Amino acid transport into cultured McCoy cells infected with Chlamydia trachomatis

Amino acid transport into McCoy cells infected with strains representative of the two major biovars of Chlamydia trachomatis has been studied to determine if uptake is increased during infection. Preliminary work suggested that the transport systems L, A/ASC (for neutral amino acid transport), N (for transport of Asn, Gln, and His) and y+ (for cationic amino acids) were present in McCoy cells. With lymphogranuloma venereum biovar strain 434, little difference in the influx of representative amino acids Trp, His, and Lys or the analogue 2-aminoisobutyric acid (AIB) was observed during infection. With trachoma biovar strain DK20, a small increase in the initial entry rate and equilibrium concentration of each amino acid was found. McCoy cells appear to have great capacity for concentrating amino acids, which might obviate the need for transport induction by chlamydiae under conditions favoring the growth of infectious organisms.

Chlamydial development is adversely affected by minor changes in amino acid supply, blood plasma amino acid levels, and glucose deprivation

This study has demonstrated the extreme sensitivity of Chlamydia trachomatis growing in McCoy cells to small changes in external amino acid supply. In the absence of cycloheximide, a decrease in the amino acid concentration of medium to 75% of control values was sufficient to induce the growth of enlarged chlamydial forms of reduced infectivity. Morphology became more distorted and the yield of infectious particles from inclusions declined as medium amino acid levels were further reduced. These events correlated with a general decline in intracellular amino acids, as measured by high-performance liquid chromatography, suggesting that chlamydiae require a minimum concentration of each amino acid for normal development. Cycloheximide enhanced the production of normal organisms and increased infectivity yield in media, suggesting that the drug increased the available pool of amino acids. This was supported by intracellular amino acid analyses. Aberrant forms with reduced infectivity were also induced during supply of infected cell cultures with medium containing blood plasma amino acid concentrations, supporting the proposal that nutrient levels in vivo could promote abnormal chlamydial development. Markedly abnormal forms were also observed during glucose deprivation, providing further evidence that aberrant development is a general stress-related response.

Novel bisphosphonate inhibitors of phosphoglycerate kinase

A series of novel, conformationally-restrained bisphosphonate analogues of 1,3-bisphosphoglyceric acid 1 have been synthesised and evaluated as inhibitors of 3-PGK. They are competitive inhibitors of the human enzyme and, especially for certain alpha-halophosphonic acid analogues, both Ki and IC50 values extend into the submicromolar range.

The flux control coefficient of carnitine palmitoyltransferase I on palmitate beta-oxidation in rat hepatocyte cultures

Two important factors that determine the flux of hepatic beta-oxidation of long-chain fatty acids are the availability of fatty acid and the activity of carnitine palmitoyltransferase I (CPT I). Using Metabolic Control Analysis, the flux control coefficient of CPT I in rat hepatocyte monolayers was determined by titration with 2-[6-(4-chlorophenoxy)hexyl]oxirane-2-carboxylate (Etomoxir), which is converted to Etomoxir-CoA, an irreversible inhibitor of CPT I. We measured CPT I activity and flux through beta-oxidation at 0.2 mM and 1.0 mM palmitate to simulate substrate concentrations in fed and fasted states. Rates of beta-oxidation were 4.5-fold higher at 1. 0 mM palmitate compared with 0.2 mM palmitate. Flux control coefficients of CPT I, estimated by two independent methods, were similar: 0.67 and 0.79 for 0.2 mM palmitate, and 0.68 and 0.77 for 1 mM palmitate. It is concluded that the regulatory potential of CPT I is similar at low and high physiological concentrations of palmitate.

Etomoxir, sodium 2-[6-(4-chlorophenoxy)hexyl] oxirane-2-carboxylate, inhibits triacylglycerol depletion in hepatocytes and lipolysis in adipocytes

The effects of etomoxir, an inhibitor of mitochondrial long-chain fatty acid oxidation, on triacylglycerol metabolism in rat hepatocytes and adipocytes were investigated. Etomoxir inhibited the depletion of triacylglycerol stores in hepatocytes incubated without exogenous fatty acids and inhibited lipolysis in adipocytes. The effects on hepatocytes could be attributed to two mechanisms. At low concentrations (1-10 microM) R-etomoxir increased fatty acid esterification by inhibition of beta-oxidation. This effect was specific for the R-enantiomer and was associated with increased triacylglycerol secretion. At higher concentrations (50-100 microM) RS-etomoxir inhibited lipolysis and triacylglycerol secretion, independently of inhibition of carnitine palmitoyl-transferase I. These effects of RS-etomoxir on triacylglycerol metabolism and lipolysis may contribute to the chronic hypolipidaemic effects of etomoxir in vivo.

