The alpha-keto acids of branched-chain amino acids: simplified derivatization for physiological samples and complete separation as quinoxalinols by packed column gas chromatography

The application of stable-isotope tracers to study human musculoskeletal protein turnover: a tale of bag filling and bag enlargement

The nutritional regulation of protein and amino acid balance in human skeletal muscle carried out by the authors with Mike Rennie is reviewed in the context of a simple physiological model for the regulation of the maintenance and growth of skeletal muscle, the "Bag Theory". Beginning in London in the late 1970s the work has involved the use of stable isotopes to probe muscle protein synthesis and breakdown with two basic experimental models, primed-dose continuous tracer infusions combined with muscle biopsies and arterio-venous (A-V) studies across a limb, most often the leg, allowing both protein synthesis and breakdown as well as net balance to be measured. In this way, over a 30 year period, the way in which amino acids and insulin mediate the anabolic effect of a meal has been elaborated in great detail confirming the original concepts of bag filling within the muscle endomysial "bag", which is limited by the "bag" size unless bag enlargement occurs requiring new collagen synthesis. Finally we briefly review some new developments involving ² H₂ O labelling of muscle proteins.

Age attenuates leucine oxidation after eccentric exercise

Aging may alter protein metabolism during periods of metabolic and physiologic challenge. The purpose of this study was to assess the effects of age on whole-body amino acid turnover in response to eccentric exercise and hyperglycemia-induced hyperinsulinemia. 16 healthy men were divided into young (N=8) and older (N=8) groups. Protein metabolism was assessed using a [1-13C]-leucine isotopic tracer approach. Measures were obtained under fasted basal conditions and during 3-h hyperglycemic clamps that were performed without (control) and 48 h after eccentric exercise. Exercise reduced leucine oxidation in the younger men (P<0.05), but not in older men. Insulin sensitivity was inversely correlated with leucine oxidation (P<0.05), and was lower in older men (P<0.05). Healthy aging is associated with an impaired capacity to adjust protein oxidation in response to eccentric exercise. The decreased efficiency of protein utilization in older men may contribute to impaired maintenance, growth, and repair of body tissues with advancing age.

Whole-body and muscle protein metabolism are not affected by acute deviations from habitual protein intake in older men: the Hormonal Regulators of Muscle and Metabolism in Aging (HORMA) Study

BACKGROUND

Acute deviations in protein intake before the quantification of protein kinetics in older humans may explain the controversy over the effects of older age on muscle protein synthesis and proteolysis rates.

OBJECTIVE

We hypothesized that an acute decrease in protein intake from the habitual intake is associated with lower muscle protein synthesis and higher proteolysis rates, whereas an acute increase in protein intake from the habitual intake is associated with higher muscle protein synthesis and lower proteolysis rates.

DESIGN

In 112 community-dwelling healthy men aged 65-90 y, we quantified resting whole-body [1,2-(13)C(2)]leucine kinetics, muscle mixed protein fractional synthesis rates (FSRs), and muscle proteasome proteolytic enzyme activities after participants consumed for 3 d controlled research meals (0.9-1.1 g protein · kg(-1) · d(-1)) that contained more or less protein than that habitually consumed and that induced alterations in nitrogen balance.

RESULTS

Protein kinetic parameters were not significantly different between the groups, despite controlled research protein intakes that were lower (-0.2 to -0.3 g · kg(-1) · d(-1)) or higher (+0.2 g · kg(-1) · d(-1)) than habitual intakes and that induced negative (-22 to -25 mg · kg(-1) · d(-1)) or positive (22-25 mg · kg(-1) · d(-1)) nitrogen balance. Within these acutely altered protein intake and nitrogen balance boundaries, a reduction in protein intake from habitual intake and induction of negative nitrogen balance were not associated with higher proteolysis or lower muscle FSR, and an acute increase in protein intake from habitual intake and induction of positive nitrogen balance were not associated with lower proteolysis or higher muscle FSR. A higher quantitative insulin sensitivity check index was associated with lower whole-body proteolysis rates.

CONCLUSIONS

The practice of acutely controlling protein intake, even at intakes lower than habitual intakes that induce negative nitrogen balance, before quantifying human protein kinetics does not significantly reduce muscle protein synthesis or increase proteolysis. Factors other than protein intake explain lower muscle protein synthesis rates with advanced age. This trial is registered at clinicaltrials.gov as NCT00183040.

Nutrient ingestion, protein intake, and sex, but not age, affect the albumin synthesis rate in humans

The aim of this study was to assess the effects of nutrient ingestion, dietary protein intake, age, and sex on the fractional synthesis rate (FSR) of albumin. Thirty-six healthy free-living individuals (8 females and 10 males aged 21-43 y and 9 females and 9 males aged 63-79 y) completed three 18-d periods of controlled feeding with protein intakes of 125% (P125, 1.00 g protein x kg(-1) x d(-1)), 94% (P94, 0.75 g protein x kg(-1) x d(-1)), and 63% (P63, 0.50 g protein x kg(-1) x d(-1)) of the recommended dietary allowance. On d 12 of each trial, postabsorptive (PA) serum albumin concentration was determined and PA and postprandial (PP) albumin FSR were estimated from the rate of l-[1- 13C] leucine incorporation into plasma albumin during an 8-h infusion. There were no age-related differences in PA and PP albumin FSR. Albumin FSR was higher PP than PA (P < 0.0001), and the increase in albumin FSR from PA to PP was smaller as dietary protein intake decreased from P125 to P94 and P63 (P < 0.05). Independent of protein intake, males had a higher albumin FSR (P < 0.05) and a greater increase in albumin FSR with feeding (P < 0.05). There was no age or dietary protein effect on serum albumin concentrations, but males had higher albumin concentrations than females (P < 0.0001). These results show that older persons are responsive to nutrient ingestion and dietary protein-related changes in albumin FSR. The greater albumin synthesis rate in males might contribute to a higher albumin concentration set point.

Trypsin secretion and turnover in patients with acute pancreatitis

Studies in humans have shown that pancreatic enzyme secretion is reduced during acute pancreatitis. It is not known, however, whether the reduction is due to impaired synthesis or disruption of the secretory pathway. The rate of secretion and turnover of trypsin was measured in 12 patients with acute pancreatitis of variable etiology and severity (median Ranson's score 2.5, range 0-5, 4 with severe necrotizing disease) and eight healthy volunteers by 4-h primed/continuous intravenous infusions of 1-(13)C-labeled l-leucine, and collection of pancreatic secretions by duodenal perfusion and sampling. Trypsin secretion was reduced from 476 +/- 73 to 153 +/- 60 U/h (means +/- SE, P = 0.005) in acute pancreatitis, with the greatest reductions being observed in patients with necrotizing disease (32 +/- 7 U/h, P = 0.003). The time for newly labeled trypsin to first appear in digestive juice was not, however, delayed in pancreatitis patients (87.2 +/- 11.1 vs. 94.7 +/- 4.9 min); on the contrary, there was an early appearance of newly labeled trypsin at 30 min in patients with severe necrotizing pancreatitis (P < 0.05). Calculated zymogen pool turnover was unchanged, but pool size was decreased (P = 0.01). Despite low rates of luminal secretion, trypsin continues to be synthesized in patients with acute pancreatitis. Our findings could be explained by post-Golgi leakage of enzymes from acinar cells or by loss of synthetic function in some cells with preservation in others.

