Isotopic enrichment of amino acids in urine following oral infusions of L-[1-(13)C]phenylalanine and L-[1-(13)C]lysine in humans: confounding effect of D-[13C]amino acids

Comparison of the precursor, amino acid oxidation, and end-product methods for the evaluation of protein turnover in senior dogs

Stable isotope methods have been used to study protein metabolism in humans; however, there application in dogs has not been frequently explored. The present study compared the methods of precursor (13C-Leucine), end-products (15N-Glycine), and amino acid oxidation (13C-Phenylalanine) to determine the whole-body protein turnover rate in senior dogs. Six dogs (12.7 ± 2.6 years age, 13.6 ± 0.6 kg bodyweight) received a dry food diet for maintenance and were subjected to all the above-mentioned methods in succession. To establish 13C and 15N kinetics, according to different methodologies blood plasma, urine, and expired air were collected using a specifically designed mask. The volume of CO2 was determined using respirometry. The study included four methods viz. 13C-Leucine, 13C-Phenylalanine evaluated with expired air, 13C-Phenylalanine evaluated with urine, and 15N-Glycine, with six dogs (repetitions) per method. Data was subjected to variance analysis and means were compared using the Tukey test (P<0.05). In addition, the agreement between the methods was evaluated using Pearson correlation and Bland-Altman statistics. Protein synthesis (3.39 ± 0.33 g.kg-0,75. d-1), breakdown (3.26 ± 0.18 g.kg-0.75.d-1), and flux estimations were similar among the four methods of study (P>0.05). However, only 13C-Leucine and 13C-Phenylalanine (expired air) presented an elevated Pearson correlation and concordance. This suggested that caution should be applied while comparing the results with the other methodologies.

Protein Requirements of Elderly Chinese Adults Are Higher than Current Recommendations

BACKGROUND

Due to a lack of research data on the protein requirements of the elderly in China, the estimated average requirement (EAR) and the recommended nutrient intake (RNI) of protein in the elderly remain the same as those in young and middle-aged people at 0.98 g/(kg·d).

OBJECTIVE

The objective of this study was to determine the protein requirements of healthy Chinese adults >65y old through use of the indicator amino acid oxidation (IAAO) method.

METHODS

Seven healthy adult men and 7 healthy adult women participated in the study, with protein intakes ranging from 0.3 to 1.8 g/(kg·d). The diets were isocaloric and provided energy at a 1.5 resting energy expenditure. Protein was given based on the lactalbumin. Phenylalanine and tyrosine were added to protein doses of 0.3-1.5 g/kg according to the highest dose of protein content [1.8 g/(kg·d)]. Phenylalanine and tyrosine concentrations were kept constant at each protein dose. The mean protein requirement was determined by applying a nonlinear mixed-effects model analysis to the F13CO2, which identified a breakpoint in F13CO2 in response to graded amounts of protein. This trial was registered with the Chinese clinical trial registry as ChiCTR-BOC-17010930.

RESULTS

Protein EAR and RNI for healthy elderly Chinese adults were determined to be 0.91 and 1.17 g/(kg·d), respectively, based on the indicator amino acid oxidation technique.

CONCLUSIONS

The estimates of protein requirements for Chinese adults >65 y in the present study are 3.4% and 19.4% higher than the current estimated requirements, 0.88 g/(kg·d) for EAR and 0.98 g/(kg·d) for RNI.

The significance of d-isomers in stable isotope studies in humans is dependent on the age of the subject and the amino acid tracer

d-Amino acids (d-AAs) in stable isotope tracers may result in erroneous estimates of enrichment, particularly if urine is used as a surrogate for plasma enrichment. Previous studies suggest that a d-AA content of less than 0.2% will not result in significant error in studies with adult humans. To describe the effects of d-AA content of less than 0.2%, in 3 different AA tracers, on isotope enrichment in urine and plasma, arginine, proline, and phenylalanine (Phe) tracers were given enterally to human neonates. Enrichment was measured in urine and plasma using chiral chromatography and tandem mass spectrometry. The Phe tracer was also given parenterally to human neonates and enterally to children and adults to further characterize the d-AA effect. All isotopes had a confirmed d-AA content of less than 0.2%. Labeled d-arginine resulted in an overestimate for enrichment of 20% in plasma and 87% in urine. A smaller effect was seen for d-Phe, which resulted in a 5% overestimate for plasma and 40% in urine. d-Proline had no significant effect. Using the same Phe tracer, a developmental effect was found, with a reduction in the overestimate in children compared with infants and no effect on enrichment in adults. Investigators using commercially produced, stable isotope-labeled AAs need to be aware that there is no safe level of d contamination; a d-AA content less than 0.2% may result in significant overestimate for enrichment, even in plasma, for infants and children. This source of error can be avoided by the use of chiral chromatography.

Indicator amino acid oxidation is not affected by period of adaptation to a wide range of lysine intake in healthy young men

The number of days of adaptation to a specific amino acid intake required prior to the determination of amino acid requirements using the indicator amino acid oxidation method (IAAO) is still in debate. In this study, our objective was to determine whether adaptation for 8 h, 3 d, and 7 d to a wide range of lysine intakes had any effect on the oxidation of the indicator amino acid, l-[1-(13)C]phenylalanine, to (13)CO(2) (F(13)CO(2)). Five healthy young men randomly received each of 4 levels of lysine (5, 20, 35, and 70 mg x kg(-1) x d(-1)) along with an amino acid mixture to achieve a protein intake of 1.0 g x kg(-1) x d(-1) and energy intake of 1.5x resting energy expenditure during 4 separate 7-d study periods. IAAO studies were conducted on d 1, 3, and 7. During each study day, oral consumption of l-[1-(13)C]phenylalanine was followed by collection of breath for F(13)CO(2) and plasma for measurement of phenylalanine enrichment. F(13)CO(2) was affected by lysine intake but did not differ among adaptation periods of 8 h, 3 d, or 7 d. Phenylalanine flux was not significantly affected by period of adaptation. These results suggest that the minimally invasive IAAO model, where participants are adapted prior to protein intake for 2 d followed by study day adaptation to the test amino acid intake for 8 h, may be sufficient to estimate individual amino acid requirements in healthy young men.

Total sulfur amino acid requirement and metabolism in parenterally fed postsurgical human neonates

BACKGROUND

Except for tyrosine, the amino acid requirements of human neonates receiving parenteral nutrition (PN) have not been experimentally derived.

OBJECTIVES

The objectives were to determine the total sulfur amino acid (TSAA) requirement (methionine in the absence of cysteine) of postsurgical, PN-fed human neonates by using the indicator amino acid oxidation (IAAO) technique with L-[1-(13)C]phenylalanine as the indicator.

DESIGN

Fifteen postsurgical neonates were randomly assigned to receive 1 of 18 methionine intakes ranging from 10 to 120 mg x kg(-1) x d(-1), delivered in a customized, cysteine-free amino acid solution. Breath and urine samples were collected for the measurement of (13)CO(2) and amino acid enrichment. Blood samples were collected at baseline and after the test methionine infusion for the measurement of plasma methionine, homocysteine, cystathionine, and cysteine concentrations.

RESULTS

Breakpoint analysis determined the mean TSAA requirements to be 47.4 (95% CI: 38.7, 56.1) and 49.0 (95% CI: 39.9, 58.0) mg x kg(-1) x d(-1) with the use of oxidation and F(13)CO(2), respectively.

CONCLUSIONS

This is the first study to report the TSAA requirement of postsurgical, PN-fed human neonates. The estimated methionine requirement expressed as a proportion of the methionine content of current commercial pediatric PN solutions was 90% (range: 48-90%) of that found in the lowest methionine-containing PN solution.

An overview of phenylalanine and tyrosine kinetics in humans

The initial use of a tracer of phenylalanine was by Moss and Schoenheimer in rats in 1940 to determine that phenylalanine was hydroxylated to tyrosine, defining for the first time the primacy of this pathway. Phenylalanine and tyrosine kinetics were not measured in humans until the 1970-80s. The first application was to determine the degree of blockage of phenylalanine hydroxylation in patients with hyperphenylalanemia and phenylketonuria, but this approach was expanded to determination of phenylalanine hydroxylation in normal subjects. Far more uses have been demonstrated for measuring rates of phenylalanine disposal and tyrosine production in relatively normal subjects than in patients with in-born errors of metabolism. Key to use of tracers to determine phenylalanine and tyrosine metabolic rates has been the development of appropriate tracer models. Most applications have used relatively simple models ignoring the intracellular hydroxylation rate component. Because the liver is the primary site of hydroxylation in the body, the intracellular enrichment at the site of hydroxylation can be assessed from the tracer enrichments at isotopic steady state in rapid-turnover plasma proteins, such as Apo-B, made and secreted by the liver. Although there are potential problems with use of deuterated tracers of phenylalanine, suitable tracers are available and have been demonstrated for general measurement of phenylalanine and tyrosine kinetics in humans.

Phenylalanine kinetics differ between formula-fed and human milk-fed preterm infants

Infants fed casein-dominant formulas have higher plasma phenylalanine and tyrosine concentrations than those fed mother's milk. Conversely, elevated plasma threonine concentrations are observed in infants fed whey-dominant formulas. We recently showed that formula-fed preterm infants have a lower capacity to degrade threonine than do preterm infants fed mother's milk. We hypothesized that these same infants (n = 18) would differ in their catabolism of phenylalanine in response to phenylalanine loads provided by formulas with increasing casein content of formulas (whey:casein 60:40, 40:60, and 20:80) compared with preterm infants fed mother's milk. Plasma phenylalanine concentrations significantly rose (49, 46, 79 micromol . L(-1) for whey:casein 60:40, 40:60, and 20:80, respectively, pooled SD 8, P < 0.05); and plasma phenylalanine concentrations in infants fed mother's milk were low (40 +/- 4 micromol . L(-1)). Using [1-(13)C]phenylalanine tracer and (13)CO(2) production in breath we found that although there was a significant positive relation between phenylalanine oxidation and phenylalanine intake in formula-fed infants (r(2) = 0.43, P = 0.03), these infants were not able to increase their oxidation of phenylalanine enough to prevent a significant rise in plasma phenylalanine when fed the 20:80 formula. Compared to infants fed mother's milk, formula-fed infants had significantly lower phenylalanine oxidation (39.1 vs. 30.7% of phenylalanine intake, respectively, P < 0.05). We conclude that one of the mechanisms for the differences in plasma amino acid concentration between formula-fed and mother's milk-fed preterm infants may be in vivo down-regulated catabolism of 2 important essential amino acids (phenylalanine in addition to threonine) in formula-fed preterm infants.

Different approaches to define individual amino acid requirements

A full review of the strengths and limitations of the various methods used to define amino acid requirements is provided. The focus is on the recent development of carbon oxidation techniques such as indicator amino acid oxidation and 24-h amino acid balance to determine dietary indispensable (essential) amino acid needs in adults. All approaches depend on the change in a metabolic parameter in response to graded intake of the test amino acid. In humans, the within-subject variance is less than the between-subject variance, which has led to an appreciation of the need to study each subject across a range of intakes, above and below the mean requirement level. The data can then be analyzed using two-phase linear regression crossover and a precise population mean requirement can be determined. Several approaches have been used to define the variance of the mean requirement. Finally, a minimally invasive indicator amino acid oxidation model has been developed which allows the determination of dietary essential amino acid requirements in children and other vulnerable populations.

Kinetics of sequential metabolism from D-leucine to L-leucine via alpha-ketoisocaproic acid in rat

