Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “PHENYLALANINE”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources
PubMed1993

Phenylalanine conversion to tyrosine: comparative determination with L-[ring-2H5]phenylalanine and L-[1-13C]phenylalanine as tracers in man.

The in vivo rate of conversion of phenylalanine to tyrosine (PheOH) can be estimated using combinations of stable isotope-labeled phenylalanine and tyrosine. We have compared in four healthy adult men the rates of phenylalanine conversion to tyrosine based on the following pairs of primed, continuous tracer infusions administered simultaneously: (1) L-[ring-2H5]phenylalanine and 2H2-tyrosine with a 2H4-tyrosine prime, and (2) L-[1-13C]phenylalanine and 2H2-tyrosine with a 1-13C-tyrosine prime. Phenylalanine oxidation was determined from measurement of 13CO2 excretion in expired air. Tracers were given for 8 hours, with subjects being in the postabsorptive state during the first 3 hours and in the fed state during the remaining 5 hours. Mean (+/- SD) rates (mumol.kg-1.h-1) of phenylalanine conversion to tyrosine for fasted and fed states, respectively, were 5.1 +/- 2.9 and 6.8 +/- 3.4 with 2H5-phenylalanine and significantly higher (P < .05) at 11.1 +/- 5.6 and 12.7 +/- 7.7 with 13C-phenylalanine as tracer. Phenylalanine oxidation was 9.9 +/- 2.0 and 13.5 +/- 2.6, respectively, for fasted and fed states, and these mean values did not differ (P > .1) from the rate of phenylalanine conversion to tyrosine determined using 13C-phenylalanine. These results indicate the need for caution in interpreting kinetic aspects of phenylalanine metabolism when based on isotopic data from multideuterated phenylalanine.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed2004

In vivo studies of phenylalanine hydroxylase by phenylalanine breath test: diagnosis of tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency.

Tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency is characterized by reduction of blood phenylalanine level after a BH4-loading test. Most cases of BH4-responsive PAH deficiency include mild phenylketonuria (PKU) or mild hyperphenylalaninemia (HPA), but not all patients with mild PKU respond to BH4. We performed the phenylalanine breath test as reliable method to determine the BH4 responsiveness. Phenylalanine breath test quantitatively measures the conversion of L-[1-13C] phenylalanine to 13CO2 and is a noninvasive and rapid test. Twenty Japanese patients with HPA were examined with a dose of 10 mg/kg of 13C-phenylalanine with or without a dose of 10 mg . kg(-1) . d(-1) of BH4 for 3 d. The phenylalanine breath test [cumulative recovery rate (CRR)] could distinguish control subjects (15.4 +/- 1.5%); heterozygotes (10.3 +/- 1.0%); and mild HPA (2.74%), mild PKU (1.13 +/- 0.14%), and classical PKU patients (0.29 +/- 0.14%). The genotypes in mild PKU cases were compound heterozygotes with mild (L52S, R241C, R408Q) and severe mutations, whereas a mild HPA case was homozygote of R241C. CRR correlated inversely with pretreatment phenylalanine levels, indicating the gene dosage effects on PKU. BH4 loading increased CRR from 1.13 +/- 0.14 to 2.95 +/- 1.14% (2.6-fold) in mild PKU and from 2.74 to 7.22% (2.6-fold) in mild HPA. A CRR of 5 to 6% reflected maintenance of appropriate serum phenylalanine level. The phenylalanine breath test is useful for the diagnosis of BH4-responsive PAH deficiency and determination of the optimal dosage of BH4 without increasing blood phenylalanine level.

Adolescent↗
PubMed2001

Dietary supplements of mixtures of indispensable amino acids lacking threonine, phenylalanine or histidine increase the activity of hepatic threonine dehydrogenase, phenylalanine hydroxylase or histidase, respectively, and prevent growth depressions in chicks caused by dietary excesses of threonine, phenylalanine, or histidine*

