The liver glucose-6-phosphatase of intact microsomes is inhibited and displays sigmoid kinetics in the presence of alpha-ketoglutarate-magnesium and oxaloacetate-magnesium chelates

Magnesium treatment on methylation changes of transmembrane serine protease 2 (TMPRSS2)

OBJECTIVES

The viral entry of SARS-CoV-2 requires host-expressed TMPRSS2 to facilitate the viral spike protein priming. This study aims to test the hypothesis that magnesium (Mg) treatment leads to DNA methylation changes in TMPRSS2.

METHODS

This study is nested within the Personalized Prevention of Colorectal Cancer Trial, a double-blind 2 × 2 factorial randomized controlled trial, which enrolled 250 participants from Vanderbilt University Medical Center.

RESULTS

We found that 12 wk of personalized Mg treatment significantly increased 5-methylcytosine methylation at cg16371860 (TSS1500, promoter) by 7.2% compared to the placebo arm (decreased by 0.1%) in those ages < 65 y. The difference remained statistically significant after adjusting for age, sex, and baseline methylation as well as correction for false discovery rate (adjusted P = 0.014). Additionally, Mg treatment significantly reduced 5-hydroxymethylcytosine levels at cg26337277 (close proximity to TSS200 and the 5' untranslated region, promoter) by 2.3% compared to an increase of 7.1% in the placebo arm after adjusting for covariates in those ages < 65 y (P = 0.003). The effect remained significant at a false discovery rate of 0.10 (adjusted P = 0.088).

CONCLUSIONS

Among individuals ages < 65 y with calcium-to-magnesium intake ratios equal to or over 2.6, reducing the ratio to around 2.3 increased 5-methylcytosine modifications (i.e., cg16371860) and reduced 5-hydroxymethylcytosine modifications (i.e., cg26337277) in the TMPRSS2 gene. These findings, if confirmed, provide another mechanism for the role of Mg intervention in the prevention of COVID-19 and treatment of early and mild disease by modifying the phenotype of the TMPRSS2 genotype.

Ca:Mg Ratio, APOE Cytosine Modifications, and Cognitive Function: Results from a Randomized Trial

BACKGROUND

Deterioration of ionized calcium (Ca2+) handling in neurons could lead to neurodegenerative disease. Magnesium (Mg) antagonizes Ca during many physiologic activities, including energy metabolism and catalyzation of demethylation from 5-methylcytosine(5-mC) to 5-hydroxymethylcytosine(5-hmC).

OBJECTIVE

To test the hypothesis that actively reducing the Ca:Mg intake ratio in the diet through Mg supplementation improves cognitive function, and to test whether this effect is partially mediated by modified cytosines in Apolipoprotein E (APOE).

METHODS

This study is nested within the Personalized Prevention of Colorectal Cancer Trial (PPCCT), a double-blind 2×2 factorial randomized controlled trial, which enrolled 250 participants from Vanderbilt University Medical Center. Target doses for both Mg and placebo arms were personalized.

RESULTS

Among those aged > 65 years old who consumed a high Ca:Mg ratio diet, we found that reducing the Ca:Mg ratio to around 2.3 by personalized Mg supplementation significantly improved cognitive function by 9.1% (p = 0.03). We also found that reducing the Ca:Mg ratio significantly reduced 5-mC at the cg13496662 and cg06750524 sites only among those aged > 65 years old (p values = 0.02 and 0.03, respectively). Furthermore, the beneficial effect of reducing the Ca:Mg ratio on cognitive function in those aged over 65 years was partially mediated by reductions in 5-mC levels (i.e., cg13496662 and cg06750524) in APOE (p for indirect effect = 0.05).

CONCLUSION

Our findings suggest that, among those age 65 and over with a high dietary Ca:Mg ratio, optimal Mg status may improve cognitive function partially through modifications in APOE methylation. These findings, if confirmed, have significant implications for the prevention of cognitive aging and Alzheimer's disease.Clinical Trial Registry number and website: #100106 https://clinicaltrials.gov/ct2/show/NCT03265483.

