Liver Phenylalanine Hydroxylase Research Articles

Plasma tetrahydrobiopterin and its pharmacokinetic following oral administration

Tetrahydrobiopterin (BH 4) is widely used as a therapeutic agent in patients with BH 4 deficiencies and mild forms of phenylketonuria (PKU) and there is an increasing need for the measurement of its plasma concentrations in patients with cardiovascular disorders. We measured BH 4 and total biopterin in dithioerythritol (DTE) pretreated plasma from four adults after oral administration of BH 4 (2, 10, and 20 mg/kg body weight) using the differential iodine oxidation method. About 80% (range 64.8–92.2% ) of total biopterin was found as BH 4 when analyzed immediately after blood sampling. Compared with ascorbic acid as an antioxidant, DTE was more protective against oxidation of BH 4, particularly in samples stored over a period of 8 months. Without antioxidant (DTE or ascorbic acid) almost no BH 4 was detected. Furthermore, BH 4 and total biopterin were measured at different time intervals (up to 33 h after oral administration) and pharmacokinetic parameters T max (1–4 h), C max (258.7–259.0 nmol/L biopterin at a dosage of 10 mg/kg), and area under the curve (AUC=1708–1958 nmol *h/L up to T=10 h) were estimated. The elimination half-life time was calculated to be 3.3–5.1 h. Doubling the BH 4 dosage to 20 mg/kg resulted in 60% higher AUC while sublingual BH 4 application (2 mg/kg) resulted in 58–76% higher BH 4 plasma concentrations when compared with oral administration. These preliminary data suggest that in patients with BH 4 cofactor defects and BH 4-responsive phenylalanine hydroxylase deficiency, BH 4 should be given in at least two to three daily doses and that sublingual administration may lower the required BH 4 dosage and subsequently the cost of treatment. Due to inter individual differences in pharmacokinetic properties, in some patients with hyperphenylalaninemia and mild PKU plasma BH 4 levels may be not high enough to fully activate the liver phenylalanine hydroxylase and thus lower blood phenylalanine levels. Assessment of plasma BH 4 or total biopterin concentrations may be a good way to control the efficacy of the loading test.

L-[1- 13C] phenylalanine breath test reflects phenylalanine hydroxylase activity of the whole liver

Object The purpose of this study was to perform L-[1- 13C] phenylalanine breath test (PBT), measure phenylalanine hydroxylase (PAH) activity in liver tissue biopsies from patients, analyze the relationship between PBT results and PAH activity, and determine the time point at which measurements best reflect PAH activity in liver tissue. Methods PBT was performed in 25 patients (10 with normal liver and 15 with liver cirrhosis). After administering 10 mg/kg L-[1- 13C] phenylalanine, 300 ml of expired air was collected over 90 min at 15-min intervals. The rate of hepatic phenylalanine oxidation (% 13C dose h −1) at each time point was calculated from the amount of 13CO 2 in the breath, assuming a CO 2 production rate of 300 mmol m −2 body surface area per hour. Subsequently, we examined the relationship between the results of PBT and PAH activity. Results PAH activity of the whole liver was significantly decreased in hepatic cirrhosis patients ( P < 0.05). The results of PBT % 13C dose h −1 correlated with the PAH activity/liver, with correlation coefficients at 30, 45, and 60 min of more than 0.7, and the maximum correlation was at 30 min ( r = 0.821, P < 0.0001). % 13C cumulative excretion correlated with the PAH activity/liver with correlation coefficients of more than 0.7 after 45 min. The maximum correlation was at 90 min ( r = 0.770, P = 0.001). Conclusion PBT values reflect PAH activity in the whole liver and, in particular, the % dose h −1 at 30 min after oral administration highly correlates with PAH activity, providing an important indicator for monitoring changes in whole liver PAH activity.