Experiments were carried out to determine whether the addition of a mixture of indispensable amino acids (IAA) lacking in threonine, phenylalanine or histidine, respectively, to a nutritionally complete diet would increase the hepatic activities of the rate-limiting enzymes for catabolism of threonine, phenylalanine or histidine and prevent the adverse effects of the amino acid on growth when the dietary level of the amino acid is excessive. Week old Leghorn chicks were fed semi-purified diets containing 19% crude protein to which were added no IAA supplement or 10% crude protein from an IAA mix and 5 graded levels of either L-threonine, L-phenylalanine or L-histidine in a 2 x 5 factorial arrangement of treatments. Each amino acid was investigated in a separate experiment involving four replicate pens (seven chicks each) per diet. Weight gains and feed consumptions were determined on the fourteenth day of each experiment. The groups receiving no excess, and 1.0% or 2.0% excesses of amino acids were sampled on the fifteenth day for enzyme activities and plasma amino acid concentrations. Weight gain and/or feed consumption were lower, and plasma concentrations of threonine, phenylalanine and histidine were higher, in chicks receiving 1.5 to 2.0% dietary additions of threonine, phenylalanine, and histidine, respectively, than in chicks that did not receive these amino acids. Chicks that received the amino acids in diets that also contained the IAA supplement had better growth and feed consumption, lower plasma concentrations of threonine, phenylalanine or histidine, higher plasma concentrations of other indispensable amino acids, and higher activities of threonine dehydrogenase, phenylalanine hydroxylase, and histidase than chicks receiving excess amino acids in the absence of IAA supplements. We conclude that the dietary level of protein, not the dietary level of individual amino acids, is the primary determinant of the activity of amino acid degrading enzymes in liver. The increased activity of these enzymes may be the mechanism by which dietary protein alleviates the adverse effects of excessive levels of individual amino acids.

Journal Article↗
PubMed1998

Identification of hepatic nuclear factor 1 binding sites in the 5' flanking region of the human phenylalanine hydroxylase gene: implication of a dual function of phenylalanine hydroxylase stimulator in the phenylalanine hydroxylation system.

Phenylalanine hydroxylase stimulator (PHS) is a component of the phenylalanine hydroxylation system that is involved in the regeneration of the cofactor tetrahydrobiopterin. It is also identical to the dimerization cofactor of hepatocyte nuclear factor 1 (HNF1) (DCoH) that is able to enhance the transcriptional activity of HNF1. Moreover, it has the structural potential for binding macromolecules such as proteins and nucleic acids, consistent with its involvement in gene expression. We investigated whether PHS/DCoH could enhance the expression of phenylalanine hydroxylase (PAH). Cotransfection assays showed that DCoH itself could not transactivate the 9-kb human PAH 5' flanking fragment. However, this 9-kb fragment was transactivated by HNF1 in a dose-dependent manner with a maximum of nearly 8-fold activation; DCoH potentiated this transactivation by another 1.6-fold. The HNF1 binding sites were located at -3.5 kb in a region that is 77.5% identical to the mouse liver-specific hormone-inducible PAH gene enhancer. This study suggests a possible dual function of PHS in vivo in the human phenylalanine hydroxylation system: it is involved in the regeneration of the cofactor tetrahydrobiopterin and can also enhance the expression of the human PAH gene.

Animals↗
PubMed1994

In vitro embryotoxicity of the cysteine proteinase inhibitors benzyloxycarbonyl-phenylalanine-alanine-diazomethane (Z-Phe-Ala-CHN2) and benzyloxycarbonyl-phenylalanine-phenylalanine-diazomethane (Z-Phe-Phe-CHN2).

This study makes use of whole embryo culture to investigate the potential embryotoxicity of benzyloxycarbonyl-phenylalanine-alanine-diazomethane (Z-Phe-Ala-CHN2) and benzyloxycarbonyl-phenylalanine-phenylalanine-diazomethane (Z-Phe-Phe-CHN2), two low molecular weight, active site-directed and irreversible inhibitors of the lysosomal cysteine proteinases. Peptidyl diazomethanes are the most specific inhibitors available for lysosomal cysteine proteinases and can be hypothesized to interrupt visceral yolk sac (VYS)-mediated nutrition during early organogenesis. When added directly to the culture medium of gestational day 10-11 rat conceptuses, both compounds inhibited lysosomal cysteine proteinase activity in the VYS in a concentration-dependent fashion that correlated with the degree of embryotoxicity observed. Z-Phe-Ala-CHN2 and Z-Phe-Phe-CHN2 were also found to increase the protein content of the VYS, even though all other conceptual growth parameters decreased. This effect was dependent on the serum content of the culture medium and the exposure time. Histological examination of Z-Phe-Ala-CHN2-treated conceptuses revealed a dramatic increase in the size and number of vacuoles in the VYS endoderm epithelium, suggestive of inhibition of VYS proteolysis. At the same time, excessive cell death was observed throughout the neuroepithelium and in specific regions of the mesenchyme of the corresponding embryos. This cell death manifested morphological characteristics of apoptosis and could be detected by supravital staining with Nile Blue Sulphate. These findings provide additional evidence in support of the hypothesis that lysosomal cysteine proteinases play a critical role in VYS-mediated histiotrophic nutrition and suggest that peptidyl diazomethanes may be useful in further characterization of these enzymes. The possible direct effects of these inhibitors on embryonic cells and the relationships between interruption of VYS-mediated nutritional processes and embryonic cell death are discussed.