Magnesium Treatment on Methylation Changes of Transmembrane Serine Protease 2 (TMPRSS2)

BACKGROUND

The viral entry of SARS-CoV-2 requires host-expressed TMPRSS2 to facilitate the viral spike (S) protein priming.

OBJECTIVES

To test the hypothesis that Mg treatment leads to DNA methylation changes in TMPRSS2 .

METHODS

This study is nested within the Personalized Prevention of Colorectal Cancer Trial (PPCCT), a double-blind 2×2 factorial randomized controlled trial, which enrolled 250 participants from Vanderbilt University Medical Center. Target doses for both Mg and placebo arms were personalized.

RESULTS

We found that 12-week of personalized Mg treatment significantly increased 5-mC methylation at cg16371860 (TSS1500, promoter) by 7.2% compared to placebo arm (decreased by 0.1%) in those aged < 65 years old. The difference remained statistically significant after adjusting for age, sex and baseline methylation as well as FDR correction (FDR-adjusted P =0.014). Additionally, Mg treatment significantly reduced 5-hmC level at cg26337277 (close proximity to TSS200 and 5'UTR, promoter) by 2.3% compared to increases by 7.1% in the placebo arm after adjusting for covariates in those aged < 65 years old ( P =0.003). The effect remained significant at FDR of 0.10 (adjusted P value=0.088).

CONCLUSION

Among individuals aged younger than 65 years with the Ca:Mg intake ratios equal to or over 2.6, reducing Ca:Mg ratios to around 2.3 increased 5-mC modifications (i.e. cg16371860) and reduced 5-hmC modifications (i.e. cg26337277) in the TMPRSS2 gene. These findings, if confirmed, provide another mechanism for the role of Mg intervention for the prevention of COVID-19 and treatment of early and mild disease by modifying the phenotype of the TMPRSS2 genotype.

Intestinal glucose metabolism revisited

It is long known that the gut can contribute to the control of glucose homeostasis via its high glucose utilization capacity. Recently, a novel function in intestinal glucose metabolism (gluconeogenesis) was described. The intestine notably contributes to about 20-25% of total endogenous glucose production during fasting. More importantly, intestinal gluconeogenesis is capable of regulating energy homeostasis through a communication with the brain. The periportal neural system senses glucose (produced by intestinal gluconeogenesis) in the portal vein walls, which sends a signal to the brain to modulate hunger sensations and whole body glucose homeostasis. Relating to the mechanism of glucose sensing, the role of the glucose receptor SGLT3 has been strongly suggested. Moreover, dietary proteins mobilize intestinal gluconeogenesis as a mandatory link between their detection in the portal vein and their effect of satiety. In the same manner, dietary soluble fibers exert their anti-obesity and anti-diabetic effects via the induction of intestinal gluconeogenesis. FFAR3 is a key neural receptor involved in the specific sensing of propionate to activate a gut-brain reflex arc triggering the induction of the gut gluconeogenic function. Lastly, intestinal gluconeogenesis might also be involved in the rapid metabolic improvements induced by gastric bypass surgeries of obesity.

Reduced cellular Mg²⁺ content enhances hexose 6-phosphate dehydrogenase activity and expression in HepG2 and HL-60 cells

We have reported that Mg(2+) dynamically regulates glucose 6-phosphate entry into the endoplasmic reticulum and its hydrolysis by the glucose 6-phosphatase in liver cells. In the present study, we report that by modulating glucose 6-phosphate entry into the endoplasmic reticulum of HepG2 cells, Mg(2+) also regulates the oxidation of this substrate via hexose 6-phosphate dehydrogenase (H6PD). This regulatory effect is dynamic as glucose 6-phosphate entry and oxidation can be rapidly down-regulated by the addition of exogenous Mg(2+). In addition, HepG2 cells growing in low Mg(2+) show a marked increase in hexose 6-phosphate dehydrogenase mRNA and protein expression. Metabolically, these effects on hexose 6-phosphate dehydrogenase are important as this enzyme increases intra-reticular NADPH production, which favors fatty acid and cholesterol synthesis. Similar effects of Mg(2+) were observed in HL-60 cells. These and previously published results suggest that in an hepatocyte culture model changes in cytoplasmic Mg(2+) content regulates glucose 6-phosphate utilization via glucose 6 phosphatase and hexose-6 phosphate dehydrogenase in alternative to glycolysis and glycogen synthesis. This alternative regulation might be of relevance in the transition from fed to fasted state.