Reducing plasma HIV RNA improves muscle amino acid metabolism

We reported (Yarasheski KE, Zachwieja JJ, Gischler J, Crowley J, Horgan MM, and Powderly WG. Am J Physiol Endocrinol Metab 275: E577-E583, 1998) that AIDS muscle wasting was associated with an inappropriately low rate of muscle protein synthesis and an elevated glutamine rate of appearance (Ra Gln). We hypothesized that high plasma HIV RNA caused dysregulation of muscle amino acid metabolism. We determined whether a reduction in HIV RNA (> or =1 log) increased muscle protein synthesis rate and reduced R(a) Gln and muscle proteasome activity in 10 men and 1 woman (22-57 yr, 60-108 kg, 17-33 kg muscle) with advanced HIV (CD4 = 0-311 cells/microl; HIV RNA = 10-375 x 10(3) copies/ml). We utilized stable isotope tracer methodologies ([13C]Leu and [15N]Gln) to measure the fractional rate of mixed muscle protein synthesis and plasma Ra Gln in these subjects before and 4 mo after initiating their first or a salvage antiretroviral therapy regimen. After treatment, median CD4 increased (98 vs. 139 cells/microl, P = 0.009) and median HIV RNA was reduced (155,828 vs. 100 copies/ml, P = 0.003). Mixed muscle protein synthesis rate increased (0.062 +/- 0.005 vs. 0.078 +/- 0.006%/h, P = 0.01), Ra Gln decreased (387 +/- 33 vs. 323 +/- 15 micromol.kg fat-free mass(-1).h(-1), P = 0.04), and muscle proteasome chymotrypsin-like catalytic activity was reduced 14% (P = 0.03). Muscle mass was only modestly increased (1 kg, P = not significant). We estimated that, for each 10,000 copies/ml reduction in HIV RNA, approximately 3 g of additional muscle protein are synthesized per day. These findings suggest that reducing HIV RNA increases muscle protein synthesis and reduces muscle proteolysis, but muscle protein synthesis relative to whole body protein synthesis rate is not restored to normal, so muscle mass is not substantially increased.

Gas chromatographic-mass spectrometric determination of alpha-ketoisocaproic acid and [2H7]alpha-ketoisocaproic acid in plasma after derivatization with N-phenyl-1,2-phenylenediamine

A method for determination of alpha-ketoisocaproic acid (KIC) and [4,5,5,5,6,6,6-2H7]alpha-ketoisocaproic acid ([2H7]KIC) in rat plasma was developed using gas chromatography-mass spectrometry-selected ion monitoring (GC-MS-SIM). [5,5,5-2H3]alpha-Ketoisocaproic acid ([2H3]KIC) was used as an analytical internal standard to account for losses associated with the extraction, derivatization and chromatography. The keto acids were extracted by cation-exchange chromatography using BondElut SCX cartridge and derivatized with N-phenyl-1,2-phenylenediamine to form N-phenylquinoxalinone derivatives. Quantitation was performed by SIM of the respective molecular ions at m/z 278, 281 and 285 for the derivatives of KIC, [2H3]KIC and [2H7]KIC on the electron impact method. The limit of detection was found to be 70 fmol per injection (S/N=3) and the limit of quantitation for [2H7]KIC was around 50 nM in rat plasma. Endogenous KIC concentrations in 50 microl of rat plasma were measured with relative intra- and inter-day precision of 4.0% and 3.3%, respectively. The intra- and inter-day precision for [2H7]KIC spiked to rat plasma in the range of 0.1 to 10 microM gave good reproducibility with relative standard deviation (RSD) of 6.5% and 5.4%, respectively. The intra- and inter-day relative errors (RE) for [2H7]KIC were less than 6.4% and 3.8%, respectively. The method was applied to determine the plasma concentration of [2H7]KIC after an intravenous administration of [2H7]KIC in rat.

Resistance exercise acutely increases MHC and mixed muscle protein synthesis rates in 78-84 and 23-32 yr olds

We determined whether short-term weight-lifting exercise increases the synthesis rate of the major contractile proteins, myosin heavy chain (MHC), actin, and mixed muscle proteins in nonfrail elders and younger women and men. Fractional synthesis rates of mixed, MHC, and actin proteins were determined in seven healthy sedentary 23- to 32-yr-old and seven healthy 78- to 84-yr-old participants in paired studies done before and at the end of a 2-wk weight-lifting program. The in vivo rate of incorporation of 1-[(13)C]leucine into vastus lateralis MHC, actin, and mixed proteins was determined using a 14-h constant intravenous infusion of 1-[(13)C]leucine. Before exercise, the mixed and MHC fractional synthetic rates were lower in the older than in the younger participants (P < or = 0.04). Baseline actin protein synthesis rates were similar in the two groups (P = not significant). Over a 2-wk period, participants completed ten 1- to 1. 5-h weight-lifting exercise sessions: 2-3 sets per day of 9 exercises, 8-12 repetitions per set, at 60-90% of maximum voluntary muscle strength. At the end of exercise, MHC and mixed protein synthetic rates increased in the younger (88 and 121%) and older participants (105 and 182%; P < 0.001 vs. baseline). These findings indicate that MHC and mixed protein synthesis rates are reduced more than actin in advanced age. Similar to that of 23-32 yr olds, the vastus lateralis muscle in 78-84 yr olds retains the capacity to increase MHC and mixed protein synthesis rates in response to short-term resistance exercise.

Resistance exercise training increases mixed muscle protein synthesis rate in frail women and men >/=76 yr old

Muscle atrophy (sarcopenia) in the elderly is associated with a reduced rate of muscle protein synthesis. The purpose of this study was to determine if weight-lifting exercise increases the rate of muscle protein synthesis in physically frail 76- to 92-yr-old women and men. Eight women and 4 men with mild to moderate physical frailty were enrolled in a 3-mo physical therapy program that was followed by 3 mo of supervised weight-lifting exercise. Supervised weight-lifting exercise was performed 3 days/wk at 65-100% of initial 1-repetition maximum on five upper and three lower body exercises. Compared with before resistance training, the in vivo incorporation rate of [(13)C]leucine into vastus lateralis muscle protein was increased after resistance training in women and men (P < 0.01), although it was unchanged in five 82 +/- 2-yr-old control subjects studied two times in 3 mo. Maximum voluntary knee extensor muscle torque production increased in the supervised resistance exercise group. These findings suggest that muscle contractile protein synthetic pathways in physically frail 76- to 92-yr-old women and men respond and adapt to the increased contractile activity associated with progressive resistance exercise training.

