The phenylalanine hydroxylating system from mammalian liver

Clinical, genetic, and experimental research of hyperphenylalaninemia

Hyperphenylalaninemia (HPA) is the most common amino acid metabolism defect in humans. It is an autosomal-recessive disorder of the phenylalanine (Phe) metabolism, in which high Phe concentrations and low tyrosine (Tyr) concentrations in the blood cause phenylketonuria (PKU), brain dysfunction, light pigmentation and musty odor. Newborn screening data of HPA have revealed that the prevalence varies worldwide, with an average of 1:10,000. Most cases of HPA result from phenylalanine hydroxylase (PAH) deficiency, while a small number of HPA are caused by defects in the tetrahydrobiopterin (BH4) metabolism and DnaJ heat shock protein family (Hsp40) member C12 (DNAJC12) deficiency. Currently, the molecular pathophysiology of the neuropathology associated with HPA remains incompletely understood. Dietary restriction of Phe has been highly successful, although outcomes are still suboptimal and patients find it difficult to adhere to the treatment. Pharmacological treatments, such as BH4 and phenylalanine ammonia lyase, are available. Gene therapy for HPA is still in development.

Phenylketonuria

Phenylketonuria (PKU; also known as phenylalanine hydroxylase (PAH) deficiency) is an autosomal recessive disorder of phenylalanine metabolism, in which especially high phenylalanine concentrations cause brain dysfunction. If untreated, this brain dysfunction results in severe intellectual disability, epilepsy and behavioural problems. The prevalence varies worldwide, with an average of about 1:10,000 newborns. Early diagnosis is based on newborn screening, and if treatment is started early and continued, intelligence is within normal limits with, on average, some suboptimal neurocognitive function. Dietary restriction of phenylalanine has been the mainstay of treatment for over 60 years and has been highly successful, although outcomes are still suboptimal and patients can find the treatment difficult to adhere to. Pharmacological treatments are available, such as tetrahydrobiopterin, which is effective in only a minority of patients (usually those with milder PKU), and pegylated phenylalanine ammonia lyase, which requires daily subcutaneous injections and causes adverse immune responses. Given the drawbacks of these approaches, other treatments are in development, such as mRNA and gene therapy. Even though PAH deficiency is the most common defect of amino acid metabolism in humans, brain dysfunction in individuals with PKU is still not well understood and further research is needed to facilitate development of pathophysiology-driven treatments.

Investigation of amino acids and minerals in Chinese breast milk

BACKGROUND

The nutrients in human milk, particularly amino acids and minerals, are important for infant growth and development. Since there are few reports of amino acids and minerals in Chinese breast milk, we conducted this study to establish a representative preliminary database of breast milk nutrients in Chinese breast milk. In this study, we collected breast milk from healthy mothers in seven cities in western, southern and central China. The composition, content and proportion of total amino acids and ten elements (potassium, sodium, calcium, magnesium, iron, zinc, manganese, copper, selenium and phosphorus) in human milk in different lactation stages were investigated.

RESULTS

In this study, it was found that the content of total essential amino acids (671.47 mg 100 mL^-1 ) in Chinese breast milk was higher compared with the European Society for Paediatric Gastroenterology Hepatology and Nutrition (ESPGHAN) (574 mg 100 mL^-1 ), but the content of leucine (LEU) (129.01 mg 100 mL^-1 ) and cysteine (CYS) (20.31 mg 100 mL^-1 ) was much lower than that recommended by ESPGHAN. Moreover, it was found that the content of most of these ten elements decreased during lactation, and the content of calcium in Chinese breast milk was lower compared with ESPGHAN. In addition, the content of selenium (7.23-20.55 mg 1000 mL^-1 ) in breast milk from the three cities Nanchang, Shanghai and Guangzhou in China was much higher than that recommended by ESPGHAN.

CONCLUSIONS

In a word, amino acids and minerals in Chinese human milk showed a significant difference from other countries. Human milk meal or infant food should be regulated to meet the requirements of the infant and to maintain the balance of the amino acids and minerals. © 2020 Society of Chemical Industry.