Proinsulin C-peptide potentiates the inhibitory action of insulin on glucose synthesis in primary cultured rabbit kidney-cortex tubules: Metabolic studies

Effects of equimolar concentrations of proinsulin C-peptide and insulin on glucose synthesis were studied in primary cultures of rabbit kidney-cortex tubules grown in the presence of alanine, glycerol, and octanoate. The rhodamine-labeled C-peptide entered renal tubular cells and localized in nuclei, both in the presence and absence of insulin; preincubations with the unlabeled compound inhibited internalization. C-peptide did not affect glucose formation when added alone but potentiated the inhibitory action of insulin by about 20% due to a decrease in flux through glucose-6-phosphate isomerase (GPI) and (or) glucose-6-phosphatase (G6Pase). GPI inhibition was caused by: (i) increased intracellular contents of fructose-1,6-bisphosphate and fructose-1-phosphate, inhibitors of the enzyme and (ii) reduced level of the phosphorylated GPI, which exhibits higher enzymatic activity in the presence of casein kinase 2. A decrease in flux through G6Pase, due to diminished import of G6P by G6P-transporter from the cytoplasm into endoplasmic reticulum lumen, is also suggested. The data show for the first time that in the presence of insulin and C-peptide, both GPI and G6P-ase may act as regulatory enzymes of renal gluconeogenic pathway.

Glycogen autophagy

Glycogen autophagy, which includes the sequestration and degradation of cell glycogen in the autophagic vacuoles, is a selective process under conditions of demand for the massive hepatic production of glucose, as in the postnatal period. It represents a link between autophagy and glycogen metabolism. The formation of autophagic vacuoles in the hepatocytes of newborn animals is spatially and biochemically related to the degradation of cell glycogen. Many molecular elements and signaling pathways including the cyclic AMP/cyclic AMP-dependent protein kinase and the phosphoinositides/TOR pathways are implicated in the control of this process. These two pathways may converge on the same target to regulate glycogen autophagy.

Studies on glycogen autophagy: effects of phorbol myristate acetate, ionophore A23187, or phentolamine

The effects of agents that could manipulate the lysosomal calcium such as phorbol myristate acetate, ionophore A23187, and phentolamine on the lysosomal glycogen degradation were studied by electron microscopy, morphometric analysis, and biochemical assays in newborn rat hepatocytes. Phorbol myristate acetate, which promotes the input of calcium to lysosomes, increased the total volume of autophagic vacuoles and the activity of lysosomal glycogen-hydrolyzing acid alpha 1,4 glucosidase and decreased the fractional volume of undigested glycogen inside the autophagic vacuoles and also decreased the activity of acid mannose 6-phosphatase. Ionophore A23187, which releases lysosomal calcium, produced opposite results in these enzyme activities. Phentolamine, an alpha-adrenergic blocking agent which interferes with the generation of phosphoinositides and may activate the lysosomal calcium uptake pump, increased the total volume of autophagic vacuoles and the activity of lysosomal glycogen-hydrolyzing acid glucosidase and decreased the fractional volume of undigested glycogen inside the autophagic vacuoles. The results of this study constitute evidence that changes in lysosomal calcium may influence certain aspects of autophagy, including the degradation of glycogen inside the autophagic vacuoles. They also support our previous postulate [Kalamidas and Kotoulas (2000a,b) Histol Histopathol 15:29-35, 1011-1018] that stimulation of autophagic mechanisms in newborn rat hepatocytes may be associated with acid mannose 6-phosphatase activity-deficient lysosomes.

