Thermal stability of microsomal glucose-6-phosphatase

Three thiol groups are important for the activity of the liver microsomal glucose-6-phosphatase system. Unusual behavior of one thiol located in the glucose-6-phosphate translocase

Liver microsomal glucose-6-phosphatase (Glc-6-Pase) is a multicomponent system involving both substrate and product carriers and a catalytic subunit. We have investigated the inhibitory effect of N-ethylmaleimide (NEM), a rather specific sulfhydryl reagent, on rat liver Glc-6-Pase activity. Three thiol groups are important for Glc-6-Pase system activity. Two of them are located in the glucose-6-phosphate (Glc-6-P) translocase, and one is located in the catalytic subunit. The other transporters (phosphate and glucose) are not affected by NEM treatment. The NEM alkylation of the catalytic subunit sulfhydryl residue is prevented by preincubating the disrupted microsomes with saturating concentrations of substrate or product. This suggests either that the modified cysteine is located in the protein active site or that substrate binding hides the thiol group via a conformational change in the enzyme structure. Two other thiols important for the Glc-6-Pase system activity are located in the Glc-6-P translocase and are more reactive than the one located in the catalytic subunit. The study of the NEM inhibition of the translocase has provided evidence of the existence of two distinct areas in the protein that can behave independently, with conformational changes occurring during Glc-6-P binding to the transporter. The recent cloning of a human putative Glc-6-P carrier exhibiting homologies with bacterial phosphoester transporters, such as Escherichia coli UhpT (a Glc-6-P translocase), is compatible with the fact that two cysteine residues are important for the bacterial Glc-6-P transport.

Direct evidence for the involvement of two glucose 6-phosphate-binding sites in the glucose-6-phosphatase activity of intact liver microsomes. Characterization of T1, the microsomal glucose 6-phosphate transport protein by a direct binding assay

S 5627 is a synthetic analogue of chlorogenic acid. S 5627 is a potent linear competitive inhibitor of glucose 6-phosphate (Glc-6-P) hydrolysis by intact microsomes (Ki = 41 nM) but is without effect on the enzyme in detergent- or NH4OH-disrupted microsomes. 3H-S 5627 was synthesized and used as a ligand in binding studies directed at characterizing T1, the Glc-6-P transporter. Binding was evaluated using Ca2+-aggregated microsomes, which can be sedimented at low g forces. Aside from a modest reduction in K values for both substrate and S 5627, Ca2+ aggregation had no effect on glucose-6-phosphatase (Glc-6-Pase). Scatchard plots of binding data are readily fit to a simple "two-site" model, with Kd = 21 nM for the high affinity site and Kd = 2 microM for the low affinity site. Binding to the high affinity site was competitively blocked by Glc-6-P (Ki = 9 microM), whereas binding was unaffected by mannose-6-phosphate, Pi, and PPi and only modestly depressed by 2-deoxy-D-glucose 6-phosphate, a poor substrate for Glc-6-Pase in intact microsomes. Thus the high affinity 3H-S 5627 binding site fits the criteria for T1. Permeabilization of the membrane with 0.3% (3-[(chloramidopropyl)-dimethylammonio]-1-propanesulfonate) activated Glc-6-Pase and broadened its substrate specificity, but it did not significantly alter the binding of 3H-S 5627 to the high affinity sites or the ability of Glc-6-P to block binding. These data demonstrate unequivocally that two independent Glc-6-P binding sites are involved in the hydrolysis of Glc-6-P by intact microsomes. The present findings are the strongest and most direct evidence to date against the notion that the substrate specificity and the intrinsic activity of Glc-6-Pase in native membranes are determined by specific conformational constraints imposed on the enzyme protein. These data constitute compelling evidence for the role of T1 in Glc-6-Pase activity.

