Arginine metabolism and creatine biosynthesis in Yucatan miniature piglets

Creatine and creatine phosphate are amino acid-derived compounds that are necessary to meet short-term energy requirements. Creatine is a potent neuromodulator such that it is critical for neurological development in neonates. Neonates receive creatine in mother's milk. However, up to 77% of the daily creatine requirement must be synthesized endogenously. Creatine synthesis involves the conversion of arginine to guanidinoacetic acid (GAA) via L-arginine:glycine amidinotransferase (AGAT). The subsequent conversion of GAA to creatine requires methionine and guanidinoacetate N-methyltransferase (GAMT). Following preterm birth, total parenteral nutrition (PN) is often required as a means of nutritional support. However, creatine is not a component of pediatric PN. In this situation, the entire creatine requirement must be met by de novo synthesis which consequently must create considerable demand for the amino acid precursors. Poor arginine status is common in neonates during PN, and this may compromise optimal creatine accretion. In this thesis, the capacity of the neonatal piglet to synthesize creatine was addressed, particularly when dietary creatine was not supplied. The data from first experiment demonstrated that PN support with creatine led to greater creatine concentrations in plasma and tissues, suggesting that neonates receiving PN may not be able to sustain optimal creatine accretion. The second major objective of this thesis was to investigate whether the low creatine accretion in piglets fed creatine-free diets was due to limited enzyme capacity or limited substrate availability. Using a multiple isotope tracer method, we determined that inadequate dietary arginine and methionine, not enzyme capacity, limits GAA and creatine biosynthesis. Lastly, because we measured high AGAT activity in kidney and pancreas, we investigated how the plasma concentrations of precursor amino acids (arginine/citrulline) and creatine affected the release of GAA and creatine from these organs as well as from the intestine. We determined both arginine and citrulline can provide precursors for renal GAA synthesis. However, in the pancreas and intestine, citrulline cannot provide arginine to serve as a precursor for GAA. Overall, the results of this thesis clearly demonstrate that dietary concentrations of arginine and methionine are important mediators of creatine synthesis; the absence of dietary creatine, such as during PN feeding, must be taken into consideration when determining dietary requirements of neonates.