Whole-body protein metabolism assessed by leucine and glutamine kinetics in adult patients with active celiac disease

To assess the effect of increased renewal of intestinal epithelial cells on leucine and glutamine (Gln) turnover, 4-hour intravenous infusions of L-[1-(13)C]leucine and L-[2-(15)N]Gln were administered to five adult patients with active celiac disease in the postabsorptive state. There was a 35% increase in leucine flux (micromoles per kilogram per hour) in patients (117 +/- 17) compared with healthy controls (96 +/- 11, P < .03). Gln flux was increased by 13% in patients (377 +/- 35) versus controls (335 +/- 16, P < .04). These results suggest that active celiac disease, characterized by villous atrophy and crypt cell hyperplasia, is associated with a dramatic increase in whole-body protein breakdown as assessed by 13C-leucine, which may contribute per se to the protein malnutrition status of the patients. The increase in Gln utilization as assessed by L-[2-(15)N]Gln was moderate, but may have been offset due to the villose atrophy and ensuing reduced intestinal epithelial cell mass. The results are consistent with the concept that increased renewal of intestinal epithelial cells represents a sizable fraction of whole-body protein turnover and that Gln is an important fuel for epithelial intestinal cells in vivo.

Isolation of human skeletal muscle myosin heavy chain and actin for measurement of fractional synthesis rates

Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), we have developed a simple method to isolate myosin heavy chain (MHC) and actin from small (60-80 mg) human skeletal muscle samples for the determination of their fractional synthesis rates. The amounts of MHC and actin isolated are adequate for the quantification of [13C]leucine abundance by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Fractional synthesis rates of mixed muscle protein (MMP), MHC, and actin were determined in six healthy young subjects (27 +/- 1 yr) after they received a 14-h intravenous infusion (prime = 7.58 micromol/kg body wt, constant infusion = 7.58 micromol. kg body wt-1. h-1) of [1-13C]leucine. The fractional synthesis rates of MMP, MHC, and actin were found to be 0.0468 +/- 0.0048, 0.0376 +/- 0. 0033, and 0.0754 +/- 0.0078%/h, respectively. Overall, the synthesis rate of MHC was 20% lower (P = 0.012), and the synthesis rate of actin was 61% higher (P = 0.060, not significant) than the MMP synthesis rate. The isolation of these proteins for isotope abundance analysis by GC-C-IRMS provides important information about the synthesis rates of these specific contractile proteins, as opposed to the more general information provided by the determination of MMP synthesis rates.

Increased plasma gln and Leu Ra and inappropriately low muscle protein synthesis rate in AIDS wasting

Muscle protein wasting occurs in human immunodeficiency virus (HIV)-infected individuals and is often the initial indication of acquired immunodeficiency syndrome (AIDS). Little is known about the alterations in muscle protein metabolism that occur with HIV infection. Nine subjects with AIDS wasting (CD4 < 200/mm3), chronic stable opportunistic infections (OI), and >/=10% weight loss, fourteen HIV-infected men and one woman (CD4 > 200/mm3) without wasting or OI (asymptomatic), and six HIV-seronegative lean men (control) received a constant intravenous infusion of [1-13C]leucine (Leu) and [2-15N]glutamine (Gln). Plasma Leu and Gln rate of appearance (Ra), whole body Leu turnover, disposal and oxidation rates, and [13C]Leu incorporation rate into mixed muscle protein were assessed. Total body muscle mass/fat-free mass was greater in controls (53%) than in AIDS wasting (43%; P = 0.04). Fasting whole body proteolysis and synthesis rates were increased above control in the HIV+ asymptomatic group and in the AIDS-wasting group (P = 0. 009). Whole body Leu oxidation rate was greater in the HIV+ asymptomatic group than in the control and AIDS-wasting groups (P < 0.05). Fasting mixed muscle protein synthesis rate was increased in the asymptomatic subjects (0.048%/h; P = 0.01) but was similar in AIDS-wasting and control subjects (0.035 vs. 0.037%/h). Plasma Gln Ra was increased in AIDS-wasting subjects but was similar in control and HIV+ asymptomatic subjects (P < 0.001). These findings suggest that AIDS wasting results from 1) a preferential reduction in muscle protein, 2) a failure to sustain an elevated rate of mixed muscle protein synthesis while whole body protein synthesis is increased, and 3) a significant increase in Gln release into the circulation, probably from muscle. Several interesting explanations for the increased Gln Ra in AIDS wasting exist.

The effect of human growth hormone therapy on L-(methyl-2H3)-leucine turnover and urinary pseudouridine concentration in patients with Ullrich-Turner syndrome

The effect of daily human growth hormone (hGH) injections (3 I.U./m2/day) on tissue anabolism was determined in six patients with Ullrich-Turner syndrome (XO) (8.7-19 years of age) using novel techniques such as whole body leucine kinetics during continuous infusion of L-(Methyl-2H3)-leucine and urinary pseudouridine (5-ribosyluracil) excretion on the one hand and traditional methods like serum urea and amino acid concentrations on the other. Pseudouridine is only found in ribonucleic acid (RNA) and is neither reincorporated nor catabolically broken down and is therefore considered an ideal index of whole body RNA turnover. The mean L-(Methyl-2H3)-leucine turnover of the six XO patients before hGH was 1.90 +/- 0.15 mumoles/kg per minute. After 3 months of hGH-treatment it had increased in three patients, whereas it had decreased in the other three. The results obtained with the stable isotope technique were correlated with the urinary pseudouridine concentrations (r = 0.68; P < 0.01). The growth rates were positively correlated with leucine turnover (r = 0.63; P < 0.02) and urinary pseudouridine concentration (r = 0.73; P < 0.006) as well as negatively correlated with the serum urea concentrations r = -0.62; P < 0.03). The decrease in the individual serum urea concentrations were tightly correlated with the hGH induced change in growth rate (r = -0.90; P < 0.01). The individual bone ages were negatively correlated with the hGH induced changes in leucine turnover (r = -0.77; P < 0.003) as well as with the urinary pseudouridine concentrations (r = -0.87; P < 0.0002). The hGH effect on leucine and RNA turnover, showing effectiveness only until a developmental age between 11 and 12 years, leads the discussion of the ideal moment of oestrogen supplementation when girls with Ullrich-Turner syndrome are treated with hGH in early adolescence.