Untargeted metabolomic analysis of the carotenoid-based orange coloration in Haliotis gigantea using GC-TOF-MS

Seafood coloration is typically considered an indicator of quality and nutritional value by consumers. One such seafood is the Xishi abalone (Haliotis gigantea), which displays muscle color polymorphism wherein a small subset of individuals display orange coloration of muscles due to carotenoid enrichment. However, the metabolic basis for carotenoid accumulation has not been thoroughly investigated in marine mollusks. Here, GC-TOF-MS-based untargeted metabolite profiling was used to identify key pathways and metabolites involved in differential carotenoid accumulation in abalones with variable carotenoid contents. Cholesterol was the most statistically significant metabolite that differentiated abalones with orange muscles against those with common white muscles. This observation is likely due to the competitive interactions between cholesterol and carotenoids during cellular absorption. In addition, the accumulation of carotenoids was also related to fatty acid contents. Overall, this study indicates that metabolomics can reflect physiological changes in organisms and provides a useful framework for exploring the mechanisms underlying carotenoid accumulation in abalone types.

Moderation of the relationship between Toxoplasma gondii seropositivity and trait impulsivity in younger men by the phenylalanine-tyrosine ratio

Previously, we reported that Toxoplasma gondii (T. gondii)-seropositivity is associated with higher impulsive sensation seeking in younger men. As dopaminergic and serotonergic signaling regulate impulsivity, and as T. gondii directly and indirectly affects dopaminergic signaling and induces activation of the kynurenine pathway leading to the diversion of tryptophan from serotonin production, we investigated if dopamine and serotonin precursors or the tryptophan metabolite kynurenine interact with the T. gondii-impulsivity association. In 950 psychiatrically healthy participants, trait impulsivity scores were related to T. gondii IgG seropositivity. Interactions were also identified between categorized levels of phenylalanine (Phe), tyrosine (Tyr), Phe:Tyr ratio, kynurenine (Kyn), tryptophan (Trp) and Kyn:Trp ratio, and age and gender. Only younger T. gondii-positive men with a high Phe:Tyr ratio, were found to have significantly higher impulsivity scores. There were no significant associations in other demographic groups, including women and older men. No significant effects or interactions were identified for Phe, Tyr, Kyn, Trp, or Kyn:Trp ratio. Phe:Tyr ratio, therefore, may play a moderating role in the association between T. gondii seropositivity and impulsivity in younger men. These results could potentially lead to individualized approaches to reduce impulsivity, based on combined demographic, biochemical and serological factors.

Amino acids and the kidney

The kidney has an important role in the metabolism of amino acids and control of plasma concentrations. Reabsorption by the tubules recovers about 70g/day of amino acids, derived from both the diet and metabolism in other tissues. Amino acids regulate haemodynamics and proteolysis and maintain integrity of the kidney. Abnormal plasma and muscle amino acid profiles in chronic renal failure (i.e. low essentials and tyrosine with high nonessentials) first indicated malnutrition, which can be partially corrected by supplementation. The loss of effective kidney tissue and uraemia, in addition to nutrition, have been considered in studies of phenylalanine hydroxylation used to investigate low tyrosine. Investigations in normal kidney have shown that glutamine uptake maintains acid-base homeostasis, glycine and citrulline are removed, and serine and arginine are released into the circulation. These metabolic processes are impaired in chronic renal failure. Uraemia affects most tissues and causes malnutrition, whilst acidosis activates catabolism of amino acids and proteins in muscle. Hyperinsulinaemia probably depresses plasma branchedchain amino acids and particularly valine. These abnormalities are less likely to respond to dietary supplementation.