The mitogenic response to EGF of rat hepatocytes cultured on laminin-rich gels (EHS) is blocked downstream of receptor tyrosine-phosphorylation

Hepatocytes cultured on type I collagen proliferate in response to mitogenic growth factors yet de-differentiate. By contrast, hepatocytes on laminin-rich gels (EHS) demonstrate stable differentiated functions, while DNA synthesis is negligible. Here we have confirmed that this lack of proliferation was not due to deterioration of the cells because albumin release was maintained for several weeks. Additionally it does not appear to be due to the presence of the growth inhibitor TGF beta within the gel since and anti-TGF beta antibodies did not reverse the growth blockade. The addition of EGF to cells cultured on either type I collagen or EHS gel induced increased tyrosine phosphorylation of a 180kDa protein which ran in a position identical to that of the EGF receptor detected by a specific EGF receptor antibody. We conclude that since functional EGF receptors were present, the mitogenic response of hepatocytes on EHS gels is blocked at some downstream signaling event beyond that of receptor tyrosine autophosphorylation.

The effects of a novel and selective inhibitor of tryptophan 2,3-dioxygenase on tryptophan and serotonin metabolism in the rat

The effects of a novel inhibitor 680C91 ((E)-6-fluoro-3-[2-(3- pyridyl)vinyl]-1H-indole) of the key enzyme of tryptophan catabolism tryptophan 2,3-dioxygenase (TDO) (EC 1.13.11.11), were examined on tryptophan catabolism in vitro and in vivo and on brain levels of tryptophan, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA). 680C91 was a potent (Ki = 51 nM) and selective TDO inhibitor with no inhibitory activity against indoleamine 2,3-dioxygenase (EC 1.13.11.17), monoamine oxidase A and B, 5-HT uptake and 5-HT1A,1D,2A and 2C receptors at a concentration of 10 microM. 680C91 had no effect on the binding of tryptophan to serum albumin in plasma and inhibited TDO competitively with respect to its substrate tryptophan. 680C91 inhibited the catabolism of tryptophan by rat liver cells and rat liver perfused in situ. The catabolism of L-[ring-2-14C]-tryptophan and a load dose of tryptophan (100 mg/kg) in vivo were inhibited by prior administration of 680C91. Administration of 680C91 alone produced marked increases in brain tryptophan, 5-HT and 5-HIAA. A load dose of tryptophan (100 mg/kg), producing increases in brain tryptophan 4-fold greater than that seen with 680C91, did not increase brain 5-HT and 5-HIAA to levels greater than those seen with 680C91 and produced a shorter-lasting increase in these parameters. These data therefore demonstrate the importance of TDO as a regulator of whole-body tryptophan catabolism and brain levels of tryptophan and 5-HT and suggest that a greater antidepressant efficacy might be achieved with inhibitors of TDO than tryptophan administration alone.

The effects of an inhibitor of tryptophan 2,3-dioxygenase and a combined inhibitor of tryptophan 2,3-dioxygenase and 5-HT reuptake in the rat

The effects of a novel inhibitor 680C91 ((E)-6-fluoro-3-[2-(3-pyridyl)vinyl]-1H-indole) of the key enzyme of tryptophan catabolism tryptophan 2,3-dioxygenase (TDO), and a novel inhibitor 709W92 ((E)-6-fluoro-3-[2-(4-pyridyl)vinyl]-1H-indole), of both TDO and 5-hydroxytryptamine (5-HT) reuptake, were examined on tryptophan catabolism, cerebrospinal fluid (CSF) concentrations of tryptophan and 5-HT and serotonergic-mediated physiology and behaviour in the rat. The catabolism of L-[ring-2-14C]tryptophan in vivo was completely inhibited by prior administration of 709W92. 709W92, but not 680C91, potentiated head-twitch produced by 5-hydroxytryptophan, prevented head-twitch and whole brain 5-HT depletion produced by p-chloroamphetamine and rapidly decreased dorsal raphe firing. Both 709W92 and 680C91 elevated CSF tryptophan by up to 260% of basal concentration. A maximally effective dose of 680C91 elevated a global measure of brain extracellular 5-HT (CSF 5-HT) to concentrations similar to those seen maximally after exogenous tryptophan administration (approx 170% of basal). Maximally effective doses of 709W92 increased CSF 5-HT to concentrations comparable to those seen after tryptophan and 5-HT reuptake inhibitor coadministration (approx 900% of basal) and to concentrations greater than those achieved maximally with serotonergically active antidepressant monotherapy (approx 500% of basal). 709W92 did not elevate CSF 5-HT to concentrations associated with the serotonin syndrome (approx 3000% of basal). The combined TDO inhibitor/5-HT reuptake inhibitor, 709W92, showed anxiolytic activity in the rat-pup vocalization model of anxiety. These results show that 709W92 (a novel inhibitor of both TDO and 5-HT reuptake), can produce an elevation of CSF 5-HT similar to that achieved with a serotonin reuptake inhibitor/tryptophan combination therapy but with a more sustained timecourse; such compounds may therefore have superior antidepressant efficacy in the clinic.