CONCLUSION

The protein metabolism of patients with Ullrich-Turner syndrome is influenced by hGH in an age dependent manner. In a clinical setting, pseudouridine, an easily determined derivative of ribonucleic acids, may be able to replace the tedious work with expensive stable isotopes when questions related to tissue anabolism are to be answered.

Removal of branched-chain amino acids and alpha-ketoisocaproate by haemofiltration and haemodiafiltration

Venovenous haemofiltration (VVHF) and haemodiafiltration (VVHDF) were performed with a neonatal haemo(dia)filter (Miniflow 10, Hospal) on 8 anaesthetized rabbits infused with branched-chain amino acids (leucine, isoleucine and valine) and alpha-ketoisocaproate. The branched-chain amino acids (BCAA) and alpha-ketoisocaproate blood levels were close to those previously observed in neonates with maple syrup urine disease when extracorporeal blood purification was required. VVHF and VVHDF performances were assessed with two different blood flows (Qb = 8.3 and 16.6 ml/min). VVHDF was performed with four dialysate flow rates (Qd = 0.5, 1.0, 2.0 and 3.0 L/h). Within each period, clearances of the three BCAA were strictly similar. BCAA clearances obtained by VVHF were similar to ultrafiltration rates (respectively, 0.78 +/- 0.14 and 1.79 +/- 0.28 ml/min at high and low Qb; p < 0.05). The alpha-ketoisocaproate clearances obtained by VVHF were 0.39 +/- 0.17 and 0.92 +/- 0.43 ml/min at low and high Qb (not significantly different). Whatever the Qd value, the VVHDF procedures always allowed higher BCAA and alpha-ketoisocaproate clearances as compared with the corresponding VVHF period with similar Qb. BCAA clearances obtained by VVHDF with a 0.5 L/h dialysate flow were 4.1 +/- 0.5 and 5.4 +/- 0.5 mL/min at low and high Qb, respectively. The concurrent alpha-ketoisocaproate clearances were 2.5 +/- 0.8 and 2.9 +/- 1.0 ml/min.

Glutamine supplementation of enteral nutrition: impact on whole body protein kinetics and glucose metabolism in critically ill patients

BACKGROUND

Glutamine-supplemented parenteral nutrition has been reported to attenuate the early postoperative reduction in intracellular glutamine and improve protein synthesis and nitrogen balance. We investigated the effect of an enteral formula or protein and glucose kinetics and nitrogen balance in trauma patients.

METHODS

The enteral formula (AlitraQ) provided a mean intake of 0.35 g of glutamine/kg body weight per day to 16 trauma patients and was compared with an isonitrogenous formula that provided a mean of 0.05 g of glutamine/kg body weight per day in 14 trauma patients. After 3 days of feeding, protein kinetics were measured using a 4-hour prime-continuous infusion of L-[1-13C]leucine. Glucose kinetics were measured during the same time interval using prime-continuous infusion of [U-14C]- and [6-3H]glucose.

RESULTS

Nitrogen balance was not significantly different in the two groups. There were no significant differences in protein turnover, synthesis, and breakdown between the two groups. There were no significant differences in glucose turnover, oxidation, recycling, and percent of VCO2 from glucose oxidation between the two groups.

CONCLUSIONS

Glutamine-enriched enteral formulas are well tolerated by the severely injured patient but provide no additional nutritional advantage compared with standard enteral formulas during the first 3 days of feeding immediately after trauma.

Mitochondrial function reflected by the decarboxylation of [13C]ketoisocaproate is impaired in alcoholics

Mitochondria of patients with alcoholic liver disease exhibit structural abnormalities, and mitochondria isolated from animals exposed to ethanol are functionally deficient when studied in vitro. To assess possible functional consequences of these ethanol-associated alterations in vivo, we measured mitochondrial function in alcoholics noninvasively with a breath test. A mitochondrial function, the decarboxylation of ketoisocaproate (KICA), was assessed by measuring the exhalation of 13CO2 following the administration of 1 mg/kg 2-keto[1-13C]isocaproic acid, the decarboxylation of which occurs in mitochondria. The results of the KICA breath test in 12 alcoholic subjects were compared with the results in healthy controls and patients with nonalcoholic liver disease. The peak exhalation of 13CO2 and the fraction of the administered dose decarboxylated in 120 min were both significantly lower in alcoholics than in healthy controls and patients with nonalcoholic liver disease. In alcoholics, KICA decarboxylation was impaired in the presence of normal quantitative liver function tests such as the aminopyrine breath test and galactose elimination capacity, indicating that KICA decarboxylation does not simply reflect a decreased functional hepatic mass. The enrichment of circulating KICA with [13C]KICA was similar in alcoholics and controls, indicating that a decreased bioavailability or an increased dilution of labeled KICA cannot account for the decreased exhalation of 13CO2. It is concluded that mitochondrial function as reflected by KICA decarboxylation is impaired in chronic alcoholics. The functional impairment is specific for ethanol abuse and not a reflection of decreased global hepatic function. KICA decarboxylation could thus be useful as a marker for excessive ethanol consumption.

Differential effect of insulin on whole-body proteolysis and glucose metabolism in normal-weight, obese, and reduced-obese women

The whole-body rate of proteolysis, as indicated by the postabsorptive appearance rate (Ra) of leucine, is increased in obese women. The present study was conducted to examine the hypothesis that the increased proteolysis is explained by insulin resistance, and to determine if proteolysis returns to normal when obese women reduce to normal weight. The mean basal leucine Ra was 21% higher in 31 obese women (> 135% ideal weight) than in 17 normal-weight women, and 9% higher per kilogram lean body mass ([LBM] P > .05). When 17 of the obese women reduced and stabilized at 100% to 116% of ideal weight, their mean basal leucine Ra decreased 17% (7%/kg LBM) and was not significantly different from that of the normal-weight control group. Insulin (40 mU/m2/min) was infused for 2 hours while maintaining euglycemia in eight normal-weight, 14 obese, and eight reduced-obese subjects. Glucose disposal per kilogram LBM was 29% lower in obese than in normal-weight subjects (P < .05) and was normal in the reduced-obese subjects. Insulin suppressed the leucine Ra an average of 18.4% in the control group, 20.4% in the obese group, and 24.1% in the reduced-obese group. Suppression of the leucine Ra by insulin did not correlate with the waist to hip ratio (WHR), glucose disposal rate, or basal leucine Ra. We conclude that the increased basal proteolysis of obese women is reversed by weight loss, and is not caused by insulin resistance.