Animals↗
PubMed2002

Phenylalanine hydroxylase mutations and phenylalanine-tyrosine metabolism in heterozygotes for phenylalanine hydroxylase deficiency.

UNLABELLED: The aim of this study was to determine whether any relationship exists between the severity of mutation of the phenylalanine hydroxylase (PAH) gene and the plasma concentrations of phenylalanine (Phe) and tyrosine (Tyr) under fasting and semifasting conditions among heterozygotes in a matched case-control study. Parents of patients affected by PAH deficiency (n = 25) detected through the Italian Neonatal Screening Program and referred from January 1994 to June 2000, and parents of healthy children were investigated. In total, 68 subjects without any disease, 34 hyperphenylalaninaemia (HPA) heterozygous parents and 34 age- and gender-matched controls, were recruited. Plasma concentrations of Phe and Tyr in fasting and semifasting (1600 mg Phe oral load) conditions were the main outcome measures. DNA analysis for PAH mutations was performed in all 68 subjects. Compared with controls, heterozygotes showed higher fasting and semifasting Phe concentrations (p < 0.0001), lower semifasting Tyr concentrations (p = 0.015), lower Tyr variations (p = 0.003) and a higher Phe/Tyr ratio (p < 0.0001) in switching from fasting to semifasting conditions. Heterozygotes carrying a severe mutation showed semifasting plasma Tyr concentrations lower than controls (p = 0.019) but not significantly different from Tyr levels found in non-severe carriers (p = 0.197). The Tyr variations were minor in severe carriers than controls (p < 0.001) and non-severe carriers too, although with lower significance (p = 0.089). In six carriers of A403V mutation, parents of mild hyperphenylalaninaemics on an unrestricted diet, significant differences in variations from fasting to semifasting conditions were found compared with parents of patients on a diet. CONCLUSION: Although the great heterogeneity of PAH mutations limits any general conclusion, the results suggest that monitoring plasma Tyr variations may be more sensitive than plasma Phe in assessing the severity of PAH mutations in HPA heterozygotes.

Adult↗
PubMed1983

The contribution of phenylalanine to tyrosine metabolism in vivo. Studies in the post-absorptive and phenylalanine-loaded rat.

1. Rates of appearance and oxidation of plasma L-leucine, L-phenylalanine and L-tyrosine, as well as conversion of plasma phenylalanine into plasma tyrosine, were determined in 90-120 g rats after overnight starvation and while receiving 115-120 mumol of L-phenylalanine/h. 2. In the post-absorptive state, plasma tyrosine and phenylalanine appearances were similar, despite the fact that 22% of plasma tyrosine appearance could be attributed to the hydroxylation of phenylalanine. 3. A constant infusion of 115-120 mumol of L-phenylalanine/h did not significantly alter plasma leucine kinetics, but increased appearance of plasma phenylalanine and tyrosine. The percentage of phenylalanine and tyrosine appearance that was oxidized increased from 12.1% and 24.4% to 37.3% and 48.0% respectively. In phenylalanine-loaded rats, 72% of plasma tyrosine appearance could be attributed to the conversion of phenylalanine. 4. Whole-body tyrosine oxidation measured from a continuous infusion of either L-[14C]tyrosine or L-[14C]phenylalanine differed by 165%. 5. It can be concluded that, in the post-absorptive state, phenylalanine hydroxylation makes a substantial contribution to the plasma appearance of tyrosine and is significantly increased when phenylalanine is administered. The disposal of excess infused phenylalanine is a result of a greater percentage of plasma phenylalanine being converted into tyrosine and a greater proportion of tyrosine being further oxidized. However, apparent tyrosine oxidation rates estimated from plasma tyrosine specific radioactivities and appearance of expired 14CO2 during administration of [14C]tyrosine are underestimates of true rates, in part because tyrosine generated from phenylalanine hydroxylation is catabolized without freely equilibrating with the plasma compartment.