A quantitative analysis of the control of glutamine catabolism in rat liver cells. Use of selective inhibitors

1. At a physiological concentration of glutamine (0.5 mM), 87% of the total transport across the plasma membrane of liver cells isolated from fed rats involved the Na(+)-dependent system N; this was substantially inhibited by L-histidine. The residual Na(+)-independent component was attributed to system L on the basis of inhibition by 2-amino-2-norbornanecarboxylate and L-tryptophan. 2. Catabolism of glutamine by intact liver cells or by isolated mitochondria was inhibited by glutamate gamma-hydrazide with IC50 values of 13.7 +/- 3.5 microM and 22.6 +/- 3.8 microM respectively and a maximal inhibition of approx. 75%. The site of inhibition was identified as glutaminase; glutamate gamma-hydrazide inhibited this enzyme in cell-free extracts (IC50 37.8 +/- 7.7 microM) but had no activity against glutamate dehydrogenase or transport of glutamine, whether across mitochondrial or plasma membranes. 3. The major control site in cells from fed animals incubated with 0.5 mM L-glutamine was glutaminase (flux control coefficient 0.96). Appreciable control also resided in both plasma membrane transport systems, with coefficients of 0.51 for system N and -0.46 for system L, such that both interacted to provide a fine control of the intracellular concentration of the amino acid. Similar values were obtained by computer simulation based on theoretical determination of elasticities. 4. Previous controversy about the locus of regulation of hepatic glutamine metabolism is resolved by this distribution of control.

Uptake of acetaminophen (paracetamol) by isolated rat liver cells

The characteristics of the uptake of acetaminophen (N-acetyl-p-aminophenol or paracetamol, APAP) in incubations of isolated rat liver cells were consistent with diffusion of the drug being the predominant mechanism of APAP influx in these cells at concentrations above 0.5 mM. At lower substrate concentrations (below 0.5 mM) a saturable component was apparent. Both uptake processes could have a role in the control of the metabolism of APAP, because, at low concentrations, there was no intracellular accumulation of unconjugated drug, all the APAP entering the cell being converted to sulphate and glucuronide. After addition of drug, there was a lag phase of approximately 5 min before APAP-glucuronide and APAP-sulphate appeared in the incubation medium; during this time both conjugates accumulated inside the cells. These results have implications for our understanding of the mechanisms of APAP transport, and indicate how these processes may affect the drug's overall metabolism.

Transport of L-glutamine and L-glutamate across sinusoidal membranes of rat liver. Effects of starvation, diabetes and corticosteroid treatment

There is increasing evidence that membrane transporters for glutamine and glutamate are involved in control of liver metabolism in health and disease. We therefore investigated the effects of three catabolic states [starvation (60 h), diabetes (4 days after streptozotocin treatment) and corticosteroid (8-day dexamethasone) treatment] associated with altered hepatic amino acid metabolism on the activity of glutamine and glutamate transporters in sinusoidal membrane vesicles from livers of treated rats. In control preparations, L-[14C]glutamine uptake was largely Na(+)-dependent, but L-[14C]glutamate uptake was largely Na(+)-independent. Vmax. values for Na(+)-dependent uptake of glutamine and/or glutamate exceeded control values (by about 2- and 12-fold respectively) in liver membrane vesicles from starved (glutamine), diabetic (glutamate) or steroid-treated (glutamine and glutamate) rats. The Km values for Na(+)-dependent transport of glutamine or glutamate and the rates of their Na(+)-independent uptake were not significantly altered by any treatment. Na(+)-independent glutamate uptake appeared to include a dicarboxylate-exchange component. The patterns of inhibition of glutamine and glutamate uptake by other amino acids indicated that the apparent induction of Na(+)-dependent amino acid transport in catabolic states included increased functional expression of systems A, N (both for glutamine) and X-ag (for glutamate). The results demonstrate that conditions resulting in increased secretion of catabolic hormones (e.g. corticosteroid, glucagon) are associated with increased capacity for Na(+)-dependent transport of amino acids into liver cells from the blood. The modulation of hepatic permeability to glutamine and glutamate in these situations may control the availability of amino acids for intrahepatic metabolic processes such as ureagenesis, ammonia detoxification and gluconeogenesis.