Correlation of rat hepatic phenylalanine hydroxylase, with tetrahydrobiopterin and GTP concentrations

Hepatic phenylalanine hydroxylase is reported to be more abundant in experimentally-diabetic rats; whereas livers of animals fed a high protein diet, where gluconeogenesis also prevails, have normal amounts of this enzyme. In this study, in addition to seeking an explanation for this effect of experimental diabetes, we also examined the effects of providing alternative dietary gluconeogenic substrates. In rats fed a diet composed of 40% (w/w) glycerol, the specific activities of hepatic phenylalanine hydroxylase are decreased to about 60% of control values. There is no effect on the apparent state of phosphorylation of the enzyme. However, studies on the incorporation of radiolabelled leucine into liver phenylalanine hydroxylase suggested that there was a decreased rate of synthesis. Similarly, animals fed a diet containing 85% (w/w) fructose also have diminished phenylalanine hydroxylase activities. Under all of the above circumstances and also in streptozotocin-induced diabetic animals, alterations in the concentrations of the hydroxylase cofactor, tetrahydrobiopterin and of GTP closely correlate with the effects on the enzyme activities. They are elevated in livers of diabetic animals and significantly diminished in livers of rats fed diets rich in glycerol or fructose. These observations suggest that in adult rat both liver tetrahydrobiopterin concentrations and the expression of hepatic phenylalanine hydroxylase are regulated by GTP [210].

Oral phenylalanine loading in dopa-responsive dystonia: a possible diagnostic test.

To determine if there is abnormal phenylalanine and biopterin metabolism in patients with dopa-responsive dystonia (DRD), we measured plasma levels of phenylalanine, tyrosine, biopterin, and neopterin at baseline, and 1, 2, 4, and 6 hours after an oral phenylalanine load (100 mg/kg). Seven adults with DRD, two severely affected children with DRD, and nine adult controls were studied. All patients had phenylalanine and tyrosine concentrations within the normal range at baseline. In the adult patients, phenylalanine levels were higher than in controls at 2, 4, and 6 hours post-load (p < 0.0005); tyrosine concentrations were lower than control levels at 1, 2, and 4 hours post-load (p < 0.05). Phenylalanine to tyrosine ratios were elevated in patients at all times post-load (p < 0.0005). Biopterin levels in the patients were decreased at baseline and 1, 2, and 4 hours post-load (p < 0.005). Pretreatment with tetrahydrobiopterin (7.5 mg/kg) normalized phenylalanine and tyrosine profiles in two adult patients. In the children with DRD, phenylalanine to tyrosine ratios were slightly elevated at baseline. Following phenylalanine loading, the phenylalanine profiles were similar to those seen in the adult patients but there was no elevation in plasma tyrosine. Baseline biopterin levels were lower in the children with DRD than in the adult patients or the controls and there was no increase in biopterin post-load. In both the children and adults with DRD, neopterin concentrations did not differ from control values at baseline or after phenylalanine load. The results are consistent with decreased liver phenylalanine hydroxylase activity due to defective synthesis of tetrahydrobiopterin in patients with DRD. The findings show that a phenylalanine load may be useful in the diagnosis of this disorder.

The role of phenylalanine in structure-function relationships of phenylalanine hydroxylase revealed by radiation target analysis.

The activity of rat liver phenylalanine hydroxylase (PAH; phenylalanine 4-monooxygenase, EC 1.14.16.1) is regulated by interaction with its substrate, phenylalanine, and its coenzyme, BH4 [tetrahydrobiopterin (6R-dihydroxypropyl-L-erythro-5,6,7,8-tetrahydropterin)]. The structural changes accompanying these interactions have been studied by radiation target analysis. PAH purified from rat liver was incubated with 2 mM phenylalanine to achieve complete activation of the enzyme. Frozen samples were irradiated with various doses of high energy electrons; samples were subsequently thawed, and several surviving properties of the enzyme were determined. Each parameter decreased as a single exponential function of radiation dose. Radiation target analysis of enzymatic activity yielded a dimeric target size. Similar radiation effects on subunit monomers and on tetrameric structure were observed. Together with results from unactivated enzyme, these data show that phenylalanine increases the interactions between the subunits in a dimer and weakens the interactions between dimers in a tetramer. These alterations prevent the natural cofactor, a tetrahydrobiopterin, from exerting a negative effect on activity.