The inhibition of gluconeogenesis by chloroquine contributes to its hypoglycaemic action

The effect of chloroquine on gluconeogenesis in isolated hepatocytes and kidney-cortex tubules of rabbit has been studied. The inhibitory action of 200 microM chloroquine was the highest in hepatocytes and renal tubules incubated with glutamine and glutamate+glycerol+octanoate, respectively, while in the presence of other substrates the drug action was less pronounced. With amino acids as substrates, the inhibition of gluconeogenesis was accompanied by a decreased glutamine production, resulting from a decline of glutamate dehydrogenase activity. A decrease in the urea production by hepatocytes incubated with chloroquine in the presence of glutamine but not NH4Cl as the source of ammonium is in agreement with this suggestion. The degree of inhibition by chloroquine of the rate of gluconeogenesis in renal tubules isolated from control rabbits was similar to that determined in diabetic animals. Chloroquine-induced changes in levels of intracellular gluconeogenic intermediates indicate a decrease in phosphoenolpyruvate carboxykinase and glucose-6-phosphatase activities probably due to increased concentration of 2-oxoglutarate, an inhibitor of these two enzymes. In view of the data, it is likely that inhibition by chloroquine of glucose formation in liver and kidney may contribute to the hypoglycaemic action of this drug. The importance of the inhibitory effect of chloroquine on glutamate dehydrogenase activity in the antihyperglycaemic action of the drug is discussed.

Regulation of glucose production by the liver

Glucose is an essential nutrient for the human body. It is the major energy source for many cells, which depend on the bloodstream for a steady supply. Blood glucose levels, therefore, are carefully maintained. The liver plays a central role in this process by balancing the uptake and storage of glucose via glycogenesis and the release of glucose via glycogenolysis and gluconeogenesis. The several substrate cycles in the major metabolic pathways of the liver play key roles in the regulation of glucose production. In this review, we focus on the short- and long-term regulation glucose-6-phosphatase and its substrate cycle counter-part, glucokinase. The substrate cycle enzyme glucose-6-phosphatase catalyzes the terminal step in both the gluconeogenic and glycogenolytic pathways and is opposed by the glycolytic enzyme glucokinase. In addition, we include the regulation of GLUT 2, which facilitates the final step in the transport of glucose out of the liver and into the bloodstream.

Role of glucose-6 phosphatase, glucokinase, and glucose-6 phosphate in liver insulin resistance and its correction by metformin

We investigated the role of glucose-6 phosphatase (Glc6Pase), glucokinase (GK), and glucose-6 phosphate (Glc6P) in liver insulin resistance, an early characteristic of type 2 diabetes, and its correction by metformin. We determined hepatic glucose production (HGP) by tracer dilution, and enzyme activities and substrate concentrations after saline or insulin perfusions during euglycemic clamps in rats fed: 1) a standard hyperglucidic diet (S); 2) a high-fat diet (HF); and 3) a high-fat diet and treated with the oral antidiabetic metformin (HF/Met). Basal HGP was similar in the 3 groups: 75+/-8, 65+/-9.5 and 71+/-3 micromol x kg(-1) x min(-1) (means+/-SEM, N=5) in S, HF and HF/Met rats, respectively. Upon insulin perfusion at 240 pmol/hr, HGP was decreased by 35% in S rats (49+/-4.5 micromol x kg(-1) x min(-1), P < 0.01 vs. basal) and 65% in HF/Met rats (23+/-10 micromol x kg(-1) x min(-1), P < 0.01 vs basal), whereas it was not decreased in HF rats (60+/-12 micromol x kg(-1) x min(-1)), revealing insulin resistance. GK activity was lower (by 65%, P < 0.01) in HF and HF/Met rats (0.8+/-0.1 and 0.9+/-0.1 U/g liver, respectively) than in S rats (2.4+/-0.3 U/g). Microsomal Glc6Pase activity was lower (by 35%, P < 0.01) in HF and HF/Met rats (0.25+/-0.01 and 0.27+/-0.02 micromol r min(-1) x mg prot x (-1), respectively) than in S rats (0.39+/-0.03 micromol x min(-1) x mg prot x (-1)). Glc6P concentration was decreased by insulin perfusion at 480 pmol/hr in S and HF/Met rats (P < 0.05 vs. saline), but not in HF rats, in agreement with insulin resistance in the latter group. However, the differential inhibitions of HGP by insulin could not be ascribed to the variations in Glc6P concentrations. Metformin was present in the liver at a concentration of 27+/-2 nmol/g wet tissue and was not detected in the plasma. These results strongly suggest that the regulation of HGP by insulin additionally involves short-term regulatory mechanism(s) of Glc6Pase, occurring in vivo, and lost under in vitro conditions. These might be impaired in HF rats, in keeping with insulin resistance of HGP, and restored by metformin.

