Reversibly catalyzes the transfer of phosphate between ATP and various phosphogens (e.g. creatine phosphate). Creatine kinase isoenzymes play a central role in energy transduction in tissues with large, fluctuating energy demands, such as skeletal muscle, heart, brain and spermatozoa. ATP + creatine = ADP + H(+) + N-phosphocreatine Exists as an octamer composed of four MTCK homodimers. In many tissues, with highest levels in brain gut and kidney. In the kidney localized primarily in the outer medulla in the thick ascending limb and distal convoluted tubule. Mitochondrial creatine kinase binds cardiolipin. Belongs to the ATP:guanido phosphotransferase family.