Metabolism 2000: the emperor needs new clothes

In Vivo Stable Isotope Measurements of Methyl Metabolism: Applications in Pathophysiology and Interventions

With intravenous infusion of doubly-labeled [2H3C-1-13C-] methionine and stable isotope enrichments in plasma free methionine and carbon dioxide in breath air, whole body transmethylation, transsulfuration, and remethylation rates can be calculated. This technique demonstrated impaired recycling as the major disturbance to explain hyperhomocysteinemia in patients with end-stage renal failure, and can be used to optimize interventions with folate, B6, and B12 supplementation in this patient group. Intravenous infusion of [2,3,3-2H3] serine has also been applied to demonstrate the appearance of [2H2]- as well as [2H1]-methionine in plasma and protein, suggesting transfer of a one-carbon group from serine via 5,10-methylenetetrahydrofolate in human hepatocyte cytosol and mitochondria, respectively. In sheep, tissue free methionine enrichments after infusion of universally labeled [U-13C] methionine showed the highest remethylation activity in postmortem investigation of jejunum, liver, and kidney tissue samples, but no such activity in muscle and brain samples. Methods to quantitate one-carbon acceptor metabolism pathways and folate metabolism have recently become available.

Integrating anticipated nutrigenomics bioscience applications with ethical aspects

Nutrigenomics is a subspecialty of nutrition science which aims to understand how gene-diet interactions influence individuals' response to food, disease susceptibility, and population health. Yet ethical enquiry into this field is being outpaced by nutrigenomics bioscience. The ethical issues surrounding nutrigenomics face the challenges of a rapidly evolving field which bring forward the additional dimension of crossdisciplinary integrative research between social and biomedical sciences. This article outlines the emerging nutrigenomics definitions and concepts and analyzes the existing ethics literature concerning personalized nutrition and presents "points to consider" over ethical issues regarding future nutrigenomics applications. The interest in nutrigenomics coincides with a shift in emphasis in medicine and biosciences toward prevention of future disease susceptibilities rather than treatment of already established disease. Hence, unique ethical issues emerge concerning the extent to which nutrigenomics can alter our relation to food, boundaries between health and disease, and the folklore of medical practice. Nutrigenomics can result in new social values, norms, and responsibilities for both individuals and societies. Nutrigenomics is not only another new application of "-omics" technologies in the context of gene-diet interactions. Nutrigenomics may fundamentally change the way we perceive human illness while shifting the focus and broadening the scope of health interventions from patients to healthy individuals. In resource- and time-limited healthcare settings, this creates unique ethical dilemmas and distributive justice issues. Ethical aspects of nutrigenomics applications should be addressed proactively, as this new science develops and increasingly coalesces with other applications of genomics in medicine and public health.

Trace element biology: the knowledge base and its application for the nutrition of individuals and populations

Impressive strides are being made in the understanding of trace element metabolism and function. This is underscored by the many contributions in these proceedings. However, not so impressive are: i) the precise recognition of mild trace element deficiencies and how to establish their functional consequences, possibly confounded by concurrent trace element inadequacies, are difficult to assess, ii) approaches to the quantitative determination of requirements for trace elements remain unsatisfactory and archaic, in so many ways, iii) our understanding of the extent of the biological basis for the variation in requirements among apparently similar individuals is poor, and iv) much needs to be learned about the quantitative extent to which genetic, epigenetic and dietary factors interact to determine the nutritional phenotype. Some ideas are presented as to how we might embrace, in the context of a reconstructive approach, the exciting new knowledge and related techniques emerging during the postgenome era and develop new paradigms for assessing trace element needs and status, and for establishing effective nutrient intake under different conditions of complex genotype-environment interactions. Metabolites are functional cellular entities and I also urge a vigorous application of metabolomics and of metabolic profiling that is closely linked with genomics, proteomics, trace element kinetics and system analysis, as components of the new integrative paradigm. We need to understand the system and its strategy, not only the molecular details of its component parts and its individual controls. An interdisciplinary research and teaching enterprise will be necessary to best achieve this aim. All of this is related to our common goal to promote, through expanded biological knowledge and its effective application, the enhanced role of trace elements for human well-being.