Competition for Tetrahydrobiopterin between Phenylalanine Hydroxylase and Nitric Oxide Synthase in Rat Liver

Tetrahydrobiopterin (BH4) is an important cofactor for two hepatic enzymes, inducible nitric oxide synthase (iNOS) and phenylalanine hydroxylase (PAH), and competition for BH4 between the two enzymes might limit hepatic iNOS or PAH activity. To test this hypothesis, we determined whether conversion of phenylalanine to tyrosine was modified by changes in NO synthase activity, and conversely whether NO synthesis was limited by the rate of phenylalanine conversion to tyrosine in rat hepatocytes and perfused livers. NO production was decreased only slightly, when flux through PAH was maximized in isolated perfused livers, and in isolated hepatocytes only when BH4 synthesis was inhibited. Increases in NO synthesis did not reduce tyrosine formation from phenylalanine. Phenylalanine markedly increased biopterin synthesis, whereas arginine had no effect. Thus, basal BH4 synthesis appears to be adequate to support iNOS activity, whereas BH4 synthesis is increased to support PAH activity.

Recombinant Human Phenylalanine Hydroxylase: Novel Regulatory and Structural Properties

Recombinant human liver phenylalanine hydroxylase (PAH) expressed inEscherichia colihas been purified to homogeneity. The recombinant enzyme exists in solution as a mixture of 80% tetramers and 20% dimers. A study of the kinetic properties of the enzyme indicates that compared to the recombinant and the native rat liver enzymes, the recombinant human enzyme is in an activated state. This conclusion is supported by the finding that its catalytic activity is only marginally stimulated by incubation with either phenylalanine or lysolecithin. In contrast, the native and the recombinant rat liver enzymes are activated 8- to 25-fold, respectively, when preincubated with phenylalanine or lysolecithin. In the absence of activators, the ratio of the hydroxylase activity in the presence of 6-methyl-5,6,7,8-tetrahydropterin compared to the activity in the presence of (6R)-5,6,7,8-tetrahydrobiopterin (BH4), which is an index of the state of activation of the enzyme, is 4 for the human recombinant PAH compared to a value of 12 for the recombinant rat liver enzyme. Furthermore, theKmfor phenylalanine in the presence of BH4is 0.050 mm, a value that is one-fifth that of the recombinant rat liver enzyme. Covalent modification of the human enzyme by phosphorylation with protein kinase A provides further evidence that the human enzyme is in a substantially activated state. Phosphorylation, which results in the incorporation of 0.6 mol of phosphate/mol of subunit, leads to only a modest activation of 1.5-fold compared to about a 3-fold activation seen after phosphorylation of the native and the recombinant rat liver enzymes. Moreover, the recombinant human liver enzyme is less sensitive than the rat liver enzyme to stimulation by lysolecithin when tryptophan is the substrate. Just as is true for the rat liver enzyme, the apparentKmvalues for tryptophan and phenylalanine vary with the pterin cofactor employed. The ability of 7-tetrahydrobiopterin (7-BH4) to substitute for the natural cofactor tetrahydrobiopterin has been studiedin vitro.The apparentKmfor 7-BH4for the recombinant human enzyme is 0.2 mmand theKmfor phenylalanine is 0.05 mm. The hydroxylase reaction is severely inhibited by 7-BH4in the presence of physiological concentrations of BH4. This inhibition can be overcome by a decrease in the concentration of phenylalanine. The implications of these novel properties of human PAH for phenylalanine homoestasis in man are discussed.

Tetrahydrobiopterin and biogenic amine metabolism in the hph-1 mouse.

