The creatine kinase (CK) enzyme in rat heart tissue appears to have a KM around 6mM of creatine as substrate. and is known to positively influence mitochondrial function as higher cytoplasmic phosphocreatine concentrations (not so much creatine per se) increase the oxidative efficiency of mitochondria This is thought to be due to the transfer of high energy phosphate groups.
Synthesis primarily takes place in the kidney and liver, with creatine then being transported to the muscles via the blood. The majority of the human body's total creatine and phosphocreatine stores is located in skeletal muscle, while the remainder is distributed in the blood, brain, and other tissues. Typically, creatine is produced endogenously at an estimated rate of about 8.3 mmol or 1 gram per day in young adults. Creatine is also obtained through the diet at a rate of about 1 gram per day from an omnivorous diet. Some small studies suggest that total muscle creatine is significantly lower in vegetarians than non-vegetarians, as expected since foods of animal origin are the primary source of creatine. However, subjects happened to show the same levels after using supplements.