Role of the phenylalanine-hydroxylating system in aromatic substance degradation and lipid metabolism in the oleaginous fungus Mortierella alpina

Mortierella alpina is a filamentous fungus commonly found in soil that is able to produce lipids in the form of triacylglycerols that account for up to 50% of its dry weight. Analysis of the M. alpina genome suggests that there is a phenylalanine-hydroxylating system for the catabolism of phenylalanine, which has never been found in fungi before. We characterized the phenylalanine-hydroxylating system in M. alpina to explore its role in phenylalanine metabolism and its relationship to lipid biosynthesis. Significant changes were found in the profile of fatty acids in M. alpina grown on medium containing an inhibitor of the phenylalanine-hydroxylating system compared to M. alpina grown on medium without inhibitor. Genes encoding enzymes involved in the phenylalanine-hydroxylating system (phenylalanine hydroxylase [PAH], pterin-4α-carbinolamine dehydratase, and dihydropteridine reductase) were expressed heterologously in Escherichia coli, and the resulting proteins were purified to homogeneity. Their enzymatic activity was investigated by high-performance liquid chromatography (HPLC) or visible (Vis)-UV spectroscopy. Two functional PAH enzymes were observed, encoded by distinct gene copies. A novel role for tetrahydrobiopterin in fungi as a cofactor for PAH, which is similar to its function in higher life forms, is suggested. This study establishes a novel scheme for the fungal degradation of an aromatic substance (phenylalanine) and suggests that the phenylalanine-hydroxylating system is functionally significant in lipid metabolism.

A new model for allosteric regulation of phenylalanine hydroxylase: implications for disease and therapeutics

The structural basis for allosteric regulation of phenylalanine hydroxylase (PAH), whose dysfunction causes phenylketonuria (PKU), is poorly understood. A new morpheein model for PAH allostery is proposed to consist of a dissociative equilibrium between two architecturally different tetramers whose interconversion requires a ∼90° rotation between the PAH catalytic and regulatory domains, the latter of which contains an ACT domain. This unprecedented model is supported by in vitro data on purified full length rat and human PAH. The conformational change is both predicted to and shown to render the tetramers chromatographically separable using ion exchange methods. One novel aspect of the activated tetramer model is an allosteric phenylalanine binding site at the intersubunit interface of ACT domains. Amino acid ligand-stabilized ACT domain dimerization follows the multimerization and ligand binding behavior of ACT domains present in other proteins in the PDB. Spectroscopic, chromatographic, and electrophoretic methods demonstrate a PAH equilibrium consisting of two architecturally distinct tetramers as well as dimers. We postulate that PKU-associated mutations may shift the PAH quaternary structure equilibrium in favor of the low activity assemblies. Pharmacological chaperones that stabilize the ACT:ACT interface can potentially provide PKU patients with a novel small molecule therapeutic.

Specificity of the acute tryptophan and tyrosine plus phenylalanine depletion and loading tests I. Review of biochemical aspects and poor specificity of current amino Acid formulations

The acute tryptophan or tyrosine plus phenylalanine depletion and loading tests are powerful tools for studying the roles of serotonin, dopamine and noradrenaline in normal subjects and those with behavioural disorders. The current amino acid formulations for these tests, however, are associated with undesirable decreases in ratios of tryptophan or tyrosine plus phenylalanine to competing amino acids resulting in loss of specificity. This could confound biochemical and behavioural findings. Compositions of current formulations are reviewed, the biochemical principles underpinning the tests are revisited and examples of unintended changes in the above ratios and their impact on monoamine function and behaviour will be demonstrated from data in the literature. The presence of excessive amounts of the 3 branched-chain amino acids Leu, Ile and Val is responsible for these unintended decreases and the consequent loss of specificity. Strategies for enhancing the specificity of the different formulations are proposed.

Specificity of the Acute Tryptophan and Tyrosine Plus Phenylalanine Depletion and Loading Tests Part II: Normalisation of the Tryptophan and the Tyrosine Plus Phenylalanine to Competing Amino Acid Ratios in a New Control Formulation

Current formulations for acute tryptophan (Trp) or tyrosine (Tyr) plus phenylalanine (Phe) depletion and loading cause undesirable decreases in ratios of Trp or Tyr + Phe to competing amino acids (CAA), thus undermining the specificities of these tests. Branched-chain amino acids (BCAA) cause these unintended decreases, and lowering their content in a new balanced control formulation in the present study led to normalization of all ratios. Four groups (n = 12 each) of adults each received one of four 50 g control formulations, with 0% (traditional), 20%, 30%, or 40% less of the BCAA. The free and total [Trp]/[CAA] and [Phe + Tyr]/[BCAA + Trp] ratios all decreased significantly during the first 5 h following the traditional formulation, but were fully normalized by the formulation containing 40% less of the BCAA. We recommend the latter as a balanced control formulation and propose adjustments in the depletion and loading formulations to enhance their specificities for 5-HT and the catecholamines.