Acute effects of resistance exercise on muscle protein synthesis rate in young and elderly men and women

Muscle mass and function are improved in the elderly during resistance exercise training. These improvements must result from alterations in the rates of muscle protein synthesis and breakdown. We determined the rate of quadriceps muscle protein synthesis using the in vivo rate of incorporation of intravenously infused [13C]leucine into mixed-muscle protein in both young (24 yr) and elderly (63-66 yr) men and women before and at the end of 2 wk of resistance exercise training. Before training, the fractional rate of muscle protein synthesis was lower in the elderly than in the young (0.030 +/- 0.003 vs. 0.049 +/- 0.004%/h; P = 0.004) but increased (P < 0.03) to a comparable rate of muscle protein synthesis in both young (0.075 +/- 0.009%/h) and elderly subjects (0.076 +/- 0.011%/h) after 2 wk of exercise. In the elderly, muscle mass, 24-h urinary 3-methylhistidine and creatinine excretion, and whole body protein breakdown rate determined during the [13C]leucine infusion were not changed after 2 wk of exercise. These findings demonstrate that, during the initial phase of a resistance exercise training program, a marked increase in quadriceps muscle protein synthesis rate occurs in elderly and young adults without an increase in the rate of whole body protein breakdown. In the elderly, this was not accompanied by an increase in urinary 3-methylhistidine excretion, an index of myofibrillar protein breakdown.

The effect of hemodialysis on protein metabolism. A leucine kinetic study

To assess the effect of hemodialysis on protein metabolism, leucine flux was measured in seven patients before, during, and after high efficiency hemodialysis using cuprophane dialyzers and bicarbonate dialysate during a primed-constant infusion of L-[1-13C]leucine. The kinetics [mumol/kg per h, mean +/- SD] are as follows: leucine appearance into the plasma leucine pool was 86 +/- 28, 80 +/- 28, and 85 +/- 25, respectively, before, during, and after dialysis. Leucine appearance into the whole body leucine pool, derived from plasma [1-13C]alpha-ketoisocaproate enrichment, was 118 +/- 31, 118 +/- 31, and 114 +/- 28 before, during, and after dialysis, respectively. In the absence of leucine intake, appearance rate reflects protein degradation, which was clearly unaffected by dialysis. Leucine oxidation rate was 17.3 +/- 7.8 before, decreased to 13.8 +/- 7.8 during, and increased to 18.9 +/- 10.3 after dialysis (P = 0.027). Leucine protein incorporation was 101 +/- 26 before, was reduced to 89 +/- 23 during, and returned to 95 +/- 23 after dialysis (P = 0.13). Leucine net balance, the difference between leucine protein incorporation and leucine release from endogenous degradation, was -17.3 +/- 7.8 before, decreased to -28.5 +/- 11.0 during, and returned to -18.9 +/- 10.3 after dialysis (P < 0.0001). This markedly more negative leucine balance during dialysis was accountable by dialysate leucine loss, which was 14.4 +/- 6.2 mumol/kg per h. These data suggest that hemodialysis using a cuprophane membrane did not acutely induce protein degradation. It was, nevertheless, a net catabolic event because protein synthesis was reduced and amino acid was lost into the dialysate.

Leucine as a regulator of whole body and skeletal muscle protein metabolism in humans

Leucine has been proposed as an in vivo regulator of protein metabolism, although the evidence for this in humans remains inconclusive. To test this hypothesis, we infused either L-leucine (154 +/- 1 mumol.kg-1 x h-1) or saline intravenously in six healthy men in two separate studies. L-Leucine infusion increased plasma concentrations of leucine and alpha-ketoisocaproate from 112 +/- 6 and 38 +/- 3 mumol/l to 480 +/- 27 (P < 0.001) and 94 +/- 13 mumol/l (P < 0.001), respectively, without any significant change in circulating insulin or C peptide levels. Leucine infusion decreased plasma concentrations of several amino acids and decreased whole body valine flux and valine oxidation (using L-[1-13C]valine as a tracer) and phenylalanine flux (using [2H5]-phenylalanine as a tracer). According to arteriovenous differences across the leg, the net balance of phenylalanine, valine, and lysine shifted toward greater retention during leucine infusion, whereas alanine balance did not change. Valine release and phenylalanine release from the leg (estimated from the dilution of respective tracers) decreased, indicating inhibition of protein degradation by leucine infusion. We conclude that leucine decreases protein degradation in humans and that this decreased protein degradation during leucine infusion contributes to the decrease in plasma essential amino acids. This study suggests a potential role for leucine as a regulator of protein metabolism in humans.

Increased protein turnover in obese women

Some previous studies have indicated that rates of proteolysis and protein synthesis are greater in obese than in lean subjects, whereas others have not supported this finding. In the present study, we have measured postabsorptive protein turnover in a large group (n = 24) of obese women to establish more conclusively whether obese women have higher rates of protein turnover than lean women (n = 12), and to determine whether obese subjects with the greatest abdominal fat accumulation or those with the most severe insulin resistance (as determined by oral glucose tolerance testing) have the highest rates of protein turnover. Leucine appearance rate (Ra) was used as an index of whole-body proteolysis, and the fraction of Ra not oxidized was used as an index of whole-body protein synthesis. Leu Ra, oxidation, and incorporation into protein after an overnight fast were approximately 25% greater in obese than in lean women, and were approximately 10% to 15% greater after dividing by lean body mass (LBM) or adjusting for LBM by analysis of covariance. Among obese women, the degree of obesity (over the range of 30% to 47% fat) was not a significant determinant of protein turnover, nor were degree of insulin resistance, visceral fat accumulation (determined by magnetic resonance imaging [MRI]), or subcutaneous abdominal fat accumulation (also determined by MRI). However, the women with the highest rates of protein turnover also had higher waist to hip circumference ratios (WHR). We conclude that even moderate obesity is associated with increased protein turnover, and that this effect is not completely explained by the higher LBM in obese subjects.

Reversed-phase high-performance liquid chromatographic analysis of branched-chain keto acid hydrazone derivatives: optimization of techniques and application to branched-chain keto acid balance studies across the forearm

A sensitive method of quantifying branched-chain keto acids in plasma and whole blood samples is described. It is based on the separation by ion-pair reversed-phase liquid chromatography of 2,4-dinitrophenylhydrazine derivatives with ultraviolet detection. The sample clean-up steps that are usually required for reversed-phase high-performance liquid chromatography are eliminated. A reduction in ketoisocaproate isomer formation is obtained by incubation of derivatives in ice. The method is reproducible (coefficient of variation 2%, n = 5, at the 200-pmol level) and the ultraviolet response is linearly related to branched-chain keto acid concentration. Recoveries are high (greater than 95%). Other keto acids do not co-elute with branched-chain keto acids. Because of its sensitivity and precision, this method can be proposed for whole blood branched-chain keto acid balance studies across organs.