Amino Acids↗
PubMed1982

The conversion of phenylalanine to tyrosine in man. Direct measurement by continuous intravenous tracer infusions of L-[ring-2H5]phenylalanine and L-[1-13C] tyrosine in the postabsorptive state.

Steady state phenylalanine and tyrosine turnover and the rate of conversion of phenylalanine of tyrosine in vivo were determined in 6 healthy postabsorptive adult volunteers. Continuous infusions of tracer amounts of L-[ring-2H5]phenylalanine were determined intravenously for 13-14 hr. After 9-10 hr, a priming dose followed by a continuous infusion of L-[1-13C]tyrosine was added and maintained, along with the [2H5]phenylalanine infusion, for 4 hr. Venous plasma samples were obtained before the initiation of each infusion and every 30 min during the course of the combined [2H5]phenylalanine and [13C]tyrosine infusion for determination of isotopic enrichments of [2H5]phenylalanine, [13C]tyrosine, and [2H4]tyrosine by gas chromatograph-mass spectrometric analysis of the N-trifluoroacetyl-, methyl ester derivatives of the amino acids. Calculated from the observed enrichments, free phenylalanine and tyrosine turnover rates were 36.1 +/- 5.1 mumole . kg-1 . h-1 and 39.8 +/- 3.5 mumole . kg-1 . h-1, respectively. Phenylalanine was converted to tyrosine at the rate of 5.83 +/- 0.59 mumole . kg-1 . h-1, accounting for approximately 16% of either the phenylalanine or the tyrosine flux. The results indicate that the normal basal steady state phenylalanine hydroxylase activity in vivo in man is lower than that obtained from phenylalanine loading studies. This supports the existence of some type of substance activation of the enzyme as reflected in the previously reported exponential relationship between phenylalanine concentration and phenylalanine hydroxylase activity in vitro. The use of continuous simultaneous infusions of tracer amounts of stable isotope-labeled phenylalanine and tyrosine provides a direct means for studying physiological regulation of phenylalanine hydroxylase activity in vivo.

Absorption↗
PubMed1974

Effect of glucagon on phenylalanine metabolism and phenylalanine-degrading enzymes in the rat.

Glucagon administered subcutaneously to rats for 10 days had no significant effect on liver phenylalanine hydroxylase activity, but induced liver dihydropteridine reductase more than twofold. In rats administered a phenylalanine load orally, glucagon treatment stimulated oxidation and depressed urinary phenylalanine excretion. These responses could not be related to an effect of glucagon on hepatic tyrosine-alpha-oxoglutarate aminotransferase activity. Even in rats with phenylalanine hydroxylase activity depressed to 50% of control values by p-chlorophenylalanine administration, glucagon treatment increased the phenylalanine-oxidation rate substantially. Although hepatic phenylalanine-pyruvate aminotransferase was increased tenfold in glucagon-treated rats, glucagon treatment did not increase urinary excretion of phenylalanine transamination products by rats given a phenylalanine load. Glucagon treatment did not affect phenylalanine uptake by the gut or liver, or the liver content of phenylalanine hydroxylase cofactor. It is suggested that dihydropteridine reductase is the rate-limiting enzyme in phenylalanine degradation in the rat, and that glucagon may regulate the rate of oxidative phenylalanine metabolism in vivo by promoting indirectly the maintenance of the phenylalanine hydroxylase cofactor in its active, reduced state.

Administration, Oral↗
PubMed1982

Regulation of phenylalanine hydroxylase activity by phenylalanine in vivo, in vitro, and in perfused rat liver.