Double blind placebo control trial of large neutral amino acids in treatment of PKU: effect on blood phenylalanine

Large neutral amino acids (LNAA) have been used on a limited number of patients with phenylketonuria (PKU) with the purpose of decreasing the influx of phenylalanine (Phe) to the brain. In an open-label study using LNAA, a surprising decline of blood Phe concentration was found in patients with PKU in metabolic treatment centres in Russia, the Ukraine, and the United States. To validate the data obtained from this trial, a short-term double-blind placebo control study was done using LNAA in patients with PKU, with the participation of three additional metabolic centres--Milan, Padua and Rio de Janeiro. The results of the short trial showed significant lowering of blood Phe concentration by an average of 39% from baseline. The data from the double-blind placebo control are encouraging, establishing proof of principle of the role of orally administered LNAA in lowering blood Phe concentrations in patients with PKU. Long-term studies will be needed to validate the acceptability, efficacy and safety of such treatment.

Structure and function of the aromatic amino acid hydroxylases

Images

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.

The role of reversible phosphorylation in the hormonal control of phenylalanine hydroxylase in isolated rat proximal kidney tubules

Reversible phosphorylation is the major mechanism underlying the short-term hormonal control of phenylalanine hydroxylase activity in the liver. We report here, for the first time, the impact of a range of hormonal effectors on both the phosphorylation state and enzymic activity of phenylalanine hydroxylase present in isolated rat proximal kidney tubules. The most potent stimulator of enzyme phosphorylation was found to be parathyroid hormone, which is known to stimulate the production of cyclic AMP in proximal-tubule cells. In addition, adrenergic amines also stimulated enzyme phosphorylation, although to a lesser extent, through interaction with a mixed alpha 1 and beta receptor population.

Heterozygous carriers of classical phenylketonuria in a sample of the Turkish population: detection by a spectrofluorimetric method

Blood obtained by finger prick from 209 presumed normal homozygotes and 42 heterozygotes for classical PKU was analyzed for plasma phenylalanine (Phe) and tyrosine (Tyr) by a fluorimetric method. Subjects were tested near midday and 3 hours after a protein-rich breakfast. The plot of Phe/Tyr (micromolar ratio) against Phe2/Tyr, permitted the detection of 11 heterozygotes among 209 controls. The accuracy of this method was checked by computation of a stepwise multivariate discriminant analysis, using Phe and Tyr (mumol/L), Phe/Tyr micromolar ratio and Phe2/Tyr as variables. Ten of the 11 subjects were recovered with a percentage of correct classification of over 90%, while one case had a percentage of 89.45%. The PKU gene frequency was found to be 1/19. This emphasizes the importance of a screening programme for PKU gene carrier status in Turkey.

Pahhph-5: a mouse mutant deficient in phenylalanine hydroxylase

Mutant mice exhibiting heritable hyperphenylalaninemia have been isolated after ethylnitrosourea mutagenesis of the germ line. We describe one mutant pedigree in which phenylalanine hydroxylase activity is severely deficient in homozygotes and reduced in heterozygotes while other biochemical components of phenylalanine catabolism are normal. In homozygotes, injection of phenylalanine causes severe hyperphenylalaninemia and urinary excretion of phenylketones but not hypertyrosinemia. Severe chronic hyperphenylalaninemia can be produced when mutant homozygotes are given phenylalanine in their drinking water. Genetic mapping has localized the mutation to murine chromosome 10 at or near the Pah locus, the structural gene for phenylalanine hydroxylase. This mutant provides a useful genetic animal model affected in the same enzyme as in human phenylketonuria.