Energy dependence of protein synthesis by isolated cestode mitochondria

Mitochondria of helminth parasites produce energy in a number of ways that vary from species to species, and, in general, do not possess a classical tricarboxylic acid (Krebs) cycle. The process of protein synthesis, which is energy dependent, is not well understood in this organelle. However, protein synthesis in helminth mitochondria seems to serve the purpose to the desired level despite its limited capacity to produce ATP compared to its mammalian counterpart. Data presented here demonstrate that helminth mitochondria can synthesize proteins in vitro and the process can be supported by energy supplied exogenously by the cytosol, by an ATP generating system utilizing a substrate such as malate, or by ATP itself.

Differential time course of liver and kidney glucose-6 phosphatase activity during long-term fasting in rat correlates with differential time course of messenger RNA level

We have studied the role of Glc6Pase mRNA abundance in the time course of Glc6Pase activity in liver and kidney during long-term fasting in rat. Refered to the mRNA level in the fed state, Glc6Pase mRNA abundance was increased by 3.5 +/- 0.5 and 3.7 +/- 0.5 times (mean +/- S.E.M., n = 5) in the 24 h and 48 h-fasted liver, respectively. Then, the liver Glc6Pase mRNA was decreased to the level of the fed liver after 72 and 96 h of fasting (1.0 +/- 0.3 and 1.4 +/- 0.3). In the kidney, Glc6Pase mRNA abundance was increased by 2.7 +/- 1.0 and 5 +/- 1.2 times at 24 and 48 h of fasting, respectively. Then, it plateaued at the level of the 48 h fasted kidney after 72 h and 96 h of fasting (4.5 +/- 1.0 and 4.3 +/- 1.0). After 24 and 48 h-refeeding, the abundance of Glc6Pase mRNA in 48 h-fasted rats was decreased to the level found in the liver and kidney of fed rats. The time course of the activity of Glc6Pase catalytic subunit during fasting and refeeding was strikingly parallel to the time course of Glc6Pase mRNA level in respective tissues. These data strongly suggest that the differential expression of Glc6Pase activity in liver and kidney in the course of fasting may be accounted for by the respective time course of mRNA abundance in both organs.

Ca2+/Mg(2+)-dependent ATPase activity in Hymenolepis diminuta mitochondria

Ca2+ and Mg2+ caused a concentration-dependent activation of ATP hydrolysis by mitochondrial membranes of Hymenolepis diminuta, a rat intestinal cestode. Ca2+ was the more potent, but Mg2+ the more effective. The Lineweaver-Burk plot yielded Km and Vmax values of 1.15 nM and 217.4 nmol Pi min-1 mg-1 protein for Ca(2+)-dependent activity, and 1.86 mM and 333.3 nmol Pi min-1 mg-1 protein for Mg(2+)-dependent activity, respectively. Neither Na+ nor K+, nor a combination of the two cations, induced the hydrolysis of ATP. Ouabain, a specific inhibitor of Na+/K+ ATPase, did not affect the rate of ATP hydrolysis induced by Mg2+ alone or in combination with Na+ or K+. The membrane-bound enzyme was not affected by neuraminidase and concanavalin A. Ca2+ and Mg2+ also induