Edison EE, Brosnan ME, Meyer C, Brosnan JT. Creatine synthesis: production of guanidinoacetate by the rat and human kidney in vivo. Am J Physiol Renal Physiol 293: F1799-F1804, 2007. First published October 10, 2007; doi:10.1152/ajprenal.00356.2007.-A fraction of the body's creatine and creatine phosphate spontaneously degrades to creatinine, which is excreted by the kidneys. In humans, this amounts to ϳ1-2 g/day and demands a comparable rate of de novo creatine synthesis. This is a two-step process in which L-arginine:glycine amidinotransferase (AGAT) catalyzes the conversion of glycine and arginine to ornithine and guanidinoacetate (GAA); guanidinoacetate methyltransferase (GAMT) then catalyzes the S-adenosylmethioninedependent methylation of GAA to creatine. AGAT is found in the kidney and GAMT in the liver, which implies an interorgan movement of GAA from the kidney to the liver. We studied the renal production of this metabolite in both rats and humans. In control rats, [GAA] was 5.9 M in arterial plasma and 10.9 M in renal venous plasma for a renal arteriovenous (A-V) difference of Ϫ5.0 M. In the rat, infusion of arginine or citrulline markedly increased renal GAA production but infusion of glycine did not. Rats fed 0.4% creatine in their diet had decreased renal AGAT activity and mRNA, an arterial plasma [GAA] of 1.5 M, and a decreased renal A-V difference for GAA of Ϫ0.9 M. In humans, [GAA] was 2.4 M in arterial plasma, with a renal A-V difference of Ϫ1.1 M. These studies show, for the first time, that GAA is produced by both rat and human kidneys in vivo. L-arginine:glycine amidinotransferase; transamidinase; amino acid metabolism CREATINE AND CREATINE PHOSPHATE act to buffer the cytosolic ATP/ADP ratio in tissues that have high and variable rates of ATP usage (e.g., skeletal and cardiac muscle). Creatine kinase catalyzes the reversible transfer of the ␥-phosphate group of ATP to the guanidino group of creatine to yield ADP and creatine phosphate.ATP ϩ Creatine 7 ADP ϩ Creatine Phosphate (Equation 1) In skeletal muscle, creatine kinase activity is high, keeping its reaction at near-equilibrium. This keeps the ADP and ATP concentrations fairly constant and buffers the cytosolic phosphorylation potential (18,27,28). Energy storage and transmission by the creatine kinase system are hypothesized to work in two ways: as a temporal buffer and as a spatial buffer. The temporal energy buffer theory is best exemplified during episodes of high-energy use, such as muscle contraction. As soon as ATP is hydrolyzed, it must be replenished. The high-energy phosphate of creatine phosphate is transferred to ADP to regenerate ATP. This leads to an accumulation of creatine that must be rephosphorylated during recovery from exercise. The spatial energy buffer theory implies that creatine phosphate acts as an energy carrier, working to transport high-energy phosphate from sites of synthesis (mitochondria) to sites of ATP utilization in the cytosol (27
Creatine is essential for normal neural development; children with inborn errors of creatine synthesis or transport exhibit neurological symptoms such as mental retardation, speech delay and epilepsy. Creatine accretion may occur through dietary intake or de novo creatine synthesis. The objective of the present study was to determine how much creatine an infant must synthesise de novo. We have calculated how much creatine an infant needs to account for urinary creatinine excretion (creatine's breakdown product) and new muscle lay-down. To measure an infant's dietary creatine intake, we measured creatine in mother's milk and in various commercially available infant formulas. Knowing the amount of milk/formula ingested, we calculated the amount of creatine ingested. We have found that a breast-fed infant receives about 9 % of the creatine needed in the diet and that infants fed cows' milk-based formula receive up to 36 % of the creatine needed. However, infants fed a soya-based infant formula receive negligible dietary creatine and must rely solely on de novo creatine synthesis. This is the first time that it has been shown that neonatal creatine accretion is largely due to de novo synthesis and not through dietary intake of creatine. This has important implications both for infants suffering from creatine deficiency syndromes and for neonatal amino acid metabolism.
During lactation, there may be a higher need for creatine replacement due to the provision of creatine to the milk. Our objectives were to: 1) quantify the creatine concentration in rat milk; 2) determine the origin of milk creatine; 3) determine the activities of the enzymes of creatine synthesis in lactating rats and pups; and 4) quantify the origin of the creatine that accumulates in rat pups. The origin of milk creatine was determined in 4 dams following the administration of (14)C-creatine by measuring the isotopic enrichment of creatine in the milk and plasma. The activities of the 2 enzymes involved in creatine synthesis were compared in lactating and virgin females (n = 7). For all experiments, the litter size was standardized to 8 pups. The data indicated that the mammary gland extracts creatine from the circulation rather than synthesizing it. This was confirmed by our failure to find substantial activities of the enzymes of creatine synthesis in mammary glands. The provision of milk creatine requires an additional 35-55% of creatine above the daily requirement by lactating rat dams. However, there was no increased creatine synthesis by these dams; the additional creatine was largely provided by hyperphagia, because creatine is present in commercial rat diet. There was a substantial accumulation of creatine in the growing pups, but only approximately 12% was obtained from milk. The great bulk of creatine accretion was via de novo synthesis by the pups, which imposed a substantial metabolic burden on them.
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