In vivo enzyme activity in inborn errors of metabolism

Translational Metabolism: A multidisciplinary approach towards precision diagnosis of inborn errors of metabolism in the omics era

The laboratory diagnosis of inborn errors of metabolism has been revolutionized in recent years, thanks to the amazing developments in the field of DNA sequencing including whole exome and whole genome sequencing (WES and WGS). Interpretation of the results coming from WES and/or WGS analysis is definitely not trivial especially since the biological relevance of many of the variants identified by WES and/or WGS, have not been tested experimentally and prediction programs like POLYPHEN-2 and SIFT are far from perfect. Correct interpretation of WES and/or WGS results can only be achieved by performing functional studies at multiple levels (different metabolomics platforms, enzymology, in vitro and in vivo flux analysis), often requires studies in model organisms like zebra fish, Caenorhabditis elegans, Saccharomyces cerevisiae, mutant mice and others, and also requires the input of many different disciplines to make this Translational Metabolism approach effective.

Flux analysis of inborn errors of metabolism

Patients with an inborn error of metabolism (IEM) are deficient of an enzyme involved in metabolism, and as a consequence metabolism reprograms itself to reach a new steady state. This new steady state underlies the clinical phenotype associated with the deficiency. Hence, we need to know the flux of metabolites through the different metabolic pathways in this new steady state of the reprogrammed metabolism. Stable isotope technology is best suited to study this. In this review the progress made in characterizing the altered metabolism will be presented. Studies done in patients to estimate the residual flux through the metabolic pathway affected by enzyme deficiencies will be discussed. After this, studies done in model systems will be reviewed. The focus will be on glycogen storage disease type I, medium-chain acyl-CoA dehydrogenase deficiency, propionic and methylmalonic aciduria, urea cycle defects, phenylketonuria, and combined D,L-2-hydroxyglutaric aciduria. Finally, new developments are discussed, which allow the tracing of metabolic reprogramming in IEM on a genome-wide scale. In conclusion, the outlook for flux analysis of metabolic derangement in IEMs looks promising.

Metabolic phenotypes of phenylketonuria. Kinetic and molecular evaluation of the Blaskovics protein loading test

BACKGROUND

As part of the German Collaborative Study of Children Treated for Phenylketonuria (PKU), a three-day protein loading test was applied to children at 6 months of age. This load elicits three principal types of blood phenylalanine (Phe) response, with types I and III clinically corresponding to classic PKU and mild hyperphenylalaninaemia not requiring diet (MHP), respectively. An intermediate type II, clinically corresponding to mild PKU, is characterized by early decline of blood Phe from above 1200 micromol/L down to levels between 600 and 1200 micromol/L at 72 h.

AIMS

Unbiased classification and kinetic and molecular characterization of the intermediate Phe response; estimation of phenotypic variability of Phe disposal.

METHOD

A kinetic model with zero-order protein synthesis and first-order rate of metabolic disposal of Phe is applied to 157 tests.

RESULTS

A model of exponentially saturated activation describes the acceleration of Phe disposal from day 1 to 3 in the intermediate type of response. Eleven of 14 p.Y414C functional hemizygotes and two of three p.R261Q homozygotes manifested this kinetic type. The rate estimates of Phe metabolic disposal differ widely in patients with identical PAH genotype, yet are highly correlated with the Phe level at 72 h.

Domino liver transplantation in maple syrup urine disease

Liver transplantation has been reported in a few cases of maple syrup urine disease (MSUD), but is controversial. Many patients with approved indications for liver transplantation die before grafts are available. A 25-yr-old man with MSUD underwent liver transplantation, and his liver was used as a domino graft for a 53-yr-old man with hepatocellular carcinoma who had low priority on the liver transplant waiting list and was unlikely to survive until routine organ procurement. Both transplants were performed as "piggy back" procedures, reconstructing the domino graft with caval segments from the cadaveric donor. Neither required veno-venous bypass. Whole body leucine oxidation was estimated by 13CO2 in breath after oral boluses of L-[1-13C]-leucine, before and after transplantation in both patients and a control subject. The surgical outcome was successful. The patient with MSUD had marked decreases in plasma branched-chain amino acids (BCAAs) and alloisoleucine (from 255 +/- 66 to 16 +/- 7 micromol/L), despite advancement of dietary protein from 6 to >40 gm/day. The domino recipient maintained near-normal levels of plasma amino acids with no detectable alloisoleucine on unrestricted diet. Leucine oxidation increased in the patient with MSUD (from 2.2 to 5.6% recovered in 4 hours) and decreased in the recipient (from 9.7 to 6.2%). Neither patient demonstrated any apparent symptoms of MSUD over more than 7 months. In conclusion, liver transplantation substantially corrects whole body BCAA metabolism in MSUD and greatly attenuates the disease. Livers from patients with MSUD may be considered as domino grafts for patients who might otherwise not survive until transplantation.