D-Leucine is considered to be converted into the L-enantiomer by two steps: oxidative deamination to form alpha-ketoisocaproic acid (KIC) and subsequent stereospecific reamination of KIC. We investigated the pharmacokinetics of leucine enantiomers and KIC in rats to evaluate how deamination of D-leucine, reamination of KIC, and decarboxylation of KIC were affected to the overall extent that converted D-leucine into the L-enantiomer. After intravenous administrations of D-[(2)H(7)]leucine, L-[(2)H(7)]leucine, or [(2)H(7)]KIC, their plasma concentrations together with endogenous L-leucine and KIC were determined by gas chromatography-mass spectrometry. The rapid appearances of [(2)H(7)]KIC and L-[(2)H(7)]leucine were observed after administration of D-[(2)H(7)]leucine, whereas no detectable amount of D-[(2)H(7)]leucine was found after administrations of [(2)H(7)]KIC or L-[(2)H(7)]leucine. The fraction of conversion from D-[(2)H(7)]leucine into [(2)H(7)]KIC (F(D-->KIC)) was estimated by using the area under the curve (AUC) of [(2)H(7)]KIC on the D-[(2)H(7)]leucine administration [AUC(KIC(D))] and that of [(2)H(7)]KIC on the [(2)H(7)]KIC administration (AUC(KIC)) to yield 70.1%. The fraction of conversion from [(2)H(7)]KIC to L-[(2)H(7)]leucine (F(KIC-->L)) was 40.2%. The fraction of conversion from D-leucine to the L-enantiomer (F(D-->L)) was considered to be the product of F(D-->KIC) and F(KIC-->L), indicating that 28.2% of D-[(2)H(7)]leucine was metabolized to L-[(2)H(7)]leucine via [(2)H(7)]KIC. These results suggested that the relatively low conversion of D-leucine into the L-enantiomer might depend on irreversible decarboxylation of KIC. Regardless of [(2)H(7)]KIC, F(D-->L) was also calculated directly using AUC(L(D)) and AUC(L) to yield 27.5%. There were no differences between the two F(D-->L) values, suggesting that almost all of the formation of L-[(2)H(7)]leucine from D-[(2)H(7)]leucine occurred via [(2)H(7)]KIC as an intermediate.

Oral and intravenous tracer protocols of the indicator amino acid oxidation method provide the same estimate of the lysine requirement in healthy men

To investigate whether splanchnic uptake of the indicator amino acid ([1-(13)C] phenylalanine) during the fed state alters the estimate of lysine requirement, five healthy men were studied at graded levels of lysine intake, with either an oral or intravenous (IV) tracer protocol, in a randomized, crossover design. Splanchnic extraction of the oral tracer was expressed as the difference between the ratio of the enrichments in urinary phenylalanine between tracer protocols. The rate of release of (13)CO(2) from (13)C-phenylalanine oxidation (F(13)CO(2)) was measured and a two-phase linear regression crossover model was applied to determine the lysine requirement. Mean splanchnic extraction of the oral tracer was approximately 19%. Although actual F(13)CO(2) was higher during oral tracer infusion (P < 0.001), the breakpoint was not different from that determined with IV infusion (P = 0.98), with both yielding a mean lysine requirement of 36.6 mg/(kg. d). The upper 95% confidence intervals were 52.5 and 53.3 mg/(kg. d) for the oral and IV isotope infusions, respectively. These results demonstrate that routes of isotope administration using the indicator amino acid oxidation technique do not affect the estimated amino acid requirement. Therefore, the indicator amino acid oxidation method using the oral route, which is less invasive and allows for studies in vulnerable groups such as infants and children, should be the preferred method for studying amino acid requirements.

The effect of graded intake of glycyl-L-tyrosine on phenylalanine and tyrosine metabolism in parenterally fed neonates with an estimation of tyrosine requirement

Although tyrosine is considered indispensable during the neonatal period, its poor solubility has limited its inclusion in parenteral amino acid solutions to less than 1% of total amino acids. Dipeptides of tyrosine are highly soluble, have been shown to be well used and safe in animal models and humans, and, therefore, may be used as an effective means of providing tyrosine in the parenterally fed neonate. The goal of the present study was to determine the tyrosine requirement of the parenterally fed neonate receiving graded intakes of glycyl-L-tyrosine as a source of tyrosine. Thirteen infants receiving adequate energy (340 +/- 38 kJ. kg(-1).d(-1)) and protein (2.4 +/- 0.4 g.kg(-1).d(-1)) were randomized to receive parenteral nutrition with one of five graded levels of glycyl-L-tyrosine. The mean requirement and safe level of intake were estimated using a 1-(13)C-phenylalanine tracer and linear regression cross-over analysis that identified a break point in the response of label appearance in breath CO(2) (F(13)CO(2)) and phenylalanine oxidation to graded tyrosine intake. Based on the mean estimates of whole-body phenylalanine oxidation, the tyrosine mean requirement and safe level of intake were found to be 74 mg.kg(-1). d(-1) and 94 mg.kg(-1).d(-1), respectively. This represents 3.1 and 3.9% of total amino acids, respectively, considerably higher than levels found in present commercially available pediatric amino acid solutions. These data raise concern regarding the adequacy of aromatic amino acid intake in the parenterally fed neonate.