The rates of whole body nitric oxide (NO) synthesis, plasma arginine flux, and de novo arginine synthesis and their relationships to urea production, were examined in a total of seven healthy adults receiving an L-amino acid diet for 6 days. NO synthesis was estimated by the rate of conversion of the ['5N] indicating that -11% of the plasma arginine flux originates via conversion of plasma citrulline to arginine. Thus, the fraction of the plasma arginine flux associated with NO and also urea synthesis in healthy humans is small, although the plasma arginine compartment serves as a significant precursor pool (54%) for whole body NO formation. This tracer model should be useful for exploring these metabolic relationships in vivo, under specific pathophysiologic states where the L-arginine-NO pathway might be altered.Arginine serves various important metabolic functions, including roles in protein synthesis, nitrogen transport and elimination (1, 2), and as the precursor of nitric oxide (NO), which is a current area of considerable research interest (3-5). Oxidation of arginine by a homodimeric group of nitric oxide synthases yields citrulline and nitric oxide in stoichiometric amounts, with each containing a nitrogen atom derived from the guanidino moiety of arginine. NO undergoes oxidative degradation to the stable end products nitrite (NO-) and nitrate (NO-). In the in vivo system, NO-is readily oxidized to NO{ via hemoglobin, and so NO is eventually detected in plasma or urine as NOK (5) and its measurement has been used to estimate the rate of whole body NO synthesis.It has been suggested that therapeutic modulation of nitric oxide production may be achieved by supplying the precursor arginine (6-9) or by inhibiting NO production with L-arginine analogs (10-12). However, before profound changes in exogenous arginine intake levels or a pharmacologic inhibition of NO formation can be safely recommended, it is desirable to gain a better understanding of the regulation of whole body arginine metabolism in human subjects and its quantitative interrelationships with the L-arginine-NO pathway.In our previous studies, using stable isotopic tracer techniques, we have investigated whole body arginine metabolism and the use of arginine for NO synthesis, by measurement of the transfer of the guanidino nitrogen of plasma arginine to urinary NO-. These studies have been carried out in healthy humans receiving variable levels of arginine intake (13-15), and in infants with pulmonary hypertension (16). We have now extended these earlier investigations of whole body arginine homeostasis and the metabolism of urea cycle intermediates by examining the kinetics of arginine metabolism throughout a continuous 24-35-hr period. Using this design, it has been possible, for the first time to our knowledge, to determine the in vivo rate of conversion of [15N]guanidino arginine to [15N]ureido citrulline and NO, presumably due to the NO synthase reaction. We compared this estimate with the rate of transfer of the guanidino nitroge...

Treatment with Ringer's ethyl pyruvate solution ameliorated structural and functional damage to the intestinal mucosa in a rat model of mesenteric ischemia/reperfusion. Ethyl pyruvate solution warrants further evaluation as a novel therapeutic agent for preventing oxidant-mediated injury in various disease states.

