Studies on a fragment of human IgM macroglobulins related to "J chain"

IgM-producing gastric plasmacytoma

A case of gastric plasmacytoma is reported in an 81-year-old woman. Immunofluorescent and immunoelectrophoretic studies showed that the tumor cells produced IgM-lambda molecules, whereas no monoclonal immunoglobulin could be detected both in the serum and in the urine. Of more than 60 reported cases of gastric plasmacytoma, only a few reports referred to the production of monoclonal immunoglobulin by the tumor cells. This case is the first one of IgM-producing gastric plasmacytoma. Moreover, the produced IgM was found to be 7S-IgM. It is extremely rare that 7S-IgM alone is detected in the absence of 19S-IgM.

Esterification of J chain and its effect on electrophoretic mobility in sodium dodecyl sulfate polyacrylamide gels

Sodium dodecyl sulfate polyacrylamide gel electrophoresis yields mobilities indicative of molecular weights of approximately 27 000 for human J chain and approximately 14 000 for nurse shark J chain, in contrast to values of approximately 15 500 and approximately 12 200, respectively, obtained by other methods. The relatively high content of acidic amino acids of human J chain as compared to nurse shark J chain suggested that the greater error in the sodium dodecyl sulfate determined molecular weight for human J chain may be due to a charge anomaly. The overall net negative charge on human J chain was decreased by forming methyl esters of the carboxyl groups, resulting in a sodium dodecyl sulfate determined molecular weight of approximately 17 700. Methylated nurse shark J chain did not show a significant difference in sodium dodecyl sulfate electrophoretic mobility from the non-methylated form. These results form the basis for a possible explanation of an occasional artifact in a widely used analytical method.

Purification of J chain after mild reduction of human immunoglobulins

Two methods are described for the purification of J chain from polymeric IgA after mild reduction without the use of alkylating or dissociating reagents. The released peptide was separated from other protein components by immunoadsorption combined with gel filtration or anionic-exchange chromatography, or both. J chain was thus obtained in a yield of about 30% of the total release. Most of it consisted of dimers (molecular weight, approximately 25,000 to 30,000) or larger polymers, but re-reduction and alkylation produced a quite homogeneous fraction that sedimented slightly more slowly than egg-white lysozyme. The purity was high enough for successful immunization. When J chain coupled to bovine serum albumin was used as an antigen, all of five rabbits showed a good immune response. Although the same principle could be used for the purification of J chain from IgM and colostral IgA, high purity was more difficult to achieve and the yield was much lower. These preparations contained an unidentified slow-moving component, and the J chain was more prone to become rapidly degraded to smaller fragments.

Site of J chain attachment to human polymeric IgA

A fragment containing J chain was released from human polymeric myeloma IgA protein by cyanogen bromide cleavage. The identity of the fragment was determined by its electrophoretic mobility and antigenic determinants. After purification by gel filtrations and DEAE-Sephadex chromatography, this fraction appeared similar (with respect to its amino acid and carbohydrate compositions and its peptide maps) to the J chain isolated from this IgA protein; the molecular weight was 17,000 +/- 100. Upon reduction and alkylation, with subsequent separation of peptides by gel filtration, three components were obtained: the largest component (molecular weight 13,400) corresponded to the N-terminal segment of J chain and contained a homoserine residue, the second corresponded to the C-terminal part of J chain with 13-18 amino acid residues, and the third corresponded to the C-terminal octapeptide of the alpha chain. The data indicate that J chain is attached to alpha chain(s) through the penultimate cysteine residue of the C-terminal octapeptide.

The molecular weight of J chains derived from human immunoglobulin M

J chain was isolated from sulphonated human immunoglobulin M molecules by electrophoresis on polyacrylamide gels. When determined by electrophoresis in sodium dodecyl sulphate-polyacrylamide gels, the molecular weight of the protein was about 27000. After suspension in 5m-guanidine hydrochloride solution for 21 days, two groups of three bands appeared on the gels. Most of the protein dissociated to components of molecular weight 15000. The molecular weight of purified J chain was also determined by ultracentrifugation. In borate-saline solution the average weight-average molecular weight was about 29000. The molecular weight slowly decreased upon prolonged exposure to guanidine hydrochloride, and after 14 days the minimum molecular weight was about 15000. Some association between chains still existed. These data suggest that J chain derived from the paraprotein exists in borate-saline solution as dimers held by strong non-covalent forces.

The incorporation of J chain into reassembled human immunoglobulin M

Human IgM (immunoglobulin M) was reduced with 24mm-mercaptoethylamine. This atreatment resulted in complete dissociation to IgMs subunits and free J chain. Intr-subunit interchain disulphide bonds remained intact. The mixture then was encouraged to reoxidize. The schlieren pattern of the reoxidized mixture showed the presence of a considerable quantity of IgM in addition to residual IgMs. The isolated reassembled IgM did not dissociate in 5m-guanidinium hydrochloride. It apparently contained the same amount of covalently attached J chain as did native IgM. The J chain was a part of the high-molecular-weight Fc fragment obtained from the reassembled IgM.