Mechanisms of impaired exercise capacity in short duration experimental hyperthyroidism

To investigate the mechanism of reduced exercise tolerance in hyperthyroidism, we characterized cardiovascular function and determinants of skeletal muscle metabolism in 18 healthy subjects aged 26 +/- 1 yr (mean +/- SE) before and after 2 wk of daily ingestion of 100 micrograms of triiodothyronine (T3). Resting oxygen uptake, heart rate, and cardiac output increased and heart rate and cardiac output at the same submaximal exercise intensity were higher in the hyperthyroid state (P less than 0.05). However, maximal oxygen uptake decreased after T3 administration (3.08 +/- 0.17 vs. 2.94 +/- 0.19 l/min; P less than 0.001) despite increased heart rate and cardiac output at maximal exercise (P less than 0.05). Plasma lactic acid concentration at an equivalent submaximal exercise intensity was elevated 25% (P less than 0.01) and the arteriovenous oxygen difference at maximal effort was reduced (P less than 0.05) in the hyperthyroid state. These effects were associated with a 21-37% decline in activities of oxidative (P less than 0.001) and glycolytic (P less than 0.05) enzymes in skeletal muscle and a 15% decrease in type IIA muscle fiber cross-sectional area (P less than 0.05). Lean body mass was reduced (P less than 0.001) and the rates of whole body leucine oxidation and protein breakdown were enhanced (P less than 0.05). Thus, exercise tolerance is impaired in short duration hyperthyroidism because of decreased skeletal muscle mass and oxidative capacity related to accelerated protein catabolism but cardiac pump function is not reduced.

Versatile stable isotope technique for the measurement of amino acids and keto acids: comparison with radioactive isotope and its use in measuring in vivo disposal rates

Tracer methods using both carbon-13 and -14 have been utilized for determination of ovine fetal amino acid disposal and the results compared in seven animals. We infused [1-13C]leucine simultaneously with [1-14C]leucine into the fetal circulation of pregnant sheep chronically catheterized during late gestation. Radioactive and stable isotope enrichments of leucine (Leu) and stable isotope enrichments of ketoisocaproic acid (KIC) in the umbilical artery and vein and the maternal artery and uterine vein were measured. Stable isotope enrichments and concentrations of both Leu and KIC were determined from a single 0.2-ml sample by the use of internal standards and electron ionization GC/MS analysis after a simple isolation and derivatization procedure. The KIC/Leu enrichment ratio was measured for the first time in fetal arterial plasma and was 0.66 +/- 0.05 (SE). Fetal leucine disposal rate was 9.0 +/- 0.5 (SE) micron/min/kg. Disposal rates determined by stable isotopes were not different from those determined by radioactive isotopes. The GC/MS stable isotope method provided higher precision in both leucine concentration and enrichment measurements and has been shown to be a general method for the determination of concentration and isotopic enrichment of other amino acids and their corresponding keto acids. Furthermore, this method is ideally suited to clinical studies where large numbers of samples of rather small volume can easily be studied with a short turnaround time.

A convenient enzymatic method for the determination of 4-methyl-2-oxopentanoate in plasma: comparison with high performance liquid chromatographic analysis

A simple and rapid spectrophotometric method for the estimation of 4-methyl-2-oxopentanoate in plasma samples by use of NAD+-dependent D-2-hydroxyisocaproate dehydrogenase from Lactobacillus casei ssp. pseudoplantarum is described. It is based on the kinetic measurement of the decrease of NADH absorbance at 334 nm. Applicability is demonstrated by comparative measurement of 4-methyl-2-oxopentanoate content in plasma of patients with maple syrup urine disease by the enzymatic and a reversed phase high performance liquid chromatographic method.

A kinetic study of leucine metabolism in severely burned patients. Comparison between a conventional and branched-chain amino acid-enriched nutritional therapy

A cross-over design study was used to examine the metabolic consequences of enteral feeding for 48 to 96 hours with either a branched-chain amino acid (BCAA)-enriched (44% BCAA) or a conventional egg protein formulation in 12 severely burned adult patients. A stable isotope labeled leucine (L-1-13C-leucine) tracer approach was used to measure leucine flux and oxidation and to estimate rates of whole body protein synthesis and breakdown. Additionally, 15N2-urea and 6,6-2H-glucose were administered to assess the status of urea and glucose kinetics with these two nutritional treatments. Average patient age was 54 years, and average burn surface area was 36%. Studies were conducted at an average of 25 days postburn. Leucine flux and oxidation were significantly (p less than 0.01, by paired t-test) elevated with BCAA feeding as compared to the egg protein formulation. However, there were no significant differences in the rates of leucine incorporation into, or release from, proteins (p greater than 0.05) between the two dietary periods. Mean rates of body protein synthesis and breakdown for each diet were about twice the rates reported for healthy young adults. Apparent nitrogen balance measurements were not statistically different (p greater than 0.1) between the two diet periods. Furthermore, urea and glucose kinetics failed to show significant differences between the two diet periods. It appears from these results that the major consequences of increased intake of leucine from the BCAA formula is an enhanced rate of leucine oxidation. In conclusion, (1) the availability of BCAAs is not rate-limiting for enhanced protein synthesis in burn patients, and (2) the use of enriched BCAA formulas in burn therapy does not appear to offer advantages over a routinely used enteral egg protein formula, at least based on the present determinations.

Plasma proline kinetics and the regulation of proline synthesis in man

A quantitative exploration of the regulation of plasma proline concentration, proline oxidation, and proline endogenous biosynthesis was undertaken utilizing a 360-minute primed continuous infusion of L-[1-13C]proline and L-[methyl-2H3]leucine in healthy, postabsorptive young men. The response of proline metabolism to the intravenous administration of two physiologic rates of L-proline, as well as the withdrawal of an L-proline infusion, were examined. The administration of L-proline at 20 mumol.kg-1.h-1 after an overnight fast resulted in a higher steady state plasma proline concentration, attained within 100 minutes, and this was associated with an increase in proline oxidation, from a baseline value of 10.9 to 16.1 mumol.kg-1.h-1 (P less than .01). Additionally, there was a decrease in proline endogenous synthesis from 15.8 (baseline) to 5.3 mumol.kg-1.h-1 (P less than .01). Administration of L-proline at 40 mumol.kg-1.h-1 after an overnight fast resulted again in a higher plasma steady state proline concentration, attained within 100 minutes and with an associated increase in proline oxidation from 13.1 to 20.0 mumol.kg-1.h-1 (P less than .01) and with a decrease in proline endogenous synthesis from 12.2 to -0.6 mumol.kg-1.h-1 (P less than 0.01). The withdrawal of L-proline after a 20 mumol.kg-1.h-1 infusion resulted in a lower plasma steady state proline level and this was accompanied by a decrease in proline oxidation from 21.2 to 18.2 mumol.kg-1.h-1 (P less than .05) and an increase in endogenous synthesis from 22.2 to 29.7 mumol.kg-1.h-1 (P less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of branched-chain ketoacids in plasma by high-performance liquid chromatography