13C breath tests: visions and realities

Breath tests have been used in research laboratories for over 25 y. Originally, the tests were based on the use of (14)C, rather than on the nonradioactive isotope, (13)C. When (13)C became widely available at a reasonable cost, research groups in the United States and Europe developed methodologies to measure (13)C abundance in samples of CO(2). The tests used a variety of substrates and measured pancreatic function, fat absorption, bacterial overgrowth and P(450) mixed-function oxidase. Thus far, the only test to be approved by the Food and Drug Administration is the (13)C-urea breath test. This manuscript describes the process by which approval is gained, and indicates the steps necessary for other tests to receive Food and Drug Administration approval.

Whole-body L-leucine oxidation in patients with variant form of maple syrup urine disease

Whole-body L-leucine oxidation was assessed in patients with maple syrup urine disease of different severity using oral L-[1-(13)C]leucine bolus tests (38 micromol/kg body weight). Residual whole-body L-leucine oxidation was estimated on the basis of the 3-h kinetics of (13)CO(2) exhalation and (13)C-isotopic enrichment in plasma 4-methyl-2-oxopentanoate using a noncompartmental mathematical approach. In four patients with classical maple syrup urine disease (two females and two males; mean age, 13 +/- 5 y; range, 7--17 y), L-leucine oxidation was too low to be measurable. In two females (aged 11 and 15 y) with a severe variant form of the disease, whole-body L-leucine oxidation was reduced to about 4% of control. In six milder variants (two females and four males; mean age +/- SD, 15 +/- 10 y; range, 6--34 y), the estimates for residual whole-body L-leucine oxidation ranged from 19 to 86% (59 +/- 24%) of control and were substantially higher than the residual branched-chain 2-oxo acid dehydrogenase complex activities in the patients' fibroblasts (10--25% of control). Possible mechanisms are considered that might contribute to a comparatively high residual in vivo L-leucine oxidation in (mild) variant maple syrup urine disease.

Renal clearance of branched-chain L-amino and 2-oxo acids in maple syrup urine disease

In maple syrup urine disease (MSUD), branched-chain L-amino (BCAA) and 2-oxo acids (BCOA) accumulate in body fluids owing to an inherited deficiency of branched-chain 2-oxo acid dehydrogenase complex activity. In MSUD, little information is available on the significance of urinary disposal of branched-chain compounds. We examined the renal clearance of leucine, valine, isoleucine and alloisoleucine, and their corresponding 2-oxo acids 4-methyl-2-oxopentanoate (KIC), 3-methyl-2-oxobutanoate (KIV), (S)-(S-KMV), and (R)-3-methyl-2-oxopentanoate (R-KMV), using pairs of plasma and urine samples (n = 63) from 10 patients with classical MSUD. The fractional renal excretion of free BCAA was in the normal range (< 0.5%) and independent of the plasma concentrations. The excretion of bound (N-acylated) BCAA was normal and not significantly dependent on the BCAA plasma concentrations. The fractional renal excretion of BCOA was in the order KIC << KIV < R-KMV < or = S-KMV (range (%): KIC 0.1-25; KIV 0.14-21.3; S-KMV 0.26-24.6; R-KMV 0.1-35.9), significantly correlated with the KIC plasma concentrations, and generally higher than that of the related BCAA. The results show that the renal excretion of free BCAA as well as of the acylated derivatives is negligible. The renal excretion of BCOA, however, to some extent counteracts increases in BCAA concentrations and thus contributes to the lowering of total BCAA pools in MSUD.

