Isoenzyme-specific thermostability of human cytosolic creatine kinase

Creatine kinase (CK) is a key enzyme involved in intracellular energy homeostasis. The distinct tissue distribution of muscle CK (MMCK) and brain CK (BBCK) implies that they function under conditions facing dissimilar environmental stresses. We found that MMCK and BBCK were significantly different in their stability and reversibility against heat stress. MMCK was more stable than BBCK, and BBCK was only marginally stable and began to inactivate at temperatures just above normal body temperature. The thermal inactivation of MMCK was fully irreversible, whereas that of BBCK was highly reversible at temperatures below 55 °C. These differences in stability were proposed to be closely correlated to the isoenzymes’ adaptation to the distinct tissue environments.