Branched-chain ketoacids were isolated from plasma or serum samples by acidification, passage through a cationic exchange resin, ether extraction, and extraction of the ether layer with phosphate buffer. The recovery of 2-[1-14C]ketoisocaproate taken through these procedures averaged 95 +/- 3%. Branched-chain ketoacids were measured by high-performance liquid chromatography using a single mobile phase (sodium phosphate:acetonitrile). In normal human subjects, mean +/- SD fasting levels of 2-ketoisocaproate, 2-keto-3-methylvalerate, and 2-ketoisovalerate were 29 +/- 8, 18 +/- 4, and 12 +/- 3 microM, respectively. In normal rats, slightly different results were found: 24 +/- 10, 19 +/- 7 and 17 +/- 6 microM, respectively. In both species, levels of each ketoacid expressed as fractions of total branched-chain ketoacids were much less variable.

Leucine and alpha-ketoisocaproate metabolism and interconversions in fed and fasted sheep

Fed and three-day-fasted sheep were infused with [1-14C] alpha-ketoisocaproate (KIC), L-[1-14C] leucine, and [14C] bicarbonate for determination of their whole-body turnovers, interconversions, and oxidation. Protein synthesis (PS), protein degradation (PD), net tissue metabolism, unidirectional utilization, and production rates also were estimated for the portal-drained viscera, liver, and hindquarters. KIC and leucine arterial concentrations (6.5 and 95 mumol X L-1) both increased with fasting. KIC turnover (9 mumol X min-1) also increased but leucine turnover (108 mumol X min-1) decreased. About 40% of KIC and 15% of leucine were oxidized, but they contributed less than 1% of whole-body CO2 production. The portal-drained viscera released KIC and leucine into the blood only in fed sheep. Hepatic net utilization of KIC and leucine (approximately 2 and 12 mumol X min-1) changed only little with fasting; thus, total splanchnic tissues utilized both in fasted sheep. Net metabolism by the hindquarters (representative of skeletal muscle) was always opposite to splanchnic metabolism. Thus, muscle must produce both KIC and leucine during fasting. In fed sheep whole-body PS, expressed as mumol X min-1 of leucine, was 92 +/- 6 and PD was 71 +/- 5. After fasting, PS decreased by 27%. Calculated liver protein metabolism was unaffected by the fast; PS (fixed and plasma) remained at about 25 and PD at about 15 mumol X min-1. However, protein metabolism by the hindquarters was sensitive to fasting; PS decreased from 30 +/- 4 in fed sheep to 20 +/- 3 mumol X min-1 after fasting and PD increased from 27 +/- 2 to 35 +/- 6 mumol X min-1. Thus, hepatic PS was maintained at the expense of muscle. If the total muscle mass of the body is considered, muscle PS contributed more than one half of whole-body PS.

Branched-chain amino acid interactions in skeletal muscle: isoleucine and L-alloisoleucine

Parenteral administration of a mixture of branched-chain amino acid (BCAA) solutions is known to alter plasma levels of the BCAA (ILE, LEU, VAL), their corresponding alpha-ketoacids (KMV, KIC, KIV) and L-alloisoleucine (ALLO), a stereoisomer of ILE. Although variously formulated mixtures of BCAA are administered, the metabolic implications of individual BCAA interactions have been only partially elucidated. Using the incubated, isolated, and intact rat epitrochlearis muscle, we measured the effect of graded increases (0.05, 0.10, 0.5, and 1.0 mM) in media concentrations of a single BCAA or ALLO on (1) the rate of decarboxylation of the other BCAA, and (2) the release of branched chain ketoacids from muscle. A graded increase of media ILE concentration to 1.0 mM changed the decarboxylation of LEU by -28%, and the release of KIC by +23%, but VAL decarboxylation increased by +25%, and the release of KIV declined by -56%. A graded increase of media LEU to 1.0 mM increased ILE decarboxylation by +146%, and its corresponding ketoacid (KMV) by +61%. However, VAL decarboxylation changed by only +25% and KIV release declined by -65%. A graded increase in media VAL to 1.0 mM accelerated ILE decarboxylation by +37%, but KMV release was unchanged. Similarly, LEU decarboxylation fell by -26%, and the release of KIC by only +6%. ALLO 0.025 mM increased ILE release by +55% but had an inconsistent effect on ILE decarboxylation and did not alter protein synthesis or degradation (estimated by phenylalanine incorporation and tyrosine release, respectively). Increasing ILE did not affect ALLO release.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of alpha-keto acids in serum and urine by high-performance liquid chromatography with fluorescence detection

A simple, rapid and highly sensitive high-performance liquid chromatographic method for the determination of alpha-keto acids in serum and urine is described. In dilute hydrochloric acid, alpha-keto acids are converted by 1,2-diamino-4,5-dimethoxybenzene into highly fluorescent quinoxalinone derivatives. The derivatives are isocratically separated simultaneously within 14 min by reversed-phase chromatography on a Radial-Pak cartridge C18 and detected fluorimetrically. The limits of detection are 10-300 fmol in an injection volume of 10 microliters (40-1200 pmol/ml of serum or urine). This sensitivity permits precise determination of several alpha-keto acids in 5 microliter of serum or urine from healthy persons, and also the determination of phenylpyruvic acid in normal urine which cannot be simultaneously determined by other methods.

In vivo estimation of muscle protein synthesis in myotonic dystrophy

The rate of muscle protein synthesis in patients with myotonic dystrophy has been studied, and results correlated with total muscle mass. Whole body and skeletal muscle protein synthesis were estimated by stable isotope methodology with a primed, continuous infusion of 1-[13C]leucine with measurement of incorporation of [13C]leucine into muscle protein in biopsy samples. Whole body leucine flux, protein synthesis, and protein breakdown were only slightly depressed, but muscle protein synthesis was markedly decreased, in myotonic dystrophy. This depression of muscle protein synthesis in myotonic dystrophy correlates with previous observations of impaired insulin-induced muscle uptake of amino acids and supports the suggestion that muscle wasting in this disease is the consequence of defective anabolism in muscle.

