Creatine is an endogenous substance that is present in every human cell. It functions as an energy storehouse. Creatine is required for physical and mental exertion. In the body, creatine is synthesized from the amino acids glycine, arginine, and methionine, primarily in the liver, kidneys, and pancreas, and it is transported from there to all the cells in the body via the bloodstream. Creatine is involved in all processes that require energy. Muscle, brain and nerve cells utilize large amounts.
The enzyme GATM (L-arginine:glycine amidinotransferase is a mitochondrial enzyme responsible for catalyzing the first rate-limiting step of creatine biosynthesis, and is primarily expressed in the kidneys and pancreas. The second enzyme in the pathway (GAMT, guanidinoacetate N-methyltransferase, is primarily expressed in the liver and pancreas.
Genetic deficiencies in the creatine biosynthetic pathway lead to various severe neurological defects.
The creatine reserves of a person who weighs 70 kg equal about 120 grams. The vast majority of creatine (app. 95%) is stored in the skeletal muscles. Creatine is primarily involved in muscle contraction. It is taken up from the blood into the cell membrane by means of a sodium-dependent creatine transporter.1
References
1) Speer, Oliver. Methylglyoxal, creatine and mitochondrial micro-compartments / by Oliver Speer. (2003). http://e-collection.ethbib.ethz.ch/view/eth:26877