Chlorogenic acid and hydroxynitrobenzaldehyde: new inhibitors of hepatic glucose 6-phosphatase

We have studied the interactions of chlorogenic acid (CHL) and 2-hydroxy-5-nitrobenzaldehyde (HNB) with the components of the rat hepatic glucose 6-phosphatase (Glc-6-Pase) system. CHL and HNB are competitive inhibitors of glucose 6-phosphate (Glc-6-P) hydrolysis in intact microsomes with Ki values of 0.26 and 0.22 mm, respectively. CHL is without effect on the enzyme of fully disrupted microsomes or the system inorganic pyrophosphatase (PPiase) activity. HNB is a potent competitive inhibitor of the system PPiase activity (Ki = 0.56 mm) and a somewhat weaker noncompetitive inhibitor of enzyme activity (Ki = 2.1 mm). These findings indicate CHL binds to T1, the Glc-6-P transporter, and HNB inhibits through interaction with both T1 and T2 the phosphate (Pi)-PPi transporter. Binding of CHL and HNB is freely reversible. However, the inhibition of both PPiase and Glc-6-Pase by HNB becomes irreversible following incubation of HNB-exposed microsomes with 2.5 mm sodium borohydride, indicating that inhibition involves the formation of a Schiff base. The presence of CHL effectively protects T1, but not T2, against the irreversible inhibition by HNB. In contrast, PPi and Pi are effective in protecting T2, but not T1. This is the first report describing an effective inhibitor of the system PPiase activity (T2). CHL is the most specific T1 inhibitor described to date.

Inhibition of the glucose-6-phosphatase system by N-bromoacetylethanolamine phosphate, a potential affinity label for auxiliary proteins

N-Bromoacetylethanolamine phosphate (BAEP) has been used previously as an affinity label to study the hexose phosphate binding sites of fructose-6-P, 2-kinase:fructose-2, 6-bisphosphatase (Sakakibara et al. (1984) J. Biol. Chem. 259, 14023-14028). We have employed this compound to probe components of the glucose-6-phosphatase system using a combination of time-dependent and immediate inhibition kinetic techniques. Inhibition of D-glucose-6-phosphate (G6P) phosphohydrolase activity of native microsomes was irreversible and time- and inhibitor-concentration-dependent. Only a partial time-dependent, irreversible inhibition of the PPi phosphohydrolase activity of native microsomes was observed. BAEP inhibited PPi:glucose phosphotransferase activity of native microsomes in a concentration-dependent, irreversible manner which was more extensive than that seen with PPi phosphohydrolase, but less extensive than was observed with G6P phosphohydrolase. Disruption of microsomal integrity by detergent-treatment either prior to incubation with BAEP or subsequent to preliminary incubation with BAEP but prior to assay for activity abolished the time-dependent inhibition. These irreversible, time- and concentration-dependent inhibitory actions of BAEP thus are manifest at a site or sites where the intact membrane-bound enzyme first makes contact with substrates G6P and PPi. An additional site of inhibition by BAEP, through relatively weak, reversible competitive inhibition at the active catalytic site, is indicated by classical steady-state kinetic analysis. The irreversible, time- and concentration-dependent inhibitions by BAEP seen with G6P and PPi as substrates strongly suggest the potential utility of radio-labeled BAEP as an affinity label for the identification and ultimate isolation and study of uncharacterized auxiliary components of the glucose-6-phosphatase system.

Characteristics and specificity of the inhibition of liver glucose-6-phosphatase by arachidonic acid. Lesser inhibitability of the enzyme of diabetic rats