Measurement of alpha-keto acids in plasma using an amino acid analyzer

A method for measuring keto acid concentrations in physiological fluids using an amino acid analyzer was developed. After preliminary deproteinization and removal of amino acids, reduction with sodium cyanoborohydride at 105 degrees C resulted in efficient conversion of the keto acids to their corresponding amino acids. In applying the technique to plasma samples, the use of MeOH for deproteinization was necessary to avoid the large losses of keto acids that occurred during precipitation of proteins with perchloric acid. The method was used to follow plasma ketoisocaproate concentrations in rat plasma after administration of leucine, and was sufficiently sensitive to detect concomitant changes in other branched-chain keto acid concentrations.

Branched-chain alpha-keto acid analysis in biological fluids: preparative clean-up by anion-exchange and analysis by capillary gas chromatography

A method is given for the quantitative analysis of the alpha-keto derivatives of the branched-chain amino acids in physiological fluids. A sample containing alpha-ketovalerate and alpha-ketocaproate as internal standards is passed through a weak anion-exchange resin at neutral pH. After washing the resin with distilled water, the alpha-keto acids are eluted with 4 M hydrochloric acid--ethanol (50:50). Quinoxalinol derivatives are prepared directly in the eluent, extracted with methylene chloride, and trimethylsilylated. Separation of the derivatives is by capillary gas chromatography on a 30 m fused-silica SE-30 column. Chromatographic separation is superior to that reported for packed column methods, thereby permitting the use of alpha-ketovalerate and alpha-ketocaproate as internal standards.

Protein synthesis in muscle measured in vivo in cachectic patients with cancer

Rates of synthesis of protein were measured in vivo in skeletal muscle and in the whole body of cachectic patients with cancer and in normal healthy men, using a tracer infusion of leucine labelled with a stable isotope. Synthesis of protein in muscle was significantly reduced in the patients with cancer (0.030 v 0.198%/hour; p less than 0.01), whereas whole body rates of protein synthesis and degradation did not differ significantly between the two groups. Thus depression of synthesis of protein in muscle appeared to be the immediate cause of muscle wasting in cancerous cachexia. Any therapeutic intervention that aims at preventing the onset of cachexia should be designed to stimulate the synthesis of protein in muscle, and measurement of turnover of protein may be used to evaluate such treatment provided that rates of protein synthesis are measured directly in specific tissues.

Effects of epinephrine infusion on leucine and alanine kinetics in humans

Infusion of epinephrine in humans increases glucose production and decreases plasma concentrations of some essential amino acids such as leucine, while not affecting the plasma concentration of the potential gluconeogenic amino acid alanine. To determine whether epinephrine alters alanine and leucine metabolism, rates of appearance (Ra) and disappearance (Rd) of glucose, alanine, and leucine were determined in postabsorptive volunteers using [3H]glucose, [2H3]alanine, [15N]leucine, and [2H3]leucine during a 180-min infusion of epinephrine (50 ng X kg-1 X min-1). Plasma glucose (90 +/- 1 to 142 +/- 5 mg/dl) and insulin (10 +/- 1 to 16 +/- 2 micrograms/ml) increased (P less than 0.05), whereas plasma alanine concentrations did not change and plasma leucine concentrations increased (127 +/- 5 to 72 +/- 3 microM). Glucose Ra increased transiently and returned to basal values by 120 min. In contrast, alanine Ra and Rd increased identically and progressively from 5.7 +/- 0.5 to 14.5 +/- 1.9 mumol X kg-1 X min-1 by 180 min. Although leucine nitrogen Ra increased transiently and returned to basal values, leucine carbon Ra and Rd decreased (P less than 0.05) during the infusion of epinephrine. The calculated rate and percent of leucine nitrogen going to alanine increased, whereas the percent of alanine nitrogen derived from leucine remained constant. The increase in alanine Ra was entirely attributable to increased de novo synthesis because proteolysis, as estimated by leucine carbon flux, decreased.

Relationship of plasma leucine and alpha-ketoisocaproate during a L-[1-13C]leucine infusion in man: a method for measuring human intracellular leucine tracer enrichment

The keto analog of leucine, alpha-ketoisocaproate (KIC), is formed intracellularly from leucine and is released, in part, into the systemic circulation. Therefore. KIC can be used to estimate intracellular leucine tracer enrichment in man during labeled-leucine tracer experiments without requiring tissue biopsy samples. This approach was studied in young, healthy, male adults maintained on different dietary protein intakes from generous (1.5 g kg-1d-1) to deficient (0.0 g kg-1d-1) for 5-7 day periods. At the end of each dietary period, the volunteers were given a primed, continuous infusion of L-[1-13C]leucine either after an overnight fast (postabsorptive state) or while being fed hourly aliquots of the same diet. The plasma concentrations of all 3 branched-chain amino and keto acid pairs were measured from early morning blood samples taken from 4 subjects at 4 different levels of protein intake. Leucine concentration showed a weak correlation, and valine concentration showed a strong correlation with protein intake; isoleucine and the 3 keto acids did not. However, each branched-chain amino acid concentration was strongly correlated with its corresponding keto acid concentration. In plasma samples obtained during the L-[1-13C]leucine infusions, the ratio of [1-13C]KIC to [1-13C]leucine enrichment ratio remained relatively constant (77 +/- 1% over the wide range of dietary protein intakes and for both the fed and postabsorptive states. For the tissues from which the plasma KIC originates, the rate of plasma leucine into cells will account for approximately 77% of the intracellular leucine flux with the remaining 23% coming primarily from leucine release via protein breakdown. The constant nature of the plasma KIC to leucine 13C enrichment ratio implies that relative changes in leucine kinetics will appear the same under many dietary circumstances regardless of whether plasma leucine or KIC enrichments are used for the calculations.

Quantitation of 2-ketoacids in biological fluids by gas chromatography chemical ionization mass spectrometry of O-trimethylsilyl-quinoxalinol derivatives

A very sensitive and specific method of 2-ketoacid determination in various biological fluids using gas chromatography chemical ionization mass spectrometry of O-trimethylsilyl-quinoxalinol derivatives is described. After derivatization with o-phenylenediamine in acidic medium and extraction, the 2-ketoacids, and the 2-ketovaleric acid used as internal standard, are silylated and resolved on a capillary column and their concentrations automatically determined by monitoring the m/z values corresponding to their respective protonated molecular ions, using ammonia as reactant gas. The detection limit is below 50 ng ml-1 of each 2-ketoacid. The recoveries of 2-ketoacids from urine and plasma were between 97 and 104%. Problems encountered with interfering substances were tested for and discussed. This method has been applied successfully to various metabolic disorders characterized by an accumulation of aliphatic 2-ketoacids. The metabolic interrelationship of branched chain 2-ketoacids and other compounds, especially pyruvate, is discussed.