Measurement of glucuronidation by isolated rat liver cells using [14C]fructose

We have developed a simple and sensitive method for the study of the relative rates of glucuronidation of compounds, in isolated liver cells, based on the incorporation of 14C from fructose into glucuronide conjugates. Liver cells from fasted rats are used to minimize any reduction of the specific activity by glycogenolysis. Although rates of glucuronidation are lower in isolated liver cells from fasted rats than in those from fed rats, because of a reduction in the concentration of UDP-glucuronic acid, it is possible to compare the rates of glucuronidation of different compounds. Radiolabelled glucuronides are separated from [14C]fructose and [14C]glucose, produced by the liver cells, by normal-phase HPLC on a polar amino-cyano column. The specific activity of the glucuronide was found to be approximately 50% of that of the [14C]fructose. Absolute amounts of glucuronide can be determined by measuring the specific activity of the [14C]glucose, also produced by liver cells from fructose, which reflects that of the glucose-6-phosphate and hence the UDP-glucuronic acid used for glucuronidation, although for the measurement of relative rates this would not be necessary. We have used this method to examine the kinetics of the glucuronidation of N-acetyl-p-aminophenol (acetaminophen), 4-nitrophenol and 1-naphthol in isolated rat liver cells. The method should be applicable to the study of the rates of glucuronidation of a range of aglycones and, unlike other methods, does not require glucuronide standards or radiolabelled aglycone.

Substrate-specificity of glutamine transporters in membrane vesicles from rat liver and skeletal muscle investigated using amino acid analogues

We investigated the effects of glutamine and histidine analogues on glutamine transport processes in membrane vesicles prepared from rat liver (sinusoidal membrane) and skeletal muscle (sarcolemma). L-[14C]Glutamine is transported in these membranes predominantly by Systems N/Nm (liver and muscle respectively), and to a lesser extent by Systems A and L (e.g. about 60, 20 and 20% of total flux respectively via Systems N, A and L at 0.05 mM-glutamine in liver membrane vesicles). The glutamine anti-metabolites 6-diazo-5-oxo-L-norleucine and acivicin were relatively poor inhibitors of glutamine uptake into liver membrane vesicles (less than 25% inhibition at 20-fold excess) and appeared primarily to inhibit System A activity (i.e. N-methylaminoisobutyric acid-inhibitable glutamine uptake). In similar experiments azaserine (also a glutamine anti-metabolite) inhibited approx. 50% of glutamine uptake, apparently by inhibition of System A and also of System L (i.e. 2-amino-2-carboxybicyclo[2,2,1]heptane-inhibitable glutamine uptake). Glutamate gamma-hydroxamate, aspartate beta-hydroxamate, histidine and N'-methylhistidine were all strong inhibitors of glutamine uptake into liver membrane vesicles (greater than 65% inhibition at 20-fold excess), but neither homoglutamine nor N'-methylhistidine produced inhibition. L-Glutamate-gamma-hydroxamate was shown to be a competitive inhibitor of glutamine transport via System N (Ki approximately 0.6 mM). Glutamine uptake in sarcolemmal vesicles showed a similar general pattern of inhibition as in liver membrane vesicles. The results highlight limits on the substrate tolerance of System N; we suggest that the presence of both an L-alpha-amino acid group and a nitrogen group with a delocalized lone-pair of electrons (amide or pyrrole type), separated by a specific intramolecular distance (C2-C4 chain equivalent), is important for substrate recognition by this transporter.

Quantitative studies of sulphate conjugation by isolated rat liver cells using [35S]sulphate

We have developed a simple, rapid and sensitive method for the study of sulphate conjugation in isolated liver cells based on the incorporation of 35S from [35S]sulphate. Excess [35S]sulphate is removed by a barium precipitation procedure, leaving [35S]sulphate conjugates in solution. We have used this method to examine the kinetics of sulphation of N-acetyl-p-aminophenol (acetaminophen), 4-nitrophenol and 1-naphthol in isolated rat liver cells. The efficiency of recovery of the sulphate conjugates was greater than 86%. The method is applicable to the quantitative study of sulphate conjugation of any substrate which forms a sulphate conjugate that is soluble in the presence of barium, without the need for standards or radiolabelled sulphate acceptors.

How does displacement of albumin-bound tryptophan cause sustained increases in the free tryptophan concentration in plasma and 5-hydroxytryptamine synthesis in brain?

Models of tryptophan catabolism and binding to serum albumin are presented to explain the observed effect of displacement of tryptophan from albumin on the concentrations of free and bound tryptophan and on the rate of 5-hydroxytryptamine (5-HT) synthesis from tryptophan in the brain. A rapid rate of dissociation of tryptophan from albumin (compared to the transit time of tryptophan through the liver) and a large fractional extraction of the free pool of tryptophan during passage through the liver are shown to be necessary factors in determining the effects observed. Because of the low fractional extraction of free tryptophan in the brain, the synthesis of 5-HT will be dependent only upon the free pool of tryptophan. Dissociation of tryptophan from albumin only causes a sustained increase in 5-HT synthesis in the brain because of the effect that this dissociation has on hepatic tryptophan catabolism and thereby on the free pool of tryptophan.