hph-1 mice, which have defective tetrahydrobiopterin biosynthesis due to decreased GTP cyclohydrolase I activity, have been used to investigate the effects of tetrahydrobiopterin deficiency on aromatic L-amino acid monooxygenases and brain monoamine metabolism. Liver tetrahydrobiopterin levels were decreased, and tetrahydrobiopterin deficiency and reduced levels of dopamine, norepinephrine, serotonin, and their metabolites in the brain occurred both pre- and postnatally. Chronic subcutaneous tetrahydrobiopterin elevated brain levels to values higher than those seen in controls but had no effect on monoamine metabolism. In vivo activities of tyrosine hydroxylase and tryptophan hydroxylase were significantly decreased. There was a 30% decrease in the in vitro activity of striatal tyrosine hydroxylase and 50% decrease in liver phenylalanine hydroxylase. Western blotting demonstrated that the lower monooxygenase activities resulted from a reduced absolute amount of tyrosine hydroxylase and phenylalanine hydroxylase protein. The findings suggest involvement of tetrahydrobiopterin in the control of the steady-state concentration of the aromatic L-amino acid monooxygenases. In addition, demonstration of central monoamine changes in the hph-1 mouse make it a possible model system for the investigation of the neuropathological mechanisms in Dopa-responsive dystonia, which has recently been linked with mutations in the gene for GTP cyclohydrolase I.

Further studies of the role of Ser-16 in the regulation of the activity of phenylalanine hydroxylase.

It was previously proposed that the activation of rat liver phenylalanine hydroxylase (EC 1.14.16.1) by cAMP-dependent protein kinase-mediated phosphorylation of Ser-16 is due to the introduction of the negatively charged phosphate group. To explore the validity of this proposal, we have applied site-directed mutagenesis to specifically replace Ser-16 with negatively charged amino acids, glutamic and aspartic; with polar uncharged amino acids, asparagine and glutamine; with the positively charged amino acid lysine; and with the nonpolar hydrophobic amino acid alanine. The wild-type and mutant enzymes were purified to homogeneity, and the importance of Ser-16 in the activation of phenylalanine hydroxylase was examined by comparing the state of activation of the phosphorylated form of the wild-type hydroxylase with that of the mutants. The kinetic studies carried out on the wild-type phosphorylated hydroxylase showed that all the activation could be accounted for by an increase in Vmax with no change in Km for either phenylalanine or the pterin cofactor. Replacement of Ser-16 with a negatively charged residue, glutamate of aspartate, resulted in the activation of the hydroxylase by 2- to 4-fold, whereas replacement with glutamine, asparagine, lysine, or alanine resulted in a much more modest increase. Further, lysolecithin was found to stimulate the phosphorylated hydroxylase and the mutant enzymes S16E and S16D by a factor of 6-7. In contrast, the mutants S16Q, S16N, and S16A all showed the same magnitude of activation as the wild-type with lysolecithin. Therefore, this study demonstrates that activation of the enzyme by phosphorylation of Ser-16 by cAMP-dependent protein kinase is due to the introduction of negative charge(s) and strongly suggests the involvement of electrostatic interaction between the regulatory and catalytic domains of the hydroxylase.

Coordinate regulation of tetrahydrobiopterin turnover and phenylalanine hydroxylase activity in rat liver cells.

This work had two purposes: (i) to determine in vivo whether liver phenylalanine hydroxylase (PAH) is regulated by its substrates phenylalanine and tetrahydrobiopterin (BH4) as studies with purified enzyme suggest and (ii) to investigate in vivo the relationship between PAH activity and BH4 turnover. We found there are two BH4 pools in hepatocytes, one that is metabolically available (free BH4) and one that is not (bound BH4). Bound BH4 appears bound to PAH; the PAH-BH4 complex has much less catalytic activity and is less readily phenylalanine activated than uncomplexed enzyme. Interconversion of activated and unactivated PAH and bound and free BH4 is driven by phenylalanine; and free BH4 concentration is determined by the state of activation and activity of PAH. In hepatocytes, BH4 and PAH (subunit) concentrations are equal, all intracellular BH4 appears to be available to PAH, and free BH4 turns over rapidly (t1/2 approximately 1 hr). There is no evidence for feedback inhibition of BH4 synthesis; the BH4 synthetic rate appears high when free BH4 concentration is high and low when free BH4 is low. The data provide support in vivo that phenylalanine and BH4 are positive and negative regulators of the activity and activation state of PAH in the proposed manner; they also imply that regulation of BH4 turnover and PAH activity are linked processes, which are both controlled by phenylalanine concentration.

