A rapid separation method for inositol phosphates and their isomers.

The stimulation of phenylalanine hydroxylation in isolated liver cells by sub-maximally effective concentrations of glucagon (less than 0.1 microM) is antagonized by insulin (0.1 nM-0.1 microM). This phenomenon is a consequence of a decrease in the glucagon-stimulated phosphorylation of phenylalanine hydroxylase from liver cells incubated in the presence of insulin. The impact of insulin on the phosphorylation state and activity of the hydroxylase is mimicked by incubation of liver cells in the presence of orthovanadate (10 microM). A series of cyclic AMP and cyclic GMP analogues enhanced phenylalanine hydroxylation: in each case insulin diminished the stimulation of flux. These results are discussed in the light of the characteristics of insulin action on other metabolic processes.     

The metabolism of L-phenylalanine and L-tyrosine by liver cells isolated from adrenalectomized rats and from streptozotocin-diabetic rats.

Flux through, and maximal activities of, key enzymes of phenylalanine and tyrosine degradation were measured in liver cells prepared from adrenalectomized rats and from streptozotocin-diabetic rats. Adrenalectomy decreased the phenylalanine hydroxylase flux/activity ratio; this was restored by steroid treatment in vivo. Changes in the phosphorylation state of the hydroxylase may mediate these effects; there was no significant change in the maximal activity of the hydroxylase. Tyrosine metabolism was enhanced by adrenalectomy; this was not related to any change in maximal activity of the aminotransferase. Steroid treatment increased the maximal activity of the aminotransferase. Both acute (3 days) and chronic (10 days) diabetes were associated with increased metabolism of phenylalanine; insulin treatment in vivo did not reverse these changes. Although elevated hydroxylase protein concentration was a major factor, changes in the enzyme phosphorylation state may contribute to differences in phenylalanine degradation in the acute and chronic diabetic states. Tyrosine metabolism, increased by diabetes, was partially restored to normal by insulin treatment in vivo. These changes can, to a large extent, be interpreted in terms of changes in the maximal activity of the aminotransferase.     

Phenylalanine hydroxylase in liver cells. Correlation of glucagon-stimulated enzyme phosphorylation with expressed activity.

Phenylalanine is transported rapidly into, but is not concentrated by, liver cells. Glucagon increased flux through phenylalanine hydroxylase; a half-maximal response was obtained at 0.7 nM. Under control conditions, 0.2-0.3 mol of phosphate were incorporated per mol of subunit of the hydroxylase at steady state. Glucagon increased this incorporation of phosphate into the hydroxylase to a maximal value of approx. 0.6 mol of phosphate per subunit; a half-maximal response was obtained at 0.3 nM. Glucagon, added simultaneously with [32P]Pi to liver cells, inhibited incorporation of 32P into the enzyme. The effects of glucagon were reproduced with dibutyryl cyclic AMP. Changes in phosphorylation correlated closely with changes in flux through phenylalanine hydroxylase in cell incubations.     

Effects of adrenergic agents, vasopressin and ionophore A23187, on the phosphorylation of, and flux through, phenylalanine hydroxylase in rat liver cells.

The adrenergic amines noradrenaline and adrenaline increased flux through phenylalanine hydroxylase by approx. 50%. This effect, which appears to be mediated by an alpha-adrenergic mechanism, was accompanied by a rapid increase in the phosphorylation of phenylalanine hydroxylase. Although ionophore A23187 mimicked the effects of the adrenergic amines, vasopressin was completely without effect on either phenylalanine hydroxylation or enzyme phosphorylation. Flux through phenylalanine hydroxylase in young rats (80 g) was insensitive to alpha-adrenergic, but sensitive to beta-adrenergic, agents. Consistent with previous observations [Fisher & Pogson (1984) Biochem. J. 219, 79-85] the present data indicate a close correlation between phosphorylation state and flux rate (i.e. enzyme activity).     

Measurement of phenylalanine hydroxylase turnover in cultured hepatoma cells.