The characteristics and site of inhibition of gluconeogenesis in rat liver cells by bacterial endotoxin. Stimulation of phosphofructokinase-1

The characteristics and site of inhibition of gluconeogenesis by endotoxin were investigated in liver cells isolated from control and endotoxin-treated rats. Endotoxin treatment was associated with inhibition (40-50%) of gluconeogenesis from lactate plus pyruvate over a range of concentrations of substrate and of oleate and with or without glucose or glucagon. Similar inhibition was observed with asparagine, proline, glutamine, alanine and a substrate mixture, but not with glycerol, glyceraldehyde, dihydroxyacetone or endogenous substrates. There was no change in cellular ATP content or in the rates of ketogenesis or ureogenesis from asparagine, proline or glutamine. Other effects on isotopic fluxes, metabolite contents, enzyme activities and control coefficients were consistent with the suggestion that the effects of endotoxin on gluconeogenesis are exerted at the level of phosphofructokinase-1, and not at phosphoenolpyruvate carboxykinase, pyruvate kinase, pyruvate carboxylase or glucokinase.

The role of insulin in the modulation of glucagon-dependent control of phenylalanine hydroxylation in isolated liver cells

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 roles of glucagon, insulin and glucocorticoid hormones in the effects of sublethal doses of endotoxin on glucose homeostasis in rats

The effects of sub-lethal doses of endotoxin on plasma glucose, glucagon, insulin, glucocorticoids and non-esterified fatty acids (NEFA) were determined in rats. Endotoxin did not change the plasma concentration of glucocorticoids, but blocked the effects of elevated glucocorticoid hormone concentrations on both plasma glucose and hepatic tryptophan dioxygenase activity. Endotoxin increased the plasma concentrations of glucose, glucagon and insulin in rats with basal glucocorticoid concentrations, and changed the observed relationships between glucose, glucagon and insulin in a manner consistent with an increased sensitivity of glucagon secretion to lowered glucose concentrations. At the highest dose of endotoxin used, 20 mg/kg over 6 hr, a substantial decrease (greater than 7-fold) in the insulin/glucagon ratio provides evidence for changes in basal (as opposed to hormone-stimulated) glucose production and/or utilisation in vivo.

The effect of fatty acids and starvation on the metabolism of gluconeogenic precursors by isolated sheep liver cells

Isolated liver cells prepared from fed sheep synthesize glucose from propionate at twice the rate observed with cells from starved animals. Addition of palmitate or palmitate + carnitine to incubations of liver cells from starved animals inhibited the rate of glucose synthesis with lactate as a precursor, but had little effect when propionate and pyruvate were substrates. Liver cells from fed and starved sheep synthesized lactate and pyruvate when incubated with propionate. Fatty acids inhibited this formation of lactate and pyruvate from propionate. It is proposed that the different responses of gluconeogenic precursors to fatty acids can be explained by the effect of reducing equivalents on the transport of carbon atoms across the mitochondrial membrane.

The role of haem in the regulation of rat liver tryptophan metabolism

At saturating concentrations of tryptophan, the activity of tryptophan 2,3-dioxygenase was the same in isolated liver cells and in extracts with added haematin. Intraperitoneal injection of haematin did not increase tryptophan oxidation in livers subsequently perfused in situ. Preincubation of liver cells with physiological concentrations of tryptophan caused maximal saturation of tryptophan 2,3-dioxygenase with haem in liver cells. In cell-free extracts tryptophan 2,3-dioxygenase exhibited complex kinetics with haem. The results have important implications for the understanding of the role of haem in tryptophan metabolism.

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.

Quantification of the importance of individual steps in the control of aromatic amino acid metabolism

The quantitative importance of the individual steps of aromatic amino acid metabolism in rat liver was determined by calculation of the respective Control Coefficients (Strengths). The Control Coefficient of tryptophan 2,3-dioxygenase for tryptophan degradation was determined in a variety of physiological conditions and with a range of activities of tryptophan 2,3-dioxygenase. The Control Coefficient varied from 0.75 with basal enzyme activity to 0.25 after maximal induction of the enzyme by dexamethasone. The remainder of the control for tryptophan degradation was associated with the transport of the amino acid across the plasma membrane, with only very small contributions from kynureninase and kynurenine hydroxylase. The Control Coefficients of tyrosine aminotransferase for tyrosine degradation were approx. 0.70 and 0.20 with basal and dexamethasone-induced tyrosine aminotransferase activities respectively; the Control Coefficients of the transport of the amino acid into the cell were 0.22 and 0.58 respectively. Phenylalanine hydroxylase was found to have a Control Coefficient for the degradation of phenylalanine of approx. 0.50 under conditions of basal enzyme activity; after maximal activation by glucagon, the Control Coefficient decreased to 0.12. The transport of phenylalanine was responsible for the remaining control in the pathway. These results have important implications, directly for the regulation of aromatic amino acid metabolism in the liver, and indirectly for the regulation of neuroamine synthesis in the brain.