The effect of arachidonic acid (delta 4Ach) on liver glucose-6-phosphatase (Glc6Pase) has been studied in vitro using untreated and detergent-treated microsomes prepared from fed and 48-h-fasted normal rats and from streptozotocin-induced diabetic rats. Glc6Pase of both untreated and detergent-treated microsomes (60 micrograms protein/ml) is inhibited by delta 4Ach in a dose-dependent manner between 10-100 microM. The inhibition is very rapid and does not depend on preincubation of microsomes in the presence of delta 4Ach. It does depend on the concentration of microsomal membranes and on the concentration of glucose 6-phosphate: it is more pronounced at low Glc6P concentrations than at high. As a consequence, the enzyme displays sigmoidal kinetics in the presence of delta 4Ach. Hill coefficients (equal to 1 in the control experiments) of about 1.4 were determined in the presence of 50 microM delta 4Ach, indicating a clear positive cooperative dependency of the Glc6Pase upon its substrate in the presence of delta 4Ach. The delta 4Ach inhibition is fully reversible in the presence of bovine serum albumin. The inhibition does not depend on the metabolism of delta 4Ach through the prostaglandin synthase (cyclooxygenase) or arachidonate 12-lipoxygenase pathways since it is not affected by indomethacin and nordihydroguaiaretic acid. Several other unsaturated fatty acids are able to inhibit the enzyme within the same concentration range. In contrast, saturated fatty acids, the arachidonic acid methyl ester and numerous other lipid compounds containing esterified unsaturated fatty acids do not inhibit Glc6Pase within the same concentration range. The enzyme of fed rats was inhibited in the same manner as the enzyme of 48-h-fasted rats. However, Glc6Pase of untreated microsomes from diabetic rats was less inhibitable by delta 4Ach than the Glc6Pase of normal rats. This difference does not persist after solubilization of the membrane lipids by detergent treatment.

Glucose-6-phosphate phosphohydrolase of detergent-treated liver microsomal membranes exhibits a specific kinetic behaviour towards glucose 6-phosphate

With the aim of questioning the apparent loss of specificity of the microsomal glucose-6-phosphate phosphohydrolase after detergent-treatment, we performed competitive inhibition experiments among the four best substrates of the enzyme, i.e. the 6-phosphates of glucose (Glc6P), mannose-6 (Man6P), glucosamine (GlcN6P) and 2-deoxyglucose (dGlc6P). The Km and Vmax of glucose-6-phosphatase (Glc6Pase) and mannose-6-phosphatase (Man6Pase), assayed either by complex formation determination of P(i) produced or by radiometric determination of [U-14C]Glc or [U-14C]Man, were very close to 1 mM and 0.64 mumol.min-1.mg-1 microsomal protein, respectively. The Km of the enzyme for GlcN6P and for dGlc6P, determined by colorimetric assay of P(i), were equal to 1.53 +/- 0.07 mM and 2.35 +/- 0.15 mM, respectively, whilst the Vmax was not different from that of Glc6Pase and Man6Pase. Unexpectedly, the Ki of Man6P (1.61 +/- 0.22 mM), GlcN6P (2.24 +/- 0.17 mM) and dGlc6P (3.40 +/- 0.07 mM) for Glc6Pase, assayed by liberation of [U-14C]Glc, were significantly (50%) higher than their Km previously determined. The Ki of Glc6P (0.66 +/- 0.05 mM) for Man6Pase, assayed by liberation of [U-14C]Man, was significantly lower than its Km previously determined. In contrast, the Ki of GlcN6P (1.55 +/- 0.05 mM) for Man6Pase, assayed by the radiometric assay, was not different from its Km previously determined. It can be inferred from these data that Glc6P phosphohydrolase exhibits specific behaviour towards Glc6P after the detergent-treatment of the microsomal membrane.

Mechanism-based strategies for protein thermostabilization

Strategies for stabilizing enzymes can be derived from a two-step model of irreversible inactivation that involves preliminary reversible unfolding, followed by an irreversible step. Reversible unfolding is best prevented by covalent immobilization, whereas methods such as covalent modification of amino acid residues or 'medium engineering' (by the addition of low-molecular-weight compounds) are effective against irreversible 'incorrect' refolding. Genetic modification of the protein sequence is the most effective approach for preventing chemical deterioration.