We show that phenylalanine is able to control the extent of activation and, as a result, the catalytic activity of rat liver phenylalanine hydroxylase in vivo, in perfused liver, and in vitro. Both phosphorylated and unphosphorylated enzyme activities are controlled by phenylalanine activation and, overall, this mechanism appears to be a major means of regulating the enzyme's activity in rat liver. At normal phenylalanine levels in vivo, phenylalanine hydroxylase is at most 1-4% activated, and phosphorylated enzyme (glucagon-induced) appears at most 5-7% activated under similar conditions. In both cases, a phenylalanine load increased the percentage of activated enzyme found in vivo to about 40% of maximal. In perfused rat livers, a plasma phenylalanine concentration of only 4 times normal induced a 4-fold increase in the amount of activated enzyme present and a corresponding functional increase in the rate of phenylalanine hydroxylation by the tissue. Under the latter conditions, more than 25% of the amino acid could be hydroxylated in a single pass through the organ. Purified phosphorylated phenylalanine hydroxylase must be activated to be catalytically active. The activation with phenylalanine, at equilibrium, is a cooperative process, and the phosphorylated enzyme is activated more rapidly at pH 6.8 and 8.0 and at lower phenylalanine concentration than the unphosphorylated species. Overall, phosphorylation appears to allow phenylalanine hydroxylase to be more easily activated at relatively low phenylalanine concentrations.

Animals↗
PubMed1975

Studies on the phenylalanine hydroxylase system in vivo. An in vivo assay based on the liberation of deuterium or tritium into the body water from ring-labeled L-phenylalanine.

The rate of release of deuterons into the body water from 2,3,4,5,6-pentadeutero-L-phenylalanine has been shown to be a valid measure of the activity of the phenylalanine hydroxylase system in vivo. At a dose of 0.5 g/kg, the rate of release of deuterons is linear for 60 to 90 min. Male rats, which had previously been shown to have 22 to 25% more phenylalanine hydroxylase activity in liver extracts than female rats, produced deuterons from deuterated phenylalanine at a rate 20 to 30% greater than female rats. p-Chlorophenylalanine, which irreversibly inhibits phenylalanine hydroxylase in vivo, caused a similar degree of inhibition of the rate of deuteron formation as was found when phenylalanine hydroxylase was measured in extracts from the same group of animals. Methotrexate, which inhibits the phenylalanine hydroxylase system by preventing regeneration of the tetrahydropteridine cofactor, caused parallel inhibition of the in vivo assay as well as when the conversion of phenylalanine to tyrosine was measured in liver slices. Randomly ring-tritiated phenylalanine can be used interchangeably with ring-deuterated phenylalanine if greater sensitivity is needed in the in vivo assay for phenylalanine hydroxylase. However, a dose of 20 to 30 muCi/kg is required. The in vivo deuterium release assay described in this paper should be useful in studying the physiological control of the phenylalanine hydroxylating system. It also may be of value in differentiating between individuals who are heterozygotes for phenylketonuria and those who are homozygotes for hyperphenylalaninemia.

Animals↗
PubMed1990

Mechanism of phenylalanine regulation of phenylalanine hydroxylase.

The mechanism of phenylalanine regulation of rat liver phenylalanine hydroxylase was studied. We show that phenylalanine "activates" phenylalanine hydroxylase, converting it from an inactive to active form, by binding at a true allosteric regulatory site. One phenylalanine molecule binds per enzyme subunit; it remains at this site during catalytic turnover and, while there, cannot be hydroxylated. Loss of phenylalanine from the site causes a loss of enzymatic activity. The rate of loss of activation is dramatically slowed by phenylalanine, which kinetically "traps" activated enzyme during relaxation from the activated to unactivated state. An empirical equation is presented which allows calculation of relaxation rates over a wide range of temperatures and phenylalanine concentrations. Kinetic trapping by phenylalanine is a novel effect. It was analyzed in detail, and its magnitude implied that phenylalanine activation involves cooperativity among all four subunits of the enzyme tetramer. A regulatory model is presented, accounting for the properties of the phenylalanine activation reaction in the forward and reverse directions and at equilibrium. Fluorescence quenching studies confirmed that activation increases the solvent accessibility of the enzyme's tryptophan residues. Physical and kinetic properties of purified phenylalanine hydroxylase from rat, rabbit, baboon, and goose liver were compared. All enzymes were remarkably alike in catalytic and regulatory properties, suggesting that control of this enzyme is similar in mammals and birds.

Animals↗
PubMed1994

Regulation of rat liver phenylalanine hydroxylase. III. Control of catalysis by (6R)-tetrahydrobiopterin and phenylalanine.

