Endogenous creatine synthesis occurs primarily in the liver, but also at a small
rate in the kidney and pancreas. The transfer of a methyl group from S-adenosylmethionine
is the irreversible reaction (committed step) during synthesis. While arginine
participates in the Urea cycle, glycine is a precursor of purine nucleotides, and methionine
contributes its methyl group to DNA and RNA; the endogenous synthesis of creatine
does not interfere with the functions of these amino acids in their respective processes
(Williams et al.1999).