Heterogeneity of Myelomia Proteins

The normal gamma globulins are notably heterogeneous in electrophoretic properties, extending in a continuous spectrum throughout a wide portion of the serum globulins in man and many other species (1). In contrast, the serum myeloma proteins found in patients with malignant plasma cell disease form discrete electrophoretic peaks. The apparent homogeneity of human myeloma proteins has been emphasized on the basis of observations made by zone electrophoresis. immiiunoelectrophoresis, and ultracentrifugation (2-4). The first evidence that some myeloma proteins might be heterogeneous came from ultracentrifugal analyses which showed that a few myeloma sera contained 9S, 11S, or 13S components, or all, as well as 6.6S components (5-10). Investigation of myeloma sera by means of starch gel electrophoresis also indicated that some myeloma proteins were heterogeneous. Owen, Got, and Silberman (11) studied 16 sera and noted that in seven, 44 per cent, the myeloma proteins were composed of several closely grouped components and that in six, 38 per cent, a less tightly banded pattern of heterogeneity was present. Flynn and Stow ( 12) reported that three of 11 myeloma proteins contained two to five components, and Fine, Creyssel. and Morel (13, 14) observed heterogeneity in some myeloma proteins of beta mobility but not in any of 15 sera with myeloma proteins of gamma globulin mobility. Laurell (15) reported that 24 per cent of 226 M components were heterogeneous on starch gel electrophoresis. Engle, WVoods, Castillo, and Pert (16) found 27 per cent of myeloma proteins had two or more components and noted that in some of these cases there was "periodicity in the placement of peaks while in others the peaks were in widely and irregularly separated regions of the gel." The basis for the heterogeneity, however, was not clear, since the components of each rnyeloma protein had not been separated and studied in detail. Askonas (17) detected five or more components on starch gel electrophoresis of the myeloma protein produced by the transplantable mouse plasma cell tumor X5563, and separated the components by diethylaminoethyl (DEAE)-cellulose chromatography. She found that the components had the same ultracentrifugal properties and concluded that the heterogeneity of the X5563 myeloma protein was due to differences in net electrical charge. Recently, 20 different mouse myeloma globulins were studied in detail, and all were found to be heterogeneous (18). Two types of heterogeneity were observed on starch gel electrophoresis. The first type, termed electrophoretic heterogeneity, was revealed by myeloma proteins composed of multiple (three or more), closely approximated, discrete bands, apparently differing in net charge. This was the same pattern descril)ed by Askonas (17) and was seen only with y-myeloma proteins. The second type of heterogeneity was characterized by the presence of several, widely spaced, less discrete myeloma protein components. This latter pattern was observed only with 182A-myeloma proteins which contained 6.6, 9, 11, and 13S components in the ultracentrifuge. It seemed probable that the migration of the larger ultracentrifugal components was impeded by the starch gel, giving rise to the slower migrating bands. On the basis of evidence that the 9, 11, and 13S proteins were made up of polymerized 6.6S components (18), it was suggested that this form of heterogeneity was due to polymer formation rather than electrophoretic differences. Preliminary studies of human myeloma proteins in this laboratory (19) were in accord with the findings of Owen (11), indicating that most myeloma proteins were heterogeneous and that two types of heterogeneity occurred, similar to those seen in the mouse. The finding that heterogeneity is the common, rather than the uncommon, feature of myeloma proteins has a dual significance. In one case, it points to the question of whether multiple myeloma is a malignancy of one cell line producing multiple proteins or whether multiple tumors (myelomas)