Effects of phenylalanine and di- and tetrahydropterins on presteady-state and steady-state catalytic behavior of rat liver phenylalanine hydroxylase are analyzed. From this and previous work (Shiman, R, Xia, T., Hill, M., and Gray, D.(1994) J. Biol. Chem. 269, 24647-24656), which analyzed binding of the same compounds to the enzyme in the absence of catalysis, a model of phenylalanine hydroxylase regulation is proposed. The mechanism appears novel in that 1) one substrate, phenylalanine, is a positive effector (activator), 2) a second substrate, (6R)-tetrahydrobiopterin (BH4), is a negative effector that blocks phenylalanine activation by forming an inactive BH4.enzyme complex, and 3) the BH4.enzyme complex sequesters BH4 and controls its metabolic availability. Reaction progress curves showing regulatory effects of BH4, 7,8-dihydrobiopterin (BH2), and phenylalanine are fit by the model with high precision. Data are presented that the high affinity pterin-binding site on unactivated phenylalanine hydroxylase is the pterin site that regulates catalysis. Occupancy of this site by BH4 or BH2 causes non-cooperative, linear inhibition of phenylalanine activation of the enzyme. All inhibitory effects of BH4 appear due to its binding at the pterin regulatory site on unactivated enzyme. BH2 inhibits by binding at the active site as well as the pterin regulatory site. 6-Methyltetrahydropterin also appears to bind at the pterin regulatory site, but its effect is only seen at high phenylalanine concentrations. Using kinetic constants measured in this and earlier work, quantitative effects of phenylalanine and BH4 regulation on the rate of the phenylalanine hydroxylase reaction in vitro and in vivo are calculated. The effects of formation of the BH4.enzyme complex on free BH4 concentration, on enzyme activity, and on regulation of the rate of phenylalanine hydroxylation in liver are discussed.

Animals↗
PubMed2001

High correlation between results of the [1-13C]-phenylalanine breath test and phenylalanine hydroxylase (EC 1.14.16.1) activity of the liver in rats.

BACKGROUND: (13)CO(2) is decreased in patients with end-stage liver disease by the [1-(13)C]-phenylalanine breath test. Decreased (13)CO(2) is supposed to be caused by the decreased ability of the liver to oxidize phenylalanine. However, no direct evidence has been reported. METHODS: The [1-(13)C]-phenylalanine breath test was performed in galactosamine hepatitis rats (n = 14) and control rats (n = 8). Plasma phenylalanine concentration before intravenous administration of [1-(13)C]-phenylalanine, the elimination rate of phenylalanine and the phenylalanine hydroxylase (PAH; EC 1.14.16.1) activity of the whole liver were examined. RESULTS: Increase of (13)CO(2) in the breath [Delta(13)CO(2) ( per thousand)] of galactosamine hepatitis rats 2 min after administration of [1-(13)C]-phenylalanine was only 1/5 of that in control rats. The concentration of plasma phenylalanine and the elimination rate of plasma phenylalanine in hepatitis rats did not show significant differences compared to control rats. On the other hand, a clear difference in the activity of PAH was observed between hepatitis rats and control rats. Delta(13)CO(2) ( per thousand) 2 min after administration of [1-(13)C]-phenylalanine was highly correlated to the PAH activity of the whole liver (r = 0.917). CONCLUSION: It was strongly indicated that decreased Delta(13)CO(2 ) in hepatitis rats was the result of decreased activity of PAH.

Animals↗
PubMed1992

Phenylalanine-induced phosphorylation and activation of rat hepatic phenylalanine hydroxylase in vivo.