Purification of human microsomal liver glucose-6-phosphatase system by affinity chromatography and immunodetection

A multiple purification of phosphohydrolase (PH) and phosphotranslocase (PT) of the human liver microsomal glucose-6-phosphatase system has been obtained by a rapid two-step procedure using affinity chromatography. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of the final products showed one major band each at 63 and 37 kDa for PH and PT respectively. The immunoblot analysis of SDS-PAGE of various purification steps for human liver using rabbit antibodies raised against the enzyme preparations also showed major bands at 63 and 37 kDa for PH and PT respectively. A major band at 260 kDa was observed by the Western blot of native PAGE of the enzyme preparation for PH. Cross-reacting materials at the positions of 63 and 37 kDa were detected only in liver, kidney and intestine. From five liver samples of patients suffering from type Ia glycogenosis there were diminished amounts of crossreacting materials at 63 kDa only in two samples. The uptake of glucose-6-phosphate has taken place in liposomes of Sepharose affinity purified products suggesting that this preparation may be a complex of PH and glucose-6-phosphate translocase.

The purification of a detergent-soluble glucose-6-phosphatase from rat liver

A highly active and soluble glucose-6-phosphatase has been purified to near homogeneity from rat liver. Successful purification has been initiated by covalent labeling of the enzyme in native rat liver microsomes with pyridoxal 5'-phosphate and NaBH4, followed by solubilization of the microsomes with Triton X-100, chromatography on phenyl-Sepharose, hydroxyapatite, DEAE-Sephacel and a second chromatography step on hydroxyapatite. The final enzyme preparation obtained was approximately 700-fold purified over the activity of starting microsomes. As judged by SDS/PAGE the purified glucose-6-phosphatase is composed of a single protein with a molecular mass of 35 kDa. The present work demonstrates that the purified glucose-6-phosphatase must be arranged in the native microsomal membrane so that it is accessible to pyridoxal 5'-phosphate from the cytoplasmic side.

Modulation of the activity of hepatic glucose-6-phosphatase by methylthioadenosine sulfoxide

Methylthioadenosine sulfoxide (MTAS), an oxidized derivative of the cell toxic metabolite methylthioadenosine has been used in elucidating the relevance of an interrelationship between the catalytic behavior and the conformational state of hepatic glucose-6-phosphatase and in characterizing the transmembrane orientation of the integral unit in the microsomal membrane. The following results were obtained: (1) Glucose 6-phosphate hydrolysis at 37 degrees C is progressively inhibited when native microsomes are treated with MTAS at 37 degrees C. In contrast, glucose 6-phosphate hydrolysis of the same MTAS-treated microsomes assayed at 0 degrees C is not inhibited. (2) Subsequent modification of the MTAS-treated microsomes with Triton X-114 reveals that glucose-6-phosphatase assayed at 37 degrees C as well as at 0 degrees C is inhibited. (3) Although excess reagent is separated by centrifugation and the MTAS-treated microsomes diluted with buffer before being modified with Triton the temperature-dependent effect of MTAS on microsomal glucose-6-phosphatase is not reversed at all. (4) In native microsomes MTAS is shown to inhibit glucose-6-phosphatase noncompetitively. The subsequent Triton-modification of the MTAS-treated microsomes, however, generates an uncompetitive type of inhibition. (5) Preincubation of native microsomes with MTAS completely prevents the inhibitory effect of 4,4'-diisothiocyanostilbene 2,2'-disulfonate (DIDS) as well as 4,4'-diazidostilbene 2,2'-disulfonate (DASS) on glucose-6-phosphatase. (6) Low molecular weight thiols and tocopherol protect the microsomal glucose-6-phosphatase against MTAS-induced inhibition. (7) Glucose-6-phosphatase solubilized and partially purified from rat liver microsomes is also affected by MTAS in demonstrating the same temperature-dependent behavior as the enzyme of MTAS-treated and Triton-modified microsomes. From these results we conclude that MTAS modulates the enzyme catalytic properties of hepatic glucose-6-phosphatase by covalent modification of reactive groups of the integral protein accessible from the cytoplasmic surface of the microsomal membrane. The temperature-dependent kinetic behavior of MTAS-modulated glucose-6-phosphatase is interpreted by the existence of distinct catalytically active enzyme conformation forms. Detergent-induced modification of the adjacent hydrophobic microenvironment additionally generates alterations of the conformational state leading to changes of the kinetic characteristics of the integral enzyme.