Modulation by pterins of the phosphorylation and phenylalanine activation of phenylalanine 4-mono-oxygenase

The interaction between phenylalanine 4-mono-oxygenase and analogues of the natural cofactor (6R)-tetrahydrobiopterin [(6R)-BH4] was studied. The rate of cyclic AMP-dependent phosphorylation of phenylalanine 4-mono-oxygenase was inhibited only by those pterins [(6R)-BH4, (6S)-BH4 and 7,8-dihydrobiopterin (BH2)] that were able to decrease the potency and efficiency of phenylalanine as an allosteric activator of the hydroxylase. Since BH2 lacks cofactor activity, this was not required to modulate either the phosphorylation or the phenylalanine-activation of the hydroxylase. Half-maximal inhibition of the phosphorylation was observed at 1.9 microM-(6R)-BH4, 9 microM-(6S)-BH4 and 17 microM-BH2. Competition experiments indicated that all three pterins acted through binding to the cofactor site of the hydroxylase. Since the phosphorylation site and the cofactor binding site are known to reside, respectively, in the N- and C-terminal domains of the hydroxylase, the pterins were able to induce an interdomain conformational change. BH2, whose dihydroxypropyl group is not subject to epimerization, and (6S)-BH4 both inhibited the phosphorylation less efficiently than did the (6R)-epimer of BH4. Pterins with different spatial arrangements of the dihydroxypropyl side chain thus appeared to elicit different conformations of the phosphorylation site. The hydroxylase reaction showed a higher apparent Km for (6S)-BH4 than for (6R)-BH4 both when the native and the phenylalanine-activated enzyme were tested. For the activated enzyme Vmax was 40% lower with the (6S)-epimer than the (6R)-epimer, also when the more rapid enzyme inactivation occurring with the former cofactor was taken into account.

The polyamine-dependent modulation of phenylalanine hydroxylase phosphorylation state and enzymic activity in isolated liver cells

The role of polyamines in the control of phenylalanine hydroxylase phosphorylation state and enzymic activity was investigated. Pre-treatment of liver cells with spermine (1 mM) abolishes the glucagon (1 nM)-stimulated increase in hydroxylase phosphorylation. Concurrently there is a decrease in phenylalanine hydroxylation flux, reflecting decreased enzyme activity; 50% inhibition occurs at approx. 10 microM-spermine. These results are discussed in the context of reports concerning the properties of protein phosphatase 2A.

Phosphopeptide analysis of phenylalanine hydroxylase isolated from liver cells exposed to hormonal stimuli

Hormonal control of the phosphorylation of phenylalanine hydroxylase was studied by using rat liver cells incubated with [32P]Pi. After immunoprecipitation from cell extracts, the hydroxylase was subjected to proteinase digestion and subsequent sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. V8-proteinase digestion yielded one major 32P-labelled fragment, of approx. 9 kDa. Chymotrypsin digestion gave five 32P-labelled fragments ranging from approx. 39 kDa to approx. 10 kDa. Noradrenaline (10 microM) and glucagon (0.1 microM) enhanced the 32P content of all peptide fragments uniformly. Phorbol ester, in contrast with ionophore A23187, did not stimulate enzyme phosphorylation or enhance phenylalanine metabolism in liver cells. These results are discussed in relation to the nature of the protein kinase(s) that mediate phosphorylation of phenylalanine hydroxylase in liver cells.