A substantially new method has been developed to measure protein turnover. Its basis is the notion that in labeling experiments a secreted protein can be used to determine the specific radioactivity of the intracellular amino acid precursor pool. To measure protein turnover in the Reuber hepatoma H4 cell line, cultures were labeled with [3H]leucine for specified periods after which phenylalanine hydroxylase was isolated and its leucine specific radioactivity determined. Serum albumin secreted by the cultures was also isolated and used to estimate the leucine precursor pool specific radioactivity. The protein half-life of phenylalanine hydroxylase could them be calculated. Experiments performed at long and short labeling times and with high and low concentrations of leucine in the medium yielded equivalent results. Phenylalanine hydroxylase half-life in the H4 cells was investigated under both normal and hydrocortisone-induced growth conditions. Average half-lives of 7.4 and 8.2 h were found for induced and uninduced cultures, respectively. Although these measured enzyme half-lives were not essentially different, the steady state level of phenylalanine hydroxylase was increased 6.2-fold upon hydrocortisone induction, from 0.076 to 0.47 microgram/10(6) cells. The results demonstrated that hydrocortisone induces phenylalanine hydroxylase in the H4 cells by causing an increase in the rate of enzyme synthesis.     

p-Chlorphenylalanine effect on phenylalanine hydroxylase in hepatoma cells in culture.

We have investigated the p-chlorophenylalanine-dependent loss of phenylalanine hydroxylase activity in cultured hepatoma cells. The similarity of the effect of p-chlorophenylalanine on phenylalanine hydroxylase in the hepatoma cells and that reported from studies in vivo indicates that the loss of phenylalanine hydroxylase activity is due to a direct interaction of the amino acid analogue with the liver. We can find no evidence that the loss of phenylalanine hydroxylase activity is due to: a direct inactivation of the hydroxylase by p-chlorophenylalanine or an inhibitor produced by p-chlorophenylalanine treatment; an effect similar to that of p-fluorophenylalanine; or leakage of enzyme from the cells during p-chlorophenylalanine treatment. The data presented indicate: (a) the p-chlorophenylalanine effect is rather specific for phenylalanine hydroxylase; (b) following p-chlorophenylalanine removal, new protein synthesis is necessary for restoration of the hydroxylase activity; (c) the rate of loss of phenylalanine hydroxylase activity after the addition of p-chlorophenylalanine is much faster than the rate of restoration of the hydroxylase activity after removal of p-chlorophenylalanine; (d) even in the presence of p-chlorophenylalanine, hydrocortisone greatly stimulates the hydroxylase activity; (e) the cell density-dependent increase of phenylalanine hydroxylase activity is blocked by p-chlorophenylalanine. A discussion of the possible mechanisms of p-chlorophenylalanine-dependent loss of phenylalanine hydroxylase is presented. To measure very low leanine-dependent loss of phenylalanine hydroxylase is presented. To measure very low levels of phenylalanine hydroxylase activity, a new procedure, based on isotope dilution, was developed for isolating the tyrosine formed during the enzymatic reaction.     

Phenylalanine positively modulates the cAMP-dependent phosphorylation and negatively modulates the vasopressin-induced and okadaic-acid-induced phosphorylation of phenylalanine 4-monooxygenase in intact rat hepatocytes.

The state of phosphorylation of phenylalanine hydroxylase was determined in isolated intact rat hepatocytes. 32P-labeled phenylalanine hydroxylase was immunoisolated from cells loaded with 32Pi or from cell extracts 'back-phosphorylated' with [gamma-32P]ATP by cAMP-dependent protein kinase. The rate of phenylalanine hydroxylase phosphorylation in cells with elevated cAMP was similar to that observed for the isolated enzyme phosphorylated by homogeneous cAMP-dependent protein kinase. The phosphorylation rate in cAMP-stimulated cells was increased up to four times (reaching 0.018 s-1) by the presence of phenylalanine, the phosphate content (mol/mol hydroxylase) increasing to 0.5 from the basal level (0.17) in 50 s. The half maximal effect of phenylalanine was obtained at a physiologically relevant concentration (110 microM). The synthetic phenylalanine hydroxylase cofactor dimethyltetrahydropterin also enhanced the cAMP-stimulated phosphorylation of phenylalanine hydroxylase, presumably by displacing the endogenous cofactor, tetrahydrobiopterin. Phenylalanine was a negative modulator of the phosphorylation of phenylalanine hydroxylase induced by incubating cells with vasopressin or with the phosphatase inhibitor okadaic acid. The same site on the phenylalanine hydroxylase was phosphorylated in response to these two agents as in response to elevated cAMP. The available evidence suggested that not only vasopressin, but also okadaic acid, acted by stimulating the multifunctional Ca2+/calmodulin-dependent protein kinase II or a kinase with closely resembling properties.     