Phenylketonuria. The in vivo hydroxylation rate of phenylalanine into tyrosine is decreased

In phenylketonuria (PKU), the enzyme phenylalanine hydroxylase is deficient, resulting in a decreased conversion of phenylalanine (Phe) into tyrosine (Tyr). The severity of the disease is expressed as the tolerance for Phe at 5 yr of age. In PKU patients it is assumed that the decreased conversion of Phe into Tyr is directly correlated with the tolerance for Phe. We investigated this correlation by an in vivo stable isotope study. The in vivo residual hydroxylation was quantitated using a primed continuous infusion of L-[ring- 2H5]Phe and L-[1-13C]Tyr and the determination of the isotopic enrichments of L-[ring-2H5]Phe, L-[ring-2H4]Tyr, and L-[1-13C]Tyr in plasma. Previous reports by Thompson and coworkers (Thompson, G.N., and D. Halliday. 1990. J. Clin. Invest. 86:317-322; Thompson, G.N., J.H. Walter, J.V. Leonard, and D. Halliday. 1990. Metabolism. 39:799-807; Treacy, E., J.J. Pitt, K. Seller, G.N. Thompson, S. Ramus, and R.G.H. Cotton. 1996. J. Inherited Metab. Dis. 19:595- 602), applying the same technique, showed normal in vivo hydroxylation rates of Phe in almost all PKU patients. Therefore, our study was divided up in two parts. First, the method was re-evaluated. Second, the correlation between the in vivo hydroxylation of Phe and the tolerance for Phe was tested in seven classical PKU patients. Very low (0.13- 0.95 micromol/kg per hour) and normal (4.11 and 6.33 micromol/kg per hour) conversion rates were found in patients and controls, respectively. Performing the infusion study twice in the same patient and wash-out studies of the labels at the end of the experiment in a patient and control showed that the method is applicable in PKU patients and gives consistent data. No significant correlation was observed between the in vivo hydroxylation rates and the tolerances. The results of this study, therefore, showed that within the group of patients with classical PKU, the tolerance does not depend on the in vivo hydroxylation.

Assessment of whole body L-leucine oxidation by noninvasive L-[1-13C]leucine breath tests: a reappraisal in patients with maple syrup urine disease, obligate heterozygotes, and healthy subjects

Suitability of a recently proposed noninvasive L-[13C]leucine breath test for assessment of whole body leucine oxidation in maple syrup urine disease (MSUD) was examined. Oral L-[1-13C]leucine loads (38 micromol/kg body weight) were performed in overnight fasted MSUD patients (n = 6, classical form), obligate heterozygote parents (n = 6), and control subjects (n = 10). Three-hour 13CO2 exhalation kinetics were evaluated using curve fitting procedures. Venous blood was obtained in most cases and analyzed for 13C-labeled plasma metabolites. In control subjects, maximal 13CO2 exhalation was reached at tmax = 55 +/- 18 min. Cumulative 13CO2 output at 3 h amounted to 4.7 +/- 0.7 micromol x (kg body weight)(-1). Estimated total 3CO2 exhalation was 7.2 +/- 1.4 micromol x (kg body weight)(-1) (19.0 +/- 3.6% of the dose). Half of this amount was expired at t1/2 = 130 +/- 18 min. The data show a considerable degree of intersubject variability. Intraindividual variability was comparable, however, when checked in two volunteers. In obligate heterozygotes, 13CO2 kinetics were similar to controls (tmax = 35 +/- 8 min, t1/2 = 95 +/- 16 min). Total 13CO2 output [5.7 +/- 1.4 micromol x (kg body weight)(-1)] tended to be in the lower control range. None of the MSUD patients under study exhibited a significant increase in 13CO2 output after load. Maximal increase of label in plasma 4-methyl-2-oxopentanoate, the physiologic precursor of 13CO2, was 16.1 +/- 3.5 MPE in control subjects. In MSUD, label dilution was increased and correlated with the patients' leucine/4-methyl-2-oxopentanoate plasma levels. Considering the generally high variability of 13CO2 output and the unstable substrate pools in MSUD, we discuss the limitations of whole body leucine oxidation measurements by noninvasive approaches.