Clinical role of pteridine therapy in tetrahydrobiopterin deficiency

In most patients with deficiency of tetrahydrobiopterin (BH4) continuous administration of BH4 or of a synthetic analogue such as 6-methyltetrahydropterin (6-MPH4) lowers plasma phenylalanine concentrations to the therapeutic range. The effective dose of BH4 varies from 1 to 2 mg kg-1 daily in patients with defective biopterin synthesis, to 5 mg kg-1 or more in patients with dihydropteridine reductase (DHPR) deficiency. The cost of 2 mg kg-1 day-1 of BH4 is comparable to the cost of a low phenylalanine diet. Higher doses of pterins given orally (20 mg kg-1) raise the levels of tetrahydropterin in cerebrospinal fluid (CSF) to normal in patients with defective biopterin synthesis in whom initial concentration of biopterin species are low. In some, but not all, such patients pterin therapy also raises CSF amine metabolite concentrations and ameliorates symptoms. High dose therapy does not appear to be effective in raising CSF pterin levels in patients with DHPR deficiency who already accumulate dihydrobiopterin (BH2) in CSF. Central folate deficiency is an additional cause of neurological deterioration in patients with DHPR deficiency who require supplementation with folate as folinic acid. It is suggested that the accumulation of BH2 in such patients competitively interferes with folate metabolism.

Hyperphenylalaninaemia caused by defects in biopterin metabolism

The hepatic phenylalanine hydroxylating system consists of three essential components, phenylalanine hydroxylase, dihydropteridine reductase and the non-protein coenzyme, tetrahydrobiopterin. The reductase and the pterin coenzyme are also essential components of the tyrosine and tryptophan hydroxylating systems. Recent studies have shown that there are three distinct forms of phenylketonuria or hyperphenylalaninaemia, each caused by the lack of one of these essential components. The variant forms of the disease that are caused by the lack of dihydropteridine reductase or tetrahydrobiopterin are characterized by severe neurological deterioration, impaired functioning of tyrosine and tryptophan hydroxylases and the resultant deficiency of tyrosine- and tryptophan-derived monoamine neurotransmitters in brain.

The influence of starvation and tryptophan administration on the metabolism of phenylalanine, tyrosine and tryptophan in isolated rat liver cells

Liver cells from fed Sprague-Dawley rats metabolized phenylalanine, tyrosine and tryptophan at rates consistent with the known kinetic properties of the first enzymes of each pathway. Starvation of rats for 48 h did not increase the maximal activities of phenylalanine hydroxylase, tryptophan 2,3-dioxygenase and tyrosine aminotransferase in liver cell extracts, when results were expressed in terms of cellular DNA. Catabolic flux through the first two enzymes was unchanged; that through the aminotransferase was elevated relatively to enzyme activity. This is interpreted in terms of changes in the concentrations of 2-oxoglutarate and glutamate. Cells from tryptophan-treated animals exhibited significant increases in the catabolism of tyrosine and tryptophan, but not of phenylalanine. The activities of tyrosine aminotransferase and tryptophan 2,3-dioxygenase were also increased, although the changes in flux and enzyme activity did not correspond exactly. These results are discussed with reference to the control of aromatic amino acid catabolism in liver; the role of substrate concentration is emphasized.

Role of the liver in the pathogenesis of cerebral disorders in phenylketonuria

Comprehensive studies on structure and function of the liver (biochemical profiles, light and electron microscopy, determination of phenylalanine hydroxylase activity) were performed in children with phenylketonuria (PKU). It was established that the liver is always involved in the pathological process. Comparison of results obtained with peculiarities of neuropsychiatric disorders revealed a dependence of the initial manifestations and the severity of PKU on the extent of enzyme deficiency. Amino acid disorders and abnormal lipid metabolism both contribute to the genesis of cerebral lesions.

Phenylalanine metabolism in isolated rat liver cells. Effects of glucagon and diabetes

1. Methods are described for monitoring the metabolic flux through phenylalanine hydroxylase, the tyrosine catabolic pathway and phenylalanine: pyruvate transaminase in isolated liver cell incubations. 2. The relationship between hydroxylase flux and phenylalanine concentration is sigmoidal. 3. Glucagon increases hydroxylase activity at low, near-physiological, substrate concentrations only. The hormone does not affect the rate of formation of phenylpyruvate. 4. Experimental diabetes (for 10 days) increases phenylalanine catabolism, and this is further increased by glucagon. 5. These results are discussed in the light of the known mechanisms for control of phenylalanine hydroxylase activity in vitro.