Bacterial lipopolysaccharide down-regulates expression of GTP cyclohydrolase I feedback regulatory protein.

GTP cyclohydrolase I feedback regulatory protein (GFRP) is a 9.7-kDa protein regulating GTP cyclohydrolase I activity in dependence of tetrahydrobiopterin and phenylalanine concentrations, thus enabling stimulation of tetrahydrobiopterin biosynthesis by phenylalanine to ensure its efficient metabolism by phenylalanine hydroxylase. Here, we were interested in regulation of GFRP expression by proinflammatory cytokines and stimuli, which are known to induce GTP cyclohydrolase I expression. Recombinant human GFRP stimulated recombinant human GTP cyclohydrolase I in the presence of phenylalanine and mediated feedback inhibition by tetrahydrobiopterin. Levels of GFRP mRNA in human myelomonocytoma (THP-1) cells remained unaltered by treatment of cells with interferon-gamma or interleukin-1beta, but were significantly down-regulated by bacterial lipopolysaccharide (LPS, 1 microg/ml), without or with cotreatment by interferon-gamma, which strongly up-regulated GTP cyclohydrolase I expression and activity. GFRP expression was also suppressed in human umbilical vein endothelial cells treated with 1 microg/ml LPS, as well as in rat tissues 7 h post intraperitoneal injection of 10 mg/kg LPS. THP-1 cells stimulated with interferon-gamma alone showed increased pteridine synthesis by addition of phenylalanine to the culture medium. Cells stimulated with interferon-gamma plus LPS, in contrast, showed phenylalanine-independent pteridine synthesis. These results demonstrate that LPS down-regulates expression of GFRP, thus rendering pteridine synthesis independent of metabolic control by phenylalanine.     

Effects of p-chlorophenylalanine and alpha-methylphenylalanine on amino acid uptake and protein synthesis in mouse neuroblastoma cells.

The phenylalanine analogues p-chlorophenylalanine and alpha-methylphenylalanine were used to inhibit phenylalanine hydroxylase in animal models for phenylketonuria. The present report examines the affects of these analogues on the metabolism of neuroblastoma cells. p-Chlorophenylalanine inhibited growth and was toxic to neuroblastoma cells. Although in vivo this analogue increased cell monoribosomes by 42%, it did not significantly affect poly(U)-directed protein synthesis in vitro. P-Chlorophenylalanine did not compete with phenylalanine or tyrosine for aminoacylation of tRNA and was therefore not substituted for those amino acids in nascent polypeptides. The initial cellular uptake of various large neutral amino acids was inhibited by this analogue but did not affect the flux of amino acids already in the cell; this suggested that an alteration of the cell's amino acid pools was not responsible for the cytotoxicity of the analogues. In contrast with p-chlorophenylalanine, alpha-methylphenylalanine did not exert these direct toxic effects because the administration of alpha-methylphenylalanine in vivo did not affect brain polyribosomes and a comparable concentration of this analogue was neither growth inhibitory nor cytotoxic to neuroblastoma cells in culture. The suitability of each analogue as an inhibitor of phenylalanine hydroxylase in animal models for phenylketonuria is discussed.     

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.     

Properties of phenylalanine hydroxylase of cultured hepatoma cells.

The kinetic and immunologic properties of phenylalanine hydroxylase of adult rat liver were compared to the properties of the similar enzyme present in cultured H4-II-E-C3 hepatoma cells. The enzymes from the two sources could not be distinguished by the Km values for either phenylalanine or 6,7-dimethyltetrahydropterin. Analysis by double immunodiffusion showed that phenylalanine hydroxylase from the two sources had identical immunologic determinants, but immunotitrations revealed a small but significant difference between the enzyme of the normal adult rat liver and the enzyme of cultured hepatoma cells. The results of double immunodiffusion and immunotitration experiments indicated also that the increased levels of phenylalanine hydroxylase seen in the hepatoma cells grown in the presence of hydrocortisone resulted from the accumulation of enzyme protein, but it could not be decided whether this accumulation resulted from an increased rate of synthesis or decreased rate of degradation.     

