Spontaneous Reoxidation of the Subunits of Immunoglobulin M (IgM)

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Abstract
The consequences of subsequent reoxidation of human IgM that had been lightly reduced or extensively reduced were studied. When IgM was reduced with 0.02 M mercaptoethylamine (MEA), only the intersubunit interchain disulfide bonds were broken. The interchain bonds within each subunit, IgMs, remained intact. The percentage of μ and L chains in the alkylated IgMs, as measured by absorbancy at 280 nm, was virtually the same as that of the parent IgM (77% μ; 23% L). Reoxidation of the MEA-produced IgMs, in the absence of alkylation, led to the formation mainly of oxidized residual IgMs and apparently IgM. The percentage of μ and L chains in the reoxidized IgMs was the same as that of alkylated IgMs. In addition, the sedimentation coefficients of alkylated and of reoxidized IgMs were identical. On the other hand, reduction of IgM with 0.1 M 2-mercaptoethanol (2-ME) resulted in cleavage of all interchain bonds in the molecule. In the absence of alkylation, spontaneous reoxidation of the mixture led to the formation of several entities intermediate in size between IgMs and IgM. The percentage of L chains, again measured by absorbance at 280 nm, in the alkylated 2-ME-produced IgMs and in reoxidized IgMs was a little lower than that of parent IgM; that of one of the faster sedimenting reoxidation products was only 12%. It was concluded that the nature of the oxidation products depended upon whether only intersubunit disulfide bonds were split or whether there also was extensive cleavage of interchain disulfide bonds within the subunits.

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