Transport of the aromatic amino acids into isolated rat liver cells. Properties of uptake by two distinct systems

The transport of the aromatic amino acids into isolated rat liver cells was studied. There was a rapid and substantial binding of the aromatic amino acids, L-alanine and L-leucine to the plasma membrane. This has important consequences for the determination of rates of transport and intracellular concentrations of the amino acids. Inhibition studies with a variety of substrates of various transport systems gave results consistent with aromatic amino acid transport being catalysed by two systems: a 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH)-insensitive aromatic D- and L-amino acid-specific system, and the L-type system (BCH-sensitive). The BCH-insensitive component of transport was Na+-independent and facilitated non-concentrative transport of the aromatic amino acids; it was unaffected by culture of liver cells for 24 h, by 48 h starvation, dexamethasone phosphate or glucagon. Kinetic properties of the BCH-inhibitable component were similar to those previously reported for the L2-system in liver cells. The BCH-insensitive component was a comparatively low-Km low-Vmax. transport system that we suggest is similar to the T-transport system previously seen only in human red blood cells. The results are discussed with reference to the importance of the T- and L-systems in the control of aromatic L-amino acid degradation in the liver.

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.

Control of phenylalanine and tyrosine metabolism by phosphorylation mechanisms

A system for the parallel determination of enzyme phosphorylation and expressed activity in rat liver cells, and its application to studies of phenylalanine hydroxylase and tyrosine aminotransferase, is described. Phenylalanine hydroxylase is phosphorylated by agents which stimulate cyclic AMP- and Ca2+-dependent protein kinase activity. The phosphorylation site(s) appear to be the same for both kinases. Phosphorylation is accompanied by increased metabolic flux at low, physiologically relevant, substrate concentrations. Insulin and spermine both inhibit the phosphorylation of the enzyme, possibly by increasing dephosphorylation. Tyrosine aminotransferase is phosphorylated in liver cell incubations but the rate is slow and insensitive to additions to the medium. No parallel changes in flux could be detected. Both enzymes are subject to complex regulatory mechanisms, short- and long-term. Their activities may be coordinated in vivo by control exerted at the level of the plasma membrane where both amino acids share the same transport processes. Determination of the control coefficients for the several components indicates that membrane transport may be a major limitation on flux.

Regulation of carnitine palmitoyltransferase activity by malonyl-CoA in mitochondria from sheep liver, a tissue with a low capacity for fatty acid synthesis

The characteristics of inhibition of carnitine palmitoyltransferase (CPT) I by malonyl-CoA were studied for the enzyme in mitochondria isolated from sheep liver, a tissue with a very low rate of fatty acid synthesis. Malonyl-CoA was as potent in inhibiting the sheep liver enzyme as in inhibiting the enzyme in rat liver mitochondria. CPT I in guinea-pig liver mitochondria was also similarly inhibited. The inhibition showed the same time-dependent characteristics previously established for the rat liver enzyme. Methylmalonyl-CoA was as effective an inhibitor of CPT I as malonyl-CoA in sheep liver mitochondria, but did not affect CPT I activity in mitochondria from rat or guinea-pig liver. The concentrations of malonyl-CoA required to inhibit CPT I in sheep liver mitochondria in vitro were similar to those found in freeze-clamped sheep liver samples (about 7 nmol of malonyl-CoA/g wet wt.). In sheep liver cells the content of malonyl-CoA was only one-tenth of that observed in vivo when glucose only was added to the incubation medium. Inclusion of acetate and/or insulin increased the malonyl-CoA content about 10-fold, to values similar to those observed in vivo. The rate of fatty acid synthesis in sheep liver cells was about 1% of that observed in rat liver, but was correlated with the concentrations of malonyl-CoA in the cells under various incubation conditions. These observations are discussed in relation to (i) the regulatory role of malonyl-CoA in tissues that have a low capacity for fatty acid synthesis, and (ii) the utilization by sheep liver of propionate as a gluconeogenic precursor.

The effect of pyridoxine deficiency on the metabolism of the aromatic amino acids by isolated rat liver cells

The total activity of three key enzymes and the flux through eight steps of aromatic amino acid metabolism have been determined in liver cells isolated from rats fed either control or pyridoxine-free diet for 5-6 weeks. The pyridoxine-free diet caused a decrease in the catabolism of tyrosine and phenylalanine because of a drop in the flux through tyrosine aminotransferase. This decrease of expressed cellular tyrosine aminotransferase activity can be fully explained in terms of loss of cofactor. Larger decreases in the catabolism of tryptophan were seen after pyridoxine deprivation. The decreased extent of tryptophan catabolism can be solely attributed to loss of cofactor or increased degradation of kynureninase. Inhibition of tryptophan 2,3-dioxygenase was seen in pyridoxine deficiency, probably because of the buildup of the kynurenine metabolites. The control strength of kynureninase, for flux through kynureninase, was calculated to be less than or equal to 0.004, but 0.41 after pyridoxine deprivation. The sensitivity of the three pathways to pyridoxine deprivation is interpreted and discussed in terms of the different affinities for pyridoxal phosphate and the control strengths of the pyridoxal phosphate-dependent enzymes, tyrosine aminotransferase and kynureninase.