Reversible inactivation of phenylalanine hydroxylase by catecholamines in cultured hepatoma cells.

Phenylalanine hydroxylase in Reuber H4 hepatoma cell cultures can be rapidly inactivated by the addition of epinephrine, norepinephrine, dopamine, or 3,4-dihydroxyphenylalanine, in order of decreasing effectiveness, to the culture medium. The enzyme was 50% inactivated in 1 hour by 25 muM (R)-epinephrine or 45 muM (R)-norepinephrine in the medium. High concentrations of epinephrine caused a 70% inactivation in 15 min. Phenylalanine hydroxylase appears to be reversibly inactivated by epinephrine within the cells; since washing the compound off the cell cultures resulted in a rapid restoration of enzyme activity (40% in 1 hour), cycloheximide had little effect on the initial rate of recovery of enzyme activity and the same amount of phenylalanine hydroxylase antigen per cell was isolated from treated and normal cultures. Both (S)- and (R)-epinephrine inactivated the enzyme, and 0.1 mM desmethylimipramine, an inhibitor of amine transport, significantly decreased the effect of epinephrine on the hydroxylase activity. The possibility, suggested by the above results, that epinephrine might be directly inactivating phenylalanine hydroxylase within the cells was supported by the finding that purified rat liver phenylalanine hydroxylase would be 50% inactivated by 1.5 muM epinephrine in 10 min.     

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.     

Cell density dependent regulation of phenylalanine hydroxylase activity in hepatoma cells : Evidence for both an active and inactive enzyme form

The cell density dependent regulation of phenylalanine hydroxylase activity in Reuber hepatoma (H4) cells growing in monolayer culture has been examined in detail. We found that 48 h or more after subculture phenylalanine hydroxylase activity in the cells is an exponential function of cell density (cells/cm²). No discontinuity in the relationship is seen with the formation of a confluent monolayer.A rapid loss or a rapid gain in enzyme activity in the cells is observed after diluting or concentrating the cell cultures. The two processes appear qualitatively different. The loss in activity is a first order process which starts at the time of subculture with the rate of loss dependent on the density of subculture. The gain in activity begins 6–8 h after subculture to a higher density; it can be blocked by cycloheximide and has a maximum rate of increase that is about 10% of the maximum rate of loss of activity.Using immunochemical procedures, we found the same amount of phenylalanine hydroxylase associated antigen in Reuber cells from low density as from high density cultures, over a range of phenylalanine hydroxylase specific activities from 0.2 to 4.2. After concentrating cells to a higher density, no increase in enzyme antigen was observed, despite a several-fold increase in enzyme activity and a requirement for protein synthesis during the process. These observations imply the presence of an active and inactive phenylalanine hydroxylase with the relative amounts of each determined by the cell density. The effects of db-cAMP are discussed. Evidence is presented here that the hydrocortisone stimulation of phenylalanine hydroxylase activity works through a different mechanism than the cell density dependent process.     

Induction of hyperphenylalaninemia in mice by ethionine and phenylalanine

Female NMRI mice were fed diets containing l-ethionine (0.1 and 0.3% w/w) and phenylalanine (3% w/w), as well as respective control diets. Ethionine, the S-ethylated analog of methionine, was shown to inhibit phenylalanine hydroxylase in vivo, whereby in vitro kinetics remained unaffected. Treatment with ethionine resulted in fatty liver, reduced ATP content of liver, and alterations in serum amino acid concentrations. In the high dosage ethionine group, for instance, concentrations of Ala, Gly, Ser, Met, and Phe were increased whereas concentrations of Lys, Asp, and Pro were decreased. Applying ethionine together with phenylalanine resulted in hyperphenylalaninemia and phenylketonuria. Feeding phenylalanine alone also led to decreased activity of phenylalanine hydroxylase and increased concentration of Phe in serum. Ethionine only had a minimal effect on body weight gain; however, the hyperphenylalaninemic condition induced by application of the high dosage of ethionine and phenylalanine induced severe loss of body weight. A disturbed protein synthesis and protein phosphorylation might be the underlying mechanism of ethionine-induced suppression of phenylalanine hydroxylase.     

