Macro creatine kinase: a study on CK-linked immunoglobulin.

We describe three cases in which creatine kinase (CK, EC 2.7.3.2) was linked to immunoglobulin in serum. In this study, its prevalence was 0.8%. Enzyme-immunofixation electrophoresis revealed that the heavy chain of CK-linked immunoglobulins was of class alpha in all cases. The light-chain type was identified as lambda in two cases and as both lambda and kappa in one case. The complexes were dissociated at pH 3.4 and re-formed with CK isoenzymes MM and MB at pH 7.4. The complex fraction obtained by gel filtration was not inhibited by anti-CK-M antibodies. Treatment of the serum with urea after heating shows residual CK-MM activity; in contrast, normal CK activity disappeared entirely after this treatment. The present study suggests that CK-linked immunoglobulins may be one of the circulating immune complexes and must be distinguished from other macro-CK in the strict sense. The results obtained show that the presence of the complexes results in false-positive CK-B activity in the immuno-inhibition test, and they may provide interesting insights into the mode of binding of the CK-linked immunoglobulins.