Luc Cynober

Ornithine α-ketoglutarate (OKG) has proved to be efficient in restoring glutamine (Gln) pools which are strongly depleted in hypercatabolic patients. Since its two components, α-ketoglutarate (αKG) and ornithine (Orn), give rise to glutamate (Glu), they are both considered as Gln precursors. The aim of this study was to assess the relative contributions of Orn and αKG to Gln generation in a rat model of burn injury. Forty-eight young Wistar rats were scalded to give a 20% burn surface area. They were fasted for 24 h and then refed by enteral nutrition for 48 h by gavages with Osmolite (Abbott-Ross, 210 kcal/kg day^–1, 1.18 N/kg day^–1) supplemented with either 5 g OKG/kg day^–1 (B-OKG), Orn (isomolar to OKG; B-Orn), αKG (isomolar to OKG; B-KG) or glycine (as an isonitrogenous control; B-Gly). Rats in the B-KG group also received glycine to make all the groups isonitrogenous. Amino acid concentrations were determined in plasma, muscles, jejunal mucosa and liver. The α-KG-enriched diet had no effect on plasma Glu content or plasma and muscle Gln content compared with the burn-injured controls. The Orn-enriched diet significantly increased (p < 0.01) muscle Glu and Gln contents but not plasma Gln content. In OKG-treated animals, plasma Gln as well as muscle Glu and Gln were significantly higher than in the control (p < 0.01), α-KG-treated (p < 0.01) and Orn-treated (p < 0.05 for muscle Gln and p < 0.01 for plasma Gln) animals. OKG was more efficient than Orn or αKG alone in restoring Gln pools in plasma and muscle, which is evidence of metabolic interaction between the two components of this molecule.

Amino acids (AAs) may be consumed at intakes above those that could be obtained from the normal diet, to promote health status in certain specific situations (e.g., sports training, aging). In this context, the relevant AAs may be used at high intake levels, which may in turn trigger adverse effects. There is little information on the adverse effects or pathophysiological consequences of excessive intakes of individual amino acids or mixtures. Hence, a series of workshops (named AAAW) are being organized to bring together experts in the fields of amino acid metabolism and nutritional effects, cell and molecular biology, toxicology, and regulatory issues and policy, with the aim of establishing a paradigm for the characterization of risks associated with specific intakes of amino acids by humans. The first 3 workshops covered general aspects concerning AAs (functions, risk characterization, differences in subpopulations, definition of surrogate markers, etc.). The fourth AAAW focused on branched-chain AAs. The articles in this supplement issue of the journal summarize the fifth workshop in the series, which focused on sulfur amino acids.

Ornithine α-ketoglutarate (OKG) is a salt formed of 2 molecules of ornithine and 1 α-ketoglutarate. Its administration improves nutritional status in chronically malnourished (e.g., elderly) and acutely malnourished patients (especially burn and trauma patients). There is evidence that OKG activity is not the simple addition of the effects of ornithine (Orn) and α-ketoglutarate (αKG), because the presence of both moieties is required to induce the generation of key metabolites such as glutamine, proline, and arginine (Arg), whereas this does not occur when one or the other is given separately. This observation is related to the fact that the main feature of Orn at the whole-body level is to be metabolized through the Orn aminotransferase-dependent pathway, whereas the simultaneous administration of Orn and αKG saturates this pathway, diverting Orn toward metabolism into Arg. For years, OKG activity has been associated with its ability to induce the secretion of anabolic hormones, such as insulin and growth hormone, and to increase glutamine and polyamine synthesis. Recent studies using chemical inhibitors of nitric oxide synthase (NOS) suggest that nitric oxide derived from Arg could be partly involved in OKG activity. The use of genetically modified animals (i.e., knockout for NOS expression) is required to confirm this hypothesis.

Young: Now we will turn to the general discussion that we have included in the program. What we thought we would like to do is give all of you an opportunity to contribute to the discussion: to think about how your presentation possibly might lead us closer to identifying, or help us to identify, upper levels of amino acid intake. But before throwing this to the audience, we are going to ask three individuals to make five minutes worth of viewpoints, comments, or reactions. And these three individuals will be Dr. Kraisid Tontisirin from FAO, Dr. John Fernstrom from University of Pittsburgh, and Dr. Kyoichi Kishi from University of Tokushima. Would you like to start, Kraisid? Say whatever you want that is useful to the purpose of this workshop.

Cynober: This discussion section deals primarily with this question: which model for which amino acid? To start the discussion, I would like to ask a question to Vicky Baracos. In the figure you showed us about the extraction of amino acids by portal-drain viscera, I noticed that there was no consumption of arginine and no production of citrulline in your piglet model. What is the reason for this? Is it related to enzyme immaturity? Based on Bachier and co-workers' studies, it seems that there is an adaptation with age for amino acids related to proline; this means that this model should be, with regard to this type of amino acid (arginine, proline, glutamate, and so on), nice for the study of amino acid supplementation in infants, but inadequate for adults.

Fürst: This discussion session is devoted to the major question: what should we do about variability in response among individuals and groups? This was the question proposed by the organizer. Personally, I am also very interested in how we consider this question in the clinical setting. The major question then is whether the variability is a disease-related problem or not. For instance, it was shown that high arginine containing immune-enhancing diets increased mortality and complications. The immediate consequence was that one of the companies withdrew a preparation with high-arginine content intended to treat critically ill patients. I would like now to open the discussion.

Hebuterne: Now it is the time for our general discussion. We will discuss the amino acid requirements in pathologic states. To introduce this debate I would like to tell you that for a clinician it is really a very important problem. As Peter Fürst said in the first lecture, there are very few data on amino acid requirements in wasting diseases and many other pathologic states.

Young: We now come to the general discussion and to begin this session I wish to pose the question: “How can my research and knowledge base lend itself to defining, for various conditions in humans, the safe range of amino acid intake (from a minimum requirement intake to a level that would result in an adverse health outcome)?” I suspect that Luc Cynober will ask you a similar question to stimulate your participation in this general discussion. However, before opening up this session it is worth recalling that over the last day and a half we have focused our attention at the cellular and subcellular level; we have heard, for example, about the regulation of gene expression, by some amino acids, and of mRNA recruitment and translation as well as the impact of amino acids, that seem to me to be at relatively high levels, on the macroautophagic process of protein degradation. We also heard about the roles of amino acids in nonproteinogenic pathways, including the fact that this role is coupled to the cellular energy state. It was really quite exciting to learn that amino acids coordinate fluxes through major metabolic pathways. This is an exciting and new vision of what amino acids do in terms of cell function and metabolism. We also focused our attention, in part, at a whole-body level, and we heard about kinetic and interorgan kinetics and flux, indicating to us the complexity of the kinetic models, especially when looking at the nonsteady state.