Regulation of rat liver phenylalanine hydroxylase. II. Substrate binding and the role of activation in the control of enzymatic activity.

Activation by phenylalanine and reduction by the co-factor (6R)-tetrahydrobiopterin (BH4) are required for formation of active liver phenylalanine hydroxylase. This work describes effects of the activation and redox state on substrate and effector recognition of this enzyme, it establishes relationships among the pterin and phenylalanine binding sites on the different forms of the enzyme, and it provides a quantitative description of the enzyme's presumptive regulatory and catalytic sites. BH4, 7,8-dihydrobiopterin (BH2), 6-methyltetrahydropterin, and 5-deaza-6-methyltetrahydropterin were found to bind to unactivated phenylalanine hydroxylase with a stoichiometry of 1/enzyme subunit and with hyperbolic kinetics; all appear to compete for the same binding site on the enzyme, and all appear to bind in the proximity of, but not to, the enzyme's non-heme iron. In the transition from unactivated to activated enzyme, phenylalanine and pterin binding is modified, a new site for phenylalanine is formed, and the pterin site is replaced by a site of greatly decreased affinity for BH4 and BH2, one which does not appear to recognize the dihydroxypropyl side chain of BH4 and BH2. The pterin- and phenylalanine-binding sites on activated phenylalanine hydroxylase appear to be part of the enzyme's active site. Despite large effects on substrate binding, neither chelator binding ability nor solvent accessibility of the iron are affected by activation; activation appears to affect the nearby environment of the enzyme's iron but not the iron itself. Studies of oxidized and reduced phenylalanine hydroxylase indicate that the redox state is not a major determinant of pterin and phenylalanine association with enzyme.

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.

Regulation of rat liver phenylalanine hydroxylase. I. Kinetic properties of the enzyme's iron and enzyme reduction site.

Tetrahydropterins react with phenylalanine hydroxylase at a redox site, a regulatory site, and the catalytic site, but neither the properties of nor relationships among these sites are well understood. We have studied the redox site using the fluorescent iron chelators 2,3-dihydroxynaphthalene and bathophenanthroline; these compounds act as site-specific reporter groups for reactions on oxidized and reduced enzyme, respectively. The chelators bind reversibly and specifically to the enzyme's iron with 1:1 stoichiometry, high affinity (Kd values approximately 1 nM), and complete quenching of their own fluorescence. The kinetic behavior of these and other iron chelators indicates that the enzyme's iron is solvent accessible and in a hydrophobic pocket of the protein. Both ferrous and ferric chelators inhibit phenylalanine hydroxylase activity. Bathophenanthroline inhibits by binding to Fe2+ on reduced, active enzyme. 2,3-Dihydroxynaphthalene inhibits by binding to Fe3+ on enzyme that is oxidized during catalysis. This oxidation occurs approximately 1/150 enzyme turnovers, and its rate is increased when p-chloro- or p-fluorophenylalanine is used as the reaction substrate. Studies of the reaction of tetrahydrobiopterin (BH4) at the enzyme's redox site showed that BH4 reduces the enzyme more slowly than 6-methyltetrahydropterin under catalytic and non-catalytic conditions. Reduction occurs at a distinct site whose binding determinants and reaction characteristics are different from those of the BH4 regulatory or catalytic sites, and phenylalanine-activated enzyme is reduced more rapidly than unactivated enzyme. In reducing phenylalanine activated enzyme, BH4 donates one electron/subunit (1/iron atom); the reduction kinetics suggest a trihydrobiopterin-free radical as a reaction intermediate.