Rats were given intraperitoneal injections of 2 mCi of carrier-free 32Pi and substances known to activate liver phenylalanine hydroxylase. After 30 min, these animals were anesthetized and their livers removed for analysis of enzyme activity, 32Pi incorporation into immunoprecipitated phenylalanine hydroxylase and [gamma-32P]ATP specific activity. Following glucagon treatment, rat liver phenylalanine hydroxylase activity was stimulated more than 6-fold when assayed in the presence of the natural cofactor, tetrahydrobiopterin (BH4). Glucagon injection also resulted in an incorporation of 0.41 mol of 32Pi/mol of hydroxylase subunit (approximately 50,000 Da). In vivo stimulation of phenylalanine hydroxylase activity and 32Pi incorporation by glucagon had been previously observed in this laboratory (Donlon, J., and Kaufman, S. (1978) J. Biol. Chem. 253, 6657-6659). However, we show for the first time in the present study that in vivo treatment with phenylalanine alone results in a 4-fold increase in the BH4-dependent activity of phenylalanine hydroxylase concomitant with a significant incorporation of phosphate into phenylalanine hydroxylase (0.51 mol of 32Pi/mol of hydroxylase subunit). It is further demonstrated in vivo that the combined treatment with phenylalanine and glucagon results in a greater than 10-fold stimulation of BH4-dependent activity and the greatest level of 32Pi incorporation (0.75 mol of 32Pi/mol of hydroxylase subunit). Phenylalanine did not produce an elevation in plasma glucagon in these animals. A model is, thereby, proposed with respect to the ligand binding effects of phenylalanine on the state of phosphorylation and activation of phenylalanine hydroxylase. The significance of these regulatory roles are considered in light of the probable physiological environment of the enzyme.

Animals↗
PubMed1983

Stereospecificity of phenylalanine plasma kinetics and hydroxylation in man following oral application of a stable isotope-labelled pseudo-racemic mixture of L- and D-phenylalanine.

L-[15N]Phenylalanine and D-[2H5]phenylalanine have been administered orally to two healthy adult volunteers as a pseudo-racemic mixture at a dose of 25 mg/kg each. After oral application, the plasma kinetics of phenylalanine and tyrosine have been followed by the combined use of high pressure liquid chromatography and field desorption mass spectrometry. Additional incubation with D-amino acid oxidase was used to determine the enantiomeric composition of the differently labelled species of phenylalanine and tyrosine. D-Phenylalanine plasma levels show a faster rise to higher maximum values compared to L-phenylalanine (D/L ratio at maximum 3.19, 3.26). L-Phenylalanine is efficiently hydroxylated to L-tyrosine. In contrast, conversion of D-phenylalanine to the L-form with subsequent hydroxylation to L-tyrosine was observed. From the plasma kinetics it is estimated that about 1/3 of the applied dose of 25 mg/kg of D-phenylalanine is converted to the L-isomer. Of the administered dose of L-phenylalanine only very small amounts are excreted into urine as such (0.25%, 0.8%), whereas a substantial amount of the D-phenylalanine dose is found in urine (27.4%, 38.0%).

Adult↗
PubMed1996

Twenty-four-hour intravenous and oral tracer studies with L-[1-13C]phenylalanine and L-[3,3-2H2]tyrosine at a tyrosine-free, generous phenylalanine intake in adults.

The daily rates of whole-body phenylalanine oxidation and hydroxylation were determined in young men receiving [1-13C]phenylalanine and [2H2]tyrosine via primed, constant intravenous (n=3) or oral (n=5) infusion for 24 consecutive hours (12-h fast followed by 12-h fed period), and given a generous phenylalanine (100 mg.kg-1.d-1), tyrosine-free, but otherwise adequate L-amino acid-based diet for 6 d before the tracer study. Our hypothesis was that subjects would be in whole-body phenylalanine equilibrium. Estimates of the daily rates of phenylalanine oxidation (phe-ox) and hydroxylation (phe-OH) were significantly higher for the subjects receiving the oral compared with intravenous tracer (P<0.01 for both comparisons), with the estimates of phe-ox obtained with oral tracer during the 12-h fast period being close to those predicted from similar 24-h leucine kinetic studies. The precision of the agreement between the measured 24-h rates of phe-ox and phe-OH compared with the predicted daily rates by extrapolation from the last hour of the 12-h fast and fifth hour of the fed period was poor. From the 24-h data, daily phenylalanine balances were estimated to be positive for both the intravenous and oral tracer protocols, although it was less positive for the oral tracer group. These results imply that the [13C]phenylalanine probe underestimated whole-body irreversible loss of phenylalanine, and suggest that daily phenylalanine balance in earlier 24-h phenylalanine-tyrosine tracer studies at low phenylalanine intakes may have been overestimated. Studies involving [13C]tyrosine as tracer will be required to further assess whole-body aromatic amino acid balance.

Administration, Oral↗