Immunocytochemical identification of phenylalanine hydroxylase and albumin in cultured hepatoma cells and isolated rat hepatocytes

Rhodamine-conjugated antibodies specific for phenylalanine hydroxylase and serum albumin were employed as cytochemical probes to identify these two proteins in H4 hepatoma cells and in isolated rat hepatocytes. Each fluorescent antibody stained the cells specifically and in a distinctive manner. In both cell types, albumin staining was discretely localized in cytoplasmic and in H4 cultures varied somewhat from cell to cell. Evidence from cultures of REB15 cells, a strain derived by cloning H4 cells in tyrosine-free medium, suggested that the staining variability of H4 cells could reflect a variability in phenylalanine hydroxylase content. Hydrocortisone-treated H4 cells and REB15 cultures contain increased amounts of phenylalanine hydroxylase; and all cells in the culture appear to be induced by the hormone. Evidence was presented to show that the albumin visualized within the isolated hepatocytes had been synthesized by these cells, and, furthermore, that quantitatively nearly all intracellular albumin in the isolated rat hepatocytes appeared to be entrained in the secretion pathway (analogous data already exist for H4 cells [Baker, R.E., and R. Shiman. 1979. J. Biol. Chem. 254:9633-9639]). By scoring specific fluorescence, 86 and 98% of the H4 cells and 89 and 98% of the isolated hepatocytes were found to contain phenylalanine hydroxylase and albumin, respectively. Therefore, almost all cells in each population appeared to synthesize both proteins. An implication of these findings is that in rat virtually all liver parenchymal cells must synthesize both phenylalanine hydroxylase and albumin.

Hepatic phenylalanine 4-monooxygenase is a phosphoprotein

Phenylalanine hydroxylase [phenylalanine 4-monooxygenase; EC 1.14.16.1; L-phenylalanine, tetrahydropteridine:oxygen oxidoreductase(4-hydroxylating)] isolated from rat liver is a phosphoprotein containing approximately 0.31 mumol of protein-bound phosphate per mumol of subunit (50,000 molecular weight). When the enzyme is further phosphorylated in the presence of ATP and a 3'5'-cyclic-AMP-dependent protein kinase (EC 2.7.1.37; ATP:protein phsophotransferase), an additional 0.7 mumol of phosphate per mumol of subunit is introduced, bringing the total phosphate content up to about 1 mumol/mumol of subunit. This phosphorylation of the enzyme in vitro is accompanied by a 2.6-fold increase in hydroxylase activity when the activity is assayed in the presence of tetrahydrobiopterin. Partial proteolytic digestion of phenylalanine hydroxylase, which previously had been shown to activate the enzyme 20- to 50-fold [Fisher, D.B. & Kaufman, S. (1973) J. Biol. Chem. 248, 4345-4353], removes almost all of the phosphate from the enzyme.

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.

The isolation and properties of phenylalanine hydroxylase from human liver

Phenylalanine hydroxylase was prepared from human foetal liver and purified 800-fold; it appeared to be essentially pure. The phenylalanine hydroxylase activity of the liver was confined to a single protein of mol.wt. approx. 108000, but omission of a preliminary filtration step resulted in partial conversion into a second enzymically active protein of mol.wt. approx. 250000. Human adult and full-term infant liver also contained a single phenylalanine hydroxylase with molecular weights and kinetic parameters the same as those of the foetal enzyme; foetal, newborn and adult phenylalanine hydroxylase are probably identical. The K(m) values for phenylalanine and cofactor were respectively one-quarter and twice those found for rat liver phenylalanine hydroxylase. As with the rat enzyme, human phenylalanine hydroxylase acted also on p-fluorophenylalanine, which was inhibitory at high concentrations, and p-chlorophenylalanine acted as an inhibitor competing with phenylalanine. Iron-chelating and copper-chelating agents inhibited human phenylalanine hydroxylase. Thiol-binding reagents inhibited the enzyme but, as with the rat enzyme, phenylalanine both stabilized the human enzyme and offered some protection against these inhibitors. It is hoped that isolation of the normal enzyme will further the study of phenylketonuria.