Use of [13C]bicarbonate infusion for measurement of CO2 production

To determine whether infusion of 13C-labeled bicarbonate can be used to measure rates of CO2 production (VCO2), seven healthy adults received 6-h primed continuous intravenous infusions of NaH13CO3 and L-[1-14C]leucine in the post-absorptive state while VCO2 was measured by indirect calorimetry. Indirect calorimetry and the use of specific activity and rate of 14CO2 expired yielded identical values of VCO2: 8.97 +/- 0.82 and 8.80 +/- 0.83 mmol/min, respectively (P = NS). The concentration of NaH13CO3 in the infusates and the 13C enrichment in breath CO2 were determined using gas chromatography-isotope ratio mass spectrometry. The rate of appearance of CO2 measured using the NaH13CO3 infusion rate and the steady-state breath 13CO2 enrichments was 11.41 +/- 1.56 mmol/min, which was higher (P < 0.001) than that determined by either of the other two methods. When corrected for the recovery of labeled CO2 during the infusion of NaH13CO3 by use of published values, rate of appearance of CO2 was 9.24 +/- 0.78 mmol/min, which did not differ from VCO2 determined using the other two methods. We conclude that infusion of NaH13CO3 can be used to determine VCO2. This method should be useful to study the oxidation of substrates in populations such as ventilator-dependent neonates, in whom indirect calorimetry is laborious and inaccurate.

Alpha-oxidation of fatty acids in fasted or diabetic rats

Induction of alpha-oxidation, a possible gluconeogenic process, which should produce odd-chain fatty acids from even-chain fatty acids, was studied in rats fasted or made diabetic with streptozotocin. When a omega-phenylated even-chain fatty acid, phenylbutyric acid (1.2 mmol/kg), was administered to rats under these conditions, a significant increase in the urinary excretion of benzoic acid, the metabolic end-product of omega-phenylated odd-chain fatty acids, was observed in fasted (3.54 +/- 0.46 mumol/day) and diabetic (6.73 +/- 2.10) rats (control, 0.58 +/- 0.43; P less than 0.001). Phenylated longer chain fatty acids, phenylhexanoic and phenyldecanoic acid, did not produce significantly more benzoic acid than did phenylbutyric acid. Although the rate of alpha-oxidation was very low compared to that of beta-oxidation, these results suggested that alpha-oxidation of fatty acids was induced under fasting or diabetic conditions, and that alpha-oxidation might take place at the butyric acid stage.

Nutrient interactions with reference to amino acid and protein metabolism in non-ruminants; particular emphasis on protein-energy relations in man

Because the regulation of protein and energy balance is of major research interest in the nutrition and physiology of humans and animals, a selected account of interactions between protein and energy is given here, with particular emphasis on studies in human subjects. The discussion begins with reference to the relations between protein and energy intakes and nitrogen balance; selected aspects of the relations between protein dynamics and energy metabolism among the various mammalian species are then considered. This leads to a brief account of oxidative amino acid catabolism and its relevance to the assessment of amino acid requirements, particularly in adult man. It is concluded that obligatory oxidative losses of amino acids can be used to predict or approximate amino acid requirements in children and adults. The nitrogen-sparing properties of carbohydrate and lipid-derived fuels are then considered. Despite the well-known and profound, yet differential, impacts of dietary protein and energy sources, and their interactions on body protein balance, there remain wide gaps in our understanding of the mechanisms responsible for their effects, such as the quantitative and mechanistic involvement of hormones, including insulin and the counter-regulatory hormones, and the roles played by the major amino acids responsible for the interorgan transport of nitrogen and the regulation of urea production. Additional studies focusing on metabolic nitrogen trafficking would significantly enhance an understanding of how protein and energy interact to achieve the efficient utilization of dietary protein for maintenance and promotion of lean body gain.

Protein and energy interactions throughout life. Metabolic basis and nutritional implications

We review selected aspects of the interactions between protein and energy in human metabolism and nutrition. Following a short account of the underlying metabolic basis for the effects of energy on protein metabolism, the contribution made by whole body protein turnover to the metabolic rate is discussed, including the relationship between protein turnover and energy metabolism at different phases of life. The effects of changes in energy metabolism and intake on the nitrogen economy of the host are also reviewed briefly and we explore the relationship between amino acid oxidation and requirements for indispensable amino acids. Interactions between energy and protein metabolism need to be investigated in greater detail and also they must be considered in relation to further attempts to establish more precisely energy and amino acid requirements of people under various circumstances.