Mechanism of inactivation of phenylalanine hydroxylase by p-chlorophenylalanine in hepatome cells in culture. Two possible models.

The mechanism by which p-chlorophenylalanine specifically reduces phenylalanine hydroxylase activity in rat liver in vivo and in Reuber H4 hepatoma cells in culture has been investigated. Chromatography on hydroxylapatite of liver extract from rats injected with p-chlorophenylalanine showed that the compound differentially affected the three normal phenylalanine hydroxylase isoenzymes (I, II, and III); isoenzymes II and III were completely absent after the treatment, but isoenzyme I was only reduced in quantity compared with normal adult rats. Normal Reuber H4 cells only possess isoenzyme I; treatment with p-chlorophenylalanine yielded a reduced level of enzyme activity which appeared to be noraml isoenzyme I by both chromatographic and kinetic criteria. There is evidence, based on immunochemical techniques, that cultures grown in the presence of p-chlorophenylalanine have significantly reduced levels of phenylalanine hydroxylase antigen, and that p-chlorophenylalanine inactivates phenylalanine hydroxylase at or near the time of enzyme synthesis. The bulk of enzyme synthesized prior to the addition of the compound appears unaffected by it. There is no indication that protein synthesis itself is affected by p-chlorophenylalanine. In addition, p-chlorophenylacetate was found to inactivate phenylalanine hydroxylase in an apparently identical manner with p-chlorophenylalanine, which almost certainly eliminates from consideration any mechanism of inactivation specifically requiring an amino acid. Finally, effects of cycloheximide and chlorophenylalanine were compared. Taken together, the data lead to two possible models for the inactivation of the enzyme. The model most consistent with all data requires (predicts) the existence of a proenzyme form of phenylalanine hydroxylase which can be specifically inactivated by p-chlorophenylalanine.     

The mechanism of the irreversible inhibition ofrat liver phenylalanine hydroxylase due to treatment with p-chlorophenylalanine. The lack of effect on turnover of phenylalanine hydroxylase.

The administration of a single dose of p-chlorophenylalanine (360 mg/kg) to rats leads to the irreversible loss of 90% of hepatic phenylalanine hydroxylase activity after 24 h. This loss of activity is not the result of either an alteration in the overall structure of the enzyme, as determined by its antigenicity, or in the total immunologically reactive protein in the liver, as tested with a specific antiserum prepared against native phenylalanine hydroxylase. Neither the rate of synthesis nor the rate of degradation of phenylalanine hydroxylase is changed by p-chlorophenylalanine (pClPhe) treatment. The half-life for the enzyme is about 2 days in control and in pClPhe-treated rats. In addition, there is no detectable incorporation of pClPhe into the phenylalanine hydroxylase molecule itself.     

The effect of streptozotocin-induced diabetes on phenylalanine hydroxylase expression in rat liver.

The impact of experimentally induced diabetes on the expression of rat liver phenylalanine hydroxylase has been investigated. A significant elevation in maximal enzymic activity was observed in diabetes. This was associated with significant increases in the amount of enzyme, the phenylalanine hydroxylase-specific translational activity of hepatic RNA and the abundance of phenylalanine hydroxylase-specific mRNA. These changes in phenylalanine hydroxylase expression were not observed when diabetes was controlled by daily injections of insulin. These results are discussed in relation to the hormonal control of phenylalanine hydroxylase gene expression.     

Effects of experimental diabetes on rat hepatic phenylalanine hydroxylase in vivo

1. The stimulated levels of phenylanine hydroxylase activity in liver extracts from streptozotocin-induced diabetic rats (Donlon and Beirne, 1982) have been correlated with an increased rate of phenylalanine catabolism in vivo. 2. The levels of hepatic phenylalanine hydroxylase protein in diabetic rats become elevated. This effect is not seen in diabetic animals concurrently treated with insulin. 3. The rate of synthesis of liver phenylalanine hydroxylase in 5-day diabetics is 260% that of control animals. 4. These observations are discussed with reference to the regulation of hepatic phenylalanine hydroxylase and phenylanine metabolism in rats.     

The effect of vanadate upon the expression of phenylalanine hydroxylase in streptozotocin-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 by 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 insulin 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.