The role of tryptophan 2,3-dioxygenase in the hormonal control of tryptophan metabolism in isolated rat liver cells. Effects of glucocorticoids and experimental diabetes

The metabolism of L-tryptophan by isolated liver cells prepared from control, adrenalectomized, glucocorticoid-treated, acute-diabetic, chronic-diabetic and insulin-treated chronic-diabetic rats was studied. Liver cells from adrenalectomized rats metabolized tryptophan at rates comparable with the minimum diurnal rates of controls, but different from rates determined for cells from control rats 4h later. Administration of dexamethasone phosphate increased the activity of tryptophan 2,3-dioxygenase (EC 1.13.11.11) 7-8-fold, and the flux through the kynurenine pathway 3-4-fold, in cells from both control and adrenalectomized rats. Increases in flux through kynureninase (EC 3.7.1.3) and to acetyl-CoA can be explained in terms of increased substrate supply from tryptophan 2,3-dioxygenase. The metabolism of tryptophan was increased 3-fold in liver cells isolated from acutely (3 days) diabetic rats, with a 7-8-fold increase in the maximal activity of tryptophan 2,3-dioxygenase. The oxidation of tryptophan to CO2 and metabolites of the glutarate pathway increased 4-5-fold, consistent with an increase in picolinate carboxylase (EC 4.1.1.45) activity. Liver cells isolated from chronic (10 days) diabetic rats metabolized tryptophan at rates comparable with those of cells from acutely diabetic rats, but with a 50% decrease in the activity of tryptophan 2,3-dioxygenase. The proportion of flux from tryptophan 2,3-dioxygenase to acetyl-CoA, however, was increased by 50%; this was indicative of further increases in the activity of picolinate carboxylase. Administration of insulin partially reversed the effects of chronic diabetes on the activity of tryptophan 2,3-dioxygenase and flux through the kynurenine pathway, but had no effect on the increased activity of picolinate carboxylase. The role of tryptophan 2,3-dioxygenase in regulating the blood tryptophan concentration is discussed with reference to its sensitivity to the above conditions.

Effect of potent antiglucocorticoids on dexamethasone induced enzymes in cultured hepatoma and rat liver cells

The antagonism by 3 synthetic steroids of the induction of tyrosine aminotransferase and tryptophan dioxygenase by dexamethasone were compared in HTC and FAZA hepatoma cells and in isolated liver cells. It was observed: (i) the sensitivity of liver cells to glucocorticoid agonists and antagonists changed in relation to time of culture; (ii) the extent of the inhibitory effect on enzyme induction depends on the nature of the antagonist; (iii) hepatoma cells, especially HTC cells, appeared more sensitive to glucocorticoid antagonism than liver cells.

The protein phosphatases involved in cellular regulation. Influence of polyamines on the activities of protein phosphatase-1 and protein phosphatase-2A

The effects of polyamines on the oligomeric forms of protein phosphatase-1 (1G), protein phosphatase-2A (2A0, 2A1 and 2A2) and their free catalytic subunits (1C and 2AC) has been studied using homogeneous enzymes isolated from rabbit skeletal muscle. Spermine increased the activity of protein phosphatase-2A towards eight of nine substrates tested. Half-maximal activation was observed at 0.2 mM with optimal effects at 1-2 mM. Above 2 mM, spermine became inhibitory. The most impressive activation of protein phosphatase-2A was obtained with glycogen synthase, especially when phosphorylated at sites-3 (8-15-fold with protein phosphatase-2A1) and phenylalanine hydroxylase (6-7-fold with protein phosphatase-2A1) as substrates. Activation of protein phosphatases 2A0, 2A1 and 2A2 was greater than that observed with 2AC. Spermine was a more potent activator than spermidine, while putrescine had only a small effect. Qualitatively similar results were obtained with five other substrates, although maximal activation was much less (1.3-3-fold with protein phosphatase-2A1). The rate of dephosphorylation of glycogen phosphorylase was decreased by spermine, inhibition being more pronounced with protein phosphatase-2AC than with 2A0, 2A1 and 2A2. Spermine (I50 = 0.1 mM with protein phosphatase-2AC) was a more potent inhibitor than spermidine (I50 = 0.9 mM) or putrescine (I50 = 8 mM). Partially purified preparations of protein phosphatases-2A0, 2A1 and 2A2 from from rat liver were affected by spermine in a similar manner to the homogeneous enzymes from rabbit skeletal muscle. Spermine did not activate protein phosphatase-1 to the same extent as protein phosphatase-2A. Greatest stimulation (2.5-fold) was again observed with glycogen synthase labelled in sites-3, with half-maximal activation at 0.2 mM and optimal effects at 1-2 mM spermine. Spermine was a much more effective stimulator than spermidine, while putrescine was ineffective. Very similar results were obtained with protein phosphatases 1G and 1C. With four other substrates maximal activation by spermine was less than 1.5-fold, while the dephosphorylation of glycogen synthase (labelled in site-2), phosphorylase kinase, pyruvate kinase and glycogen phosphorylase were inhibited. Spermine (I50 = 0.04 mM) was a more potent inhibitor of the dephosphorylation of glycogen phosphorylase than spermidine (I50 = 0.9 mM) or putrescine (I50 = 9 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The effect of experimental diabetes on phenylalanine metabolism in isolated liver cells

