Stability of macro creatine kinases and creatine kinase isoenzymes compared: heat inactivation test for determination of thermostable creatine kinases.

Creatine kinase isoenzymes showed decreasing thermal stability and increasing lability towards pH changes in the order: MM, MB, and BB. The three isoenzymes exhibited their highest stability between pH 6.5 and 7.0. At 37 degrees C and an almost physiological pH of 7.5 the decay constants were 0.025, 0.164 and 0.580 h-1 (MM, MB, and BB isoenzyme), respectively. In contrast to free creatine kinase BB, immunoglobulin-linked creatine kinase BB (macro creatine kinase BB, type 1 macro creatine kinase) showed a markedly higher stability; this accounts for the persistence of creatine kinase BB activity in macro creatine kinasaemia. In addition we identified a second type of macro creatine kinase in patients' sera, which is also thermostable. A simple heat inactivation test (20 minutes, 45 degrees C, immunoinhibition of the M-subunits) differentiates thermostable macro creatine kinases from thermolabile creatine kinases and thus completes isoenzyme diagnosis.