Products of the Tyrosine-Dependent Oxidation of Tetrahydrobiopterin by Rat Liver Phenylalanine Hydroxylase

Phenylalanine hydroxylase, a tetrahydrobiopterin (BH4)-dependent oxygenase, catalyzes the conversion of phenylalanine to tyrosine. During this physiological reaction, the oxidation of BH4 is tightly coupled to the hydroxylation of the amino acid substrate with a stoichiometry of 1 mol of BH4 oxidized for every mole of tyrosine formed. In the presence of an activator and certain analogues of either the amino acid or the pterin coenzyme, the enzyme also catalyzes a partially uncoupled reaction in which the amount of tetrahydropterin oxidized exceeds the amount of amino acid hydroxylated. With tyrosine as the amino acid analogue, the reaction is completely uncoupled, i.e., the enzyme functions as a tetrahydropterin oxidase with no net hydroxylation of the amino acid. We have investigated whether the normal pterin intermediate, a 4a-hydroxytetrahydropterin, is formed during the completely uncoupled reaction and whether oxygen is reduced exclusively to the level of water, as in the tightly coupled reaction, or whether some reduction to hydrogen peroxide occurs. We present direct evidence that both the normal hydroxytetrahydropterin intermediate and hydrogen peroxide are formed. The amount of hydrogen peroxide formed, however, is not stoichiometric with the amount of oxygen reduced, an indication that there are at least two pathways for this reduction, one leading to the formation of water and one to hydrogen peroxide. We also postulate that hydrogen peroxide and the hydroxytetrahydropterin are both derived from alternate routes of breakdown of a common precursor, the corresponding 4a-hydroperoxytetrahydropterin. By contrast, in the normal tightly coupled reaction, we suggest that this peroxy compound participates in the hydroxylation of the amino acid substrate.

Expression of rat liver phenylalanine hydroxylase in insect cells and site-directed mutagenesis of putative non-heme iron-binding sites.

Rat liver phenylalanine hydroxylase was expressed in both Escherichia coli and the Spodoptera frugiperda insect cell line, Sf9. Recombinant enzyme from E. coli was inactive and contained less than 0.1 iron atom/subunit. In contrast, recombinant enzyme expressed in Sf9 cells using a baculovirus vector was active and identical in several properties to phenylalanine hydroxylase from rat liver: the Km for 6-methyltetrahydropterin was 39 microM (compared with 35 microM for the rat liver enzyme), 1 atom of iron was "associated" per enzyme subunit, and electron paramagnetic resonance spectra showed that iron was distributed within two distinct environments. Putative iron-binding sites of phenylalanine hydroxylase were studied by mutating either histidine 284 or 289 to serine and expressing these mutant enzymes (PAH-H284S and PAH-H289S) in Sf9 cells. Mutants were expressed at levels similar to wild-type PAH, but contained < or = 0.1 iron/subunit and were inactive. Thus, both His284 and His289 apparently are required for iron binding and hydroxylation activity.

"7-tetrahydrobiopterin," a naturally occurring analogue of tetrahydrobiopterin, is a cofactor for and a potential inhibitor of the aromatic amino acid hydroxylases.

The ability of 2-amino-4-hydroxy-7-[dihydroxylpropyl-(L-erythro)-5,6,7,8-tetrahyd ropterin] ("7-tetrahydrobiopterin" or 7-BH4) to substitute for the natural cofactor tetrahydrobiopterin (BH4) has been studied in vitro in the reactions of the three mammalian aromatic amino acid hydroxylases. With rat liver phenylalanine hydroxylase, the apparent Km for 7-BH4 is 160 microM, a value that is approximately 60-fold greater than that for the natural cofactor. In contrast, the hydroxylase reaction is severely inhibited by as little as 1 microM 7-BH4 when assayed in the presence of physiological concentrations of BH4. This inhibition can be overcome either by an increase in the concentration of BH4 or a decrease in the concentration of phenylalanine. With both rat brain tryptophan hydroxylase and rat pheochromocytoma tyrosine hydroxylase, the Km value for 7-BH4 is about one order of magnitude greater than the Km for BH4. Accordingly, 7-BH4 is a poor competitive inhibitor of both tryptophan and tyrosine hydroxylase. Thus, our results suggest that the observed hyperphenylalaninemia in patients who excrete 7-BH4 in their urine may arise directly from the inhibition of phenylalanine hydroxylase by low levels of this pterin. On the other hand, it is less likely that low levels of 7-BH4 would affect the activity of tyrosine or tryptophan hydroxylase in vivo.