Chronic (10-day) diabetes was associated with increased metabolic flux through phenylalanine hydroxylase in isolated liver cells. This flux was stimulated by 0.1 microM-glucagon, but not by 10 microM-noradrenaline; 0.1 microM-insulin affected neither basal nor glucagon-stimulated flux. The increased rate of phenylalanine hydroxylation in diabetes was accompanied by parallel increases in enzyme activity (as measured with artificial cofactor) and immunoreactive-enzyme-protein content. In contrast with total protein synthesis, which decreased, phenylalanine hydroxylase synthesis persisted at the control rate in cells from diabetic animals. These findings are discussed in relation to the hormonal regulation of the hydroxylase and the known metabolic consequences of chronic diabetes.

Leucine and tryptophan metabolism in rats

The rate of tryptophan metabolism in isolated liver cells from animals fed on a high-leucine diet was greater than for cells from control animals. Leucine inhibited tryptophan metabolism and tryptophan uptake in isolated liver cells, probably by competing for membrane transport. Leucine had no effect on tryptophan 2,3-dioxygenase in vitro. 4-Methyl-2-oxovalerate increased tryptophan oxidation in incubations containing albumin, by displacing bound tryptophan and increasing the availability of the amino acid to the cell. The results suggest that, under extreme conditions, when the availability of tryptophan is low, leucine may be pellagragenic.

The role of phosphoenolpyruvate in insulin secretion: the effect of L-phenylalanine

Incubation of rat islets with phenylalanine increased the tissue content of phosophoenolpyruvate, both in the presence and in the absence of glucose. At the same time, L-phenylalanine neither stimulated nor inhibited insulin release. It is unlikely that insulin secretion is tightly coupled to the availability of phosphoenolpyruvate in rat islets.

The protein phosphatases involved in cellular regulation. Glycolysis, gluconeogenesis and aromatic amino acid breakdown in rat liver

The identities of the protein phosphatases involved in the regulation of hepatic glycolysis, gluconeogenesis and aromatic amino acid breakdown were investigated using 6-phosphofructo-1-kinase, fructose-1,6-bisphosphatase, L-pyruvate kinase, phenylalanine hydroxylase and the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase as substrates. Purified preparations of protein phosphatases-1, 2A, 2B and 2C exhibited activity towards all five substrates in vitro, although phosphatases-1 and 2B were only weakly active. Studies in liver extracts using inhibitor-2 and trifluoperazine, which inhibit protein phosphatase-1 and 2B, respectively, confirmed that these phosphatases are unlikely to be important in dephosphorylating these substrates in vivo. Sequential fractionation of rat liver extracts by anion-exchange chromatography and gel-filtration failed to resolve any protein phosphatases acting on each substrate, apart from protein phosphatases-2A and 2C. The present results, together with those described in the following paper (in this journal) indicate that under the assay conditions used, protein phosphatase-2A is the most powerful phosphatase acting on each substrate, although protein phosphatase-2C contributes a significant percentage of the activity towards 6-phosphofructo-1-kinase. No clear evidence was obtained for a role of metabolites in the regulation of dephosphorylation of the five substrates. This study reinforces our contention that only a few serine-specific and threonine-specific protein phosphatase catalytic subunits participate in cellular regulation.

The determination of flux through phenylalanine hydroxylase and homogentisate oxidase in isolated hepatocytes

The metabolic fate of [4-3H]phenylalanine incubated with isolated hepatocytes has been investigated. Analysis of 3H2O production and fluorimetric estimation of tyrosine production has allowed calculation of flux through phenylalanine hydroxylase and homogentisate oxidase. The resulting flux determinations are physiologically relevant and hormonally sensitive.

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.