|
Creatine kinase (CK) — also known as creatine phosphokinase (CPK) or phospho-creatine kinase — is an enzyme () expressed by various tissues and cell types. CK catalyses the conversion of creatine and utilizes adenosine triphosphate (ATP) to create phosphocreatine (PCr) and adenosine diphosphate (ADP). This CK enzyme reaction is reversible and thus ATP can be generated from PCr and ADP. In tissues and cells that consume ATP rapidly, especially skeletal muscle, but also brain, photoreceptor cells of the retina, hair cells of the inner ear, spermatozoa and smooth muscle, PCr serves as an energy reservoir for the rapid buffering and regeneration of ATP ''in situ'', as well as for intracellular energy transport by the PCr shuttle or circuit. Thus creatine kinase is an important enzyme in such tissues. Clinically, creatine kinase is assayed in blood tests as a marker of damage of CK-rich tissue such as in myocardial infarction (heart attack), rhabdomyolysis (severe muscle breakdown), muscular dystrophy, the autoimmune myositides and in acute renal failure. ==Types== In the cells, the "cytosolic" CK enzymes consist of two subunits, which can be either ''B'' (brain type) or ''M'' (muscle type). There are, therefore, three different isoenzymes: CK-MM, CK-BB and CK-MB. The genes for these subunits are located on different chromosomes: ''B'' on 14q32 and ''M'' on 19q13. In addition to those three ''cytosolic'' CK isoforms, there are two mitochondrial creatine kinase isoenzymes, the ''ubiquitous'' and ''sarcomeric'' form. The functional entity of the latter two mitochondrial CK isoforms is an octamer consisting of four dimers each. While mitochondrial creatine kinase is directly involved in formation of phospho-creatine from mitochondrial ATP, cytosolic CK regenerates ATP from ADP, using PCr. This happens at intracellular sites where ATP is used in the cell, with CK acting as an ''in situ'' ATP regenerator. Isoenzyme patterns differ in tissues. Skeletal muscle expresses CK-MM (98%) and low levels of CK-MB (1%). The myocardium (heart muscle), in contrast, expresses CK-MM at 70% and CK-MB at 25–30%. CK-BB is predominantly expressed in brain and smooth muscle, including vascular and uterine tissue. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「creatine kinase」の詳細全文を読む スポンサード リンク
|