Catalyzes the biosynthesis of guanidinoacetate, the immediate precursor of creatine. Creatine plays a vital role in energy metabolism in muscle tissues. May play a role in embryonic and central nervous system development. Also capable of catalyzing the synthesis of a range of neuroactive guanidino compounds by utilizing alternative amidine donors and acceptors for the transamidination reaction. glycine + L-arginine = guanidinoacetate + L-ornithine Two forms of the enzyme (denoted alpha and beta) with slightly different kinetic properties are present in cellular extracts. The molecular basis of the differences is unclear. Amine and polyamine biosynthesis; creatine biosynthesis; creatine from L-arginine and glycine: step 1/2. Homodimer. Highly expressed in the kidney and pancreas, especially in the proximal tubules of the kidney, and alpha cells of the pancreatic islets (at protein level). Moderately expressed in liver hepatocytes (at protein level). Expressed in the kidney, pancreas, liver, colon, ileum, jejunum, heart and skeletal muscle. In reproductive tissues, expressed in the testis, epididymis, ovary, oviduct and uterus. Expressed throughout the brain in neurons, astrocytes and oligodendrocytes. In E12.5 embryos, it is expressed in the middle part of the somites, hepatic primordium and wall of the dorsal aorta. Expressed in E15.5 embryos in isolated cells throughout the central nervous system, skeletal muscles, gonad primordia, caudal somites, liver and pancreas, but not in the choroid plexus, root ganglia or kidney. Expressed in skeletal muscle, kidney, pancreas, central nervous system, liver and intestine epithelial cells, but not in epidermis, dermis, olfactory epithelium, trachea, lung, stomach or heart in E18.5 embryos. Expressed throughout embryogenesis. Expression in the colon and small intestine is highest in newborns and declines with age. Expression in the kidney increases with age. Expression is induced by growth hormone and repressed by dietary intake of creatine. Belongs to the amidinotransferase family.