulin (alpha 2M) given earlier (Sottrup-Jensen, L., Stepanik, T. M., Kristensen, T., Wierzbicki, D. M., Jones, C. M., Lønblad, P. B., Magnusson, S., and Petersen, T. E. (1984) J. Biol. Chem. 259, 8318-8327) has been revised by showing that Cys255 and Cys408 in one subunit are bridged to Cys408 and Cys255, respectively, in the adjacent subunit of the proteinase binding dimer. Thus, the alpha 2M-dimer contains two interchain disulfide bridges, and the individual subunits are arranged in an antiparallel fashion. These results are the outcome of partial reduction experiments, where reduction of methylamine-treated alpha 2M with 1-8 mM mercaptoethanesulfonate at pH 8.0 resulted in the appearance of 2.6 mol of SH-groups per mol of free subunit. Apart from reduction of the two interchain bridges, the intrachain bridges Cys228-Cys276, Cys572-Cys748, Cys798-Cys826, and Cys824-Cys860 are reduced to a minor extent under these conditions. The disulfide bridge pattern of alpha 2M has been completed by showing that the alpha 2M subunit contains 11 intrachain bridges, including a bridge connecting Cys447 with Cys540.