A comparative study of Drosphila phenylalanine hydroxylase with a natural and a synthetic tetrahydropterin as cofactor

1. 1. Phenylalanine hydroxylase activity has been analyzed in Drosphila melanogaster as cofactors the natural tetradropteridine 5,6,7,8-tetrahydrobiopterin (H 4Bip) and the synthetic one 5,6-dimethyl-5,6,7,8-tetrahydropteridin (H 4Dmp). 2. 2. The apparent V max and K M for substrate and cofactor in the reaction. Similarly to what was found two times more affinity for the substrate when H 4Bip is the cofactor in the reaction. Similarly to what was found with purified rat liver phenylalanine hydroxylase, H 4Bip was the most effective cofactor, leading to 4–5 times more activity than that obtained with H 4Dmp. 3. 3. With the natural cofactor H 4Bip, no activation of the enzymes with Phe was necessary (in contrast to mammalian phenylalanine hydroxylase), and this tetrahydropteridine inhibits phenylalanine hydroxylase activity when the enzymes is exposed to it before phenylalanine addition. With the synthetic H 4Dmp, both types of preincubations led to an increase of phenylalanine hydrolase activity. 4. 4. The enzymes is highly unstable compared to mammalian phenylalanine hydrolase, even at -20°C. 5. 5. Thorax and abdomen extracts caused significant inhibition of phenylalanine hydroxylase activity from third instar larvae or newborn adult head extracts, when assayed with the synthetic cofactor H 4Dmp. This inhibitor did not happen with H 4Bip. The presence of the pteridine 7-xanthopterin in adult bodies was not the cause of this inhibition.

The effect of vanadate upon the expression of phenylalanine hydroxylase in streptozotoxin-diabetic rat liver

Induction of diabetes in rats is associated with a significant elevation in the phenylalanine hydroxylating capacity of the liver. This phenomenon reflects an increase in the abundance of both phenylalanine hydroxylase protein and phenylalanine hydroxylase-specific mRNA. These changes can be abolished by insulin-dependent control of diabetes. We show here that the control of diabetes of oral administration of sodium orthovanadate will also nullify the diabetes-related alterations in phenylalanine hydroxylase expression. In addition, diabetes-induced changes in the extent of phosphorylation of phenylalanine hydroxylase are reversed by either or vanadate treatment in vivo. These treatments also abolished the diabetes-related, approx. 30-fold, decrease in glucagon sensitivity of phenylalanine hydroxylation in isolated liver cells.

Immunological detection of phenylalanine hydroxylase protein in Drosophila melanogaster.

A monoclonal antibody raised against monkey liver phenylalanine hydroxylase (PAH) has been used to detect this protein in Drosophila melanogaster. A cross-reacting material (CRM) band of apparent molecular mass 50-52 kDa, equivalent to that deduced for the Drosophila melanogaster PAH protein based on the pah gene cDNA sequence, has been detected. This CRM was analysed throughout development and showed an equivalent pattern to that reported for PAH activity in this insect, with maxima at pupariation and at pharate adult formation. Distribution of this CRM in larval tissues, the haemolymph and the adult body is mainly restricted to the larval fat body and the adult head. Demonstration of this CRM as the PAH protein comes from the correlation between the decreased PAH enzyme activities of two mutant strains and their decreased amounts of CRM by Western blotting.

Phenylalanine hydroxylaselike antigen in human chorionic villi

An antigen similar by electrophoretic mobility to liver phenylalanine hydroxylase (PH) and cross-reacting with monoclonal antibody PH8 against liver PH was detected in extracts of soluble proteins in 6 from 23 samples of chorionic villi. An antigen with electrophoretic mobility corresponding to 40-41 kDa was detected in extracts of membrane proteins from these 23 samples by immunoblotting with monoclonal antibody PH8. Its molecular weight was similar to that of major chymotryptic peptide of human liver PH. The content of the antigen varied with samples and was less than 20 ng/mg of the extracted protein. Two-dimensional gel electrophoresis revealed only 1 spot of the antigen. The antigen did not react with monoclonal antibodies PH7 and PH9 epitopes of which were located in N-terminal fragment of liver PH. These data suggest that the antigen of membrane fraction could be a PH protein without N-terminal domain.