The fluxes of arginine and citruiline through plasma and the rate of conversion of labeled citrulline to arginine were estimated in two pilot studies (with a total of six adult subjects) and in a dietary study with five healthy young men. These latter subjects received an L-amino acid-based diet that was arginine-rich or arginine-free each for 6 days prior to conduct, on day 7, of an 8-hr (first 3 hr, fast; final 5 hr, fed) primed continuous intravenous infusion protocol using L- [guandno-93C]arginine, L-[5,5-2H2]citrulline, and L- [5,5,5-2H31leucine, as tracers. A pilot study indicated that citrulline flux was about 20% higher (P < 0.05) when determined with [ureido-13C]citrulline compared with [2H2Jcitruline, indicating recycling of the latter tracer. Mean citruilin fluxes were about 8-11 pmol kg'lhr'1 for the various metabolic/diet groups and did not differ significantly between fast and fed states or arginine-rich and arginine-free periods. Arginine fluxes (mean ± SD) were 60.2 ± 5.4 and 73.3 ± 13.9 jAmol kg"l hr'1 for fast and fed states during the arginine-rich period, respectively, and were significantly lowered (P < 0.05), by 20-40%, during the arginine-free period, especially for the fed state, where this was due largely to reduced entry of dietary arginine into plasma. The conversion of plasma citruiline to arginine approximated 5.5 ,umol*kg'l-hr-1 for the various groups and also was unaffected by arginine intake. Thus, endogenous arginine synthesis is not markedly responsive to acute alterations in arginine intake in healthy adults. We propose that argmine homeostasis is achieved largely via modulating arginine intake and/or the net rate of arginine degradation.The physiological needs by tissues and organs for arginine are met via the endogenous synthesis of arginine and/or arginine supplied by the diet. For the U.S. population the latter amounts to about 5.4 g daily per capita (1). The rates of endogenous arginine synthesis in the immature rat (2, 3), guinea pig (4), cat (5, 6), dog (7-9), chicken (10), rabbit (11), and pig (12) of nitric oxide (16) and of creatine and its participation as arginyl-tRNA in the process of ubiquitin-dependent protein degradation (17). Therefore, we have begun to use stableisotope tracer techniques to explore, noninvasively, kinetic and regulatory aspects ofarginine metabolism in adult human subjects (18,19). Here we report results of a study in young men who were given for 7 days an arginine-rich diet and then, for another 7 days, an arginine-free diet. Our kinetic model involves L-[guanidino-13C]arginine and L-[5,5-2H2]citrulline as tracers, to estimate plasma arginine and citrulline fluxes as well as the rate of transfer of plasma citrulline into the arginine pool. From the present findings, and our recent studies (19), we propose an integrative scheme of body arginine homeostasis and balance, which defines the metabolic basis for the conditional indispensability of dietary arginine under various pathophysiological conditions (1, 13, 14). MATERIALS AND METHOD...

The availability of cysteine is thought to be the rate limiting factor for synthesis of the tripeptide glutathione (GSH), based on studies in rodents. GSH status is compromised in various disease states and by certain medications leading to increased morbidity and poor survival. To determine the possible importance of dietary cyst(e)ine availability for whole blood glutathione synthesis in humans, we developed a convenient mass spectrometric method for measurement of the isotopic enrichment of intact GSH and then applied it in a controlled metabolic study. Seven healthy male subjects received during two separate 10-day periods an L-amino acid based diet supplying an adequate amino acid intake or a sulfur amino acid (SAA) (methionine and cysteine) free mixture (SAA-free). On day 10, L-[1-13 C]cysteine was given as a primed, constant i.v. infusion (3mol⅐kg ؊1 ⅐h ؊1 ) for 6 h, and incorporation of label into whole blood GSH determined by GC͞MS selected ion monitoring. The fractional synthesis rate (mean ؎ SD; day -1 ) of whole blood GSH was 0.65 ؎ 0.13 for the adequate diet and 0.49 ؎ 0.13 for the SAA-free diet (P < 0.01). Whole blood GSH was 1,142 ؎ 243 and 1,216 ؎ 162 M for the adequate and SAA-free periods (P > 0.05), and the absolute rate of GSH synthesis was 747 ؎ 216 and 579 ؎ 135 mol⅐liter ؊1 ⅐day ؊1 , respectively (P < 0.05). Thus, a restricted dietary supply of SAA slows the rate of whole blood GSH synthesis and diminishes turnover, with maintenance of the GSH concentration in healthy subjects.

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Whole blood glutathione synthesis rates are decreased, by about 60%, in critically ill septic children receiving limited nutritional support. Plasma cysteine fluxes and concentration of cysteine were increased in the septic patients, suggesting a hypermetabolic state with increased protein breakdown. The mechanisms whereby GSH synthesis rates are decreased in these patients are probably multifactorial, presumably involving an inflammatory response in the presence of limited nutritional support. The role of nutritional modulation and the use of cysteine prodrugs in maintaining GSH concentration and synthesis remain to be established.

In this introductory paper to the symposium, we consider why L-glutamate (GLU) is such an abundant biomolecule. We begin with a brief discussion of the prebiotic dawn of events and some evolutionary features of GLU in the biological and metabolic world. The properties of GLU are then examined with reference to its overall structural motif and to the reactivity of the molecule at the tautomeric 2 carbon and at the 4- and 5-C positions. This chemical viewpoint reveals that the GLU molecule offers a number of features/properties not shared by its homologs (amino adipic and aspartic acids). These properties make GLU a favorable choice for facilitating its involvement in multiple metabolic processes that play major roles in the nitrogen economy of the host, as well as serving as a nutrient, an energy-yielding substrate, a structural determinant and an excitatory molecule.

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.