Arginine and nitric oxide metabolism in critically ill septic pediatric patients

OBJECTIVE

To investigate whole body, arginine metabolism and nitric oxide synthesis rates in septic, critically ill pediatric patients.

DESIGN

Prospective study.

SETTING

Pediatric intensive care unit at a general hospital.

PATIENTS

Ten consecutive septic patients age 6-16 yrs.

INTERVENTIONS

Septic patients received an 8-hr primed, constant intravenous tracer infusion of L-[guanidino-15N2]arginine, L-[1-13C]leucine, and [13C]urea. A 24-hr urine collection was obtained for determination of [15N]nitrate enrichment (15NO3(-)) and urinary nitrogen. The next day they received an infusion of L-[5-13C]arginine and L-[5-13C-ureido, 5,5, 2H2]citrulline. Blood samples were obtained for determination of plasma isotopic enrichment of the tracers given and of derived [15N]citrulline (nitric oxide synthesis), L-[13C-guanidino 5,5, 2H2]arginine (M+3 arg) (arginine synthesis), and [15N]urea (urea formation). Data are compared with historic controls from studies in healthy young adults.

MEASUREMENTS AND MAIN RESULTS

Plasma arginine fluxes were 67 +/- 21 and 72 +/- 17 micromol x kg(-1) x hr(-1), respectively, for the [15N2 guanidino] and the [13C] arginine labels, which were not different from reported adult values. The rates of arginine oxidation were 22.9 +/- 10.8 micromol x kg(-1) x hr(-1) and were higher than arginine synthesis rates of 9.6 +/- 4.2 micromol x kg(-1) x hr(-1) (p <.01); therefore, these patients were in a negative arginine balance. The rates of nitric oxide synthesis as estimated by the [15N]citrulline method were 1.58 +/- 0.69 micromol x kg(-1) x hr(-1) for septic patients and higher (p <.05) than values of 0.96 +/- 0.1 micromol x kg(-1) x hr(-1) in healthy adults. Septic patients were in a negative protein (leucine) balance of about -1.00 +/- 0.40 g x kg(-1) x day(-1).

CONCLUSIONS

Homeostasis of plasma arginine in septic patients was impaired compared with reported adult values. The rates of arginine oxidation were increased whereas net arginine synthesis was unchanged, leading to a negative arginine balance. The rates of nitric oxide synthesis and the fraction of plasma arginine used for nitric oxide and urea formation were increased. These findings suggest that under condition of sepsis, arginine becomes essential in critically ill children.

Plasma L-5-oxoproline kinetics and whole blood glutathione synthesis rates in severely burned adult humans

Compromised glutathione homeostasis is associated with increased morbidity in various disease states. We evaluated the kinetics of L-5-oxoproline, an intermediate in the gamma-glutamyl cycle of glutathione production, in fourteen severely burned adults by use of a primed, constant intravenous infusion of L-5-[1-(13)C]oxoproline. In nine of these patients, whole blood glutathione synthesis and plasma kinetics of glycine and leucine were also measured with [(15)N]glycine and L-[(2)H(3)]leucine tracers. Patients were studied under a "basal" condition that provided a low dose of glucose and total parenteral nutrition. For comparison with control subjects, whole blood glutathione synthesis was estimated in six healthy adults. Burn patients in a basal condition showed significantly higher rates of plasma oxoproline clearance and urinary D- and L-oxoproline excretion compared with fasting healthy control subjects. Whole blood glutathione concentration and absolute synthesis rate in the basal state were lower than for control subjects. Total parenteral feeding without cysteine but with generous methionine did not affect oxoproline kinetics or whole blood glutathione synthesis. The estimated rate of glycine de novo synthesis was also lower in burn patients, suggesting a possible change in glycine availability for glutathione synthesis. The roles of precursor amino acid availability, as well as alterations in metabolic capacity, in modulating whole blood glutathione production in burns now require investigation.