Adaptation to a long term (4 weeks) arginine- and precursor (glutamate, proline and aspartate)-free diet.

BACKGROUND & AIMS: It is not known whether arginine homeostasis is negatively affected by a "long term" dietary restriction of arginine and its major precursors in healthy adults. To assess the effects of a 4-week arginine- and precursor-free dietary intake on the regulatory mechanisms of arginine homeostasis in healthy subjects. METHODS: Ten healthy adults received a complete amino acid diet for 1 week (control diet) and following a break period, six subjects received a 4-week arginine, proline, glutamate and aspartate-free diet (APF diet). The other four subjects continued for 4 weeks with the complete diet. On days 4 and 7 of the first week and days 25 and 28 of the 4-week period, the subjects received 24-h infusions of arginine, citrulline, leucine and urea tracers. RESULTS: During the 4-week APF, plasma arginine fluxes for the fed state, were significantly reduced. There were no significant differences for citrulline, leucine or urea fluxes. Arginine de novo synthesis was not affected by the APF intake. However, arginine oxidation was significantly decreased. CONCLUSIONS: In healthy adults, homeostasis of arginine under a long term arginine- and precursor-free intake is achieved by decreasing catabolic rates, while de novo arginine synthesis is maintained.

Estimating glutathione synthesis with deuterated water: a model for peptide biosynthesis.

Glutathione (GSH), an intracellular tripeptide that combats oxidative stress, must be continually replaced due to loss through conjugation and destruction. Previous methods, estimating the synthesis of GSH in vivo, used constant infusions of labeled amino acid precursors. We developed a new method based on incorporation of (2)H from orally supplied (2)H(2)O into stable C-H bonds on the tripeptide. The incorporation of (2)H(2)O into GSH was studied in rabbits over a 2-week period. The method estimated N, the maximum number of C-H bonds in GSH that equilibrate with (2)H(2)O as amino acids. GSH was analyzed by liquid chromatography/mass spectrometry after derivatization to yield GSH-N-ethylmaleimide (GSNEM). A model, which simulated the expected abundance at each mass isotopomer for the GSNEM ion at various values for N, was used to find the best fit to the data. The plateau labeling fit best a model with N=6 of a possible 10 C-H bonds. Thus, the amino acid precursors do not completely equilibrate with (2)H(2)O prior to GSH synthesis. Advantages of this new method include replacing costly amino acid infusions with the oral administration of (2)H(2)O and a statistical basis for estimating N.

Regional measurement of canine skeletal muscle blood flow by positron emission tomography with H2(15)O.

Positron emission tomography (PET) with H2(15)O was used as an in vivo, relatively noninvasive, quantitative method for measuring regional blood flow to hindlimb skeletal muscle of anesthetized dogs. A hydrooccluder positioned on the femoral artery was used to reduce flow, and high-flow states were produced by local infusion of adenosine. Three to four measurements were made in each animal. Approximately 40 mCi of H2(15)O were injected intravenously, and serial images and arterial blood samples were acquired over 2.5 min. Data analysis was performed by fitting tissue and arterial blood time-activity curves to a modified, single-compartment Kety model. The model equation was also solved on a pixel-by-pixel basis to yield maps of regional skeletal muscle blood flow. After each PET determination, flow was measured with radioactive microspheres. Results of the PET measurements demonstrated that basal flow to hindlimb skeletal muscle was 3.83 +/- 0.36 ml x min(-1) x 100 g(-1) (mean +/- SE). This value was in excellent agreement with the microsphere data, 3.73 +/- 0.32 ml x min(-1) x 100 g(-1) (P = 0.69, not significant). Adenosine infusion resulted in flows as high as 30 ml x min(-1) x 100 g(-1), and the PET and microsphere data were highly correlated over the entire range of flows (r2 = 0.98, P < 0.0001). We conclude that muscle blood flow can be accurately measured in vivo by PET with H2(15)O and that this approach offers promise for application in human studies of muscle metabolism under varying pathophysiological states.