Murine cell surface immunoglobulin (Ig) isotypes were analyzed by lactoperoxidase-catalyzed radioiodination, immunoprecipitation, and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). SDS-PAGE analysis revealed three radioactive peaks: I) IgM (µ2L2; 224,000 daltons), II) IgD (δ2L2; 185,000 daltons), and III) IgD half-molecules (δL; 105,000 daltons). The size of the δ-chain obtained from both the δ2L2 and the δL molecules was the same (68,000 daltons). The δL molecules are apparently not the result of a sulfhydryl-catalyzed reduction or disulfide interchange and are probably not generated by proteolysis, since addition of alkylating agents and protease inhibitors did not decrease the amount of δL observed. Addition of sulfhydryl-oxidizing agents also did not decrease the amount of δL, suggesting that the δL molecules cannot be reoxidized to yield δ2L2 molecules. The percentage of IgD in the δL form present on lymph node and spleen cells of A/J mice and on spleen cells of BALB/c mice was the same (∼55%). The percentage of δL on splenocytes from BDF/1 mice (2 to 7 months old) was ∼42%. Spleen cells from strains that are “IgD-deficient,” CBA/N and NZB, and spleen cells from 2-week-old BDF/1 mice, which have a low δ/µ ratio, displayed a coordinate decrease in the levels of both δ2L2 and δL. This coordinate decrease resulted in no alternation in the percentage of IgD present in the δL form (∼42%). These results, taken together, indicate that indeed the cell surface Ig of murine lymphoid cell populations consists of monomeric IgM, monomeric IgD, and IgD half-molecules.
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Leon Eidels (1979) studied this question.