We determined total CK activity with the N-acetylcysteine-activated method and residual activity after immunoinhibition of the CK-M subunits in the sera of 2018 patients consecutively admitted to our university hospital for internal diseases, and of 936 outpatients, regardless of the patients' diagnoses. We could detect not more than two types of macro CK: macro CK type 2, which we observed in the sera of 85 patients (prevalence, 3.7% for hospitalized patients), and macro CK type 1. Most patients showing macro CK type 2 were older than 50 years, but we additionally observed a second peak at 20-30 years of age. We saw no preponderance by sex. We detected macro CK type 2 predominantly in severely ill patients of all ages, mainly those with malignant tumors or cirrhosis of the liver. Our findings support the assumption that macro CK type 2 is the manifestation of mitochondrial CK in serum. Occasionally, macro CK type 2 disappeared from the circulation after amelioration of the associated disease. Its occurrence in serum nevertheless is a sign of a serious illness with high mortality but not inevitably a sign of impending death.

Summary: 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 °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 °C, immunoinhibition of the M-subunits) differentiates thermostable macro creatine kinases from thermolabile creatine kinases and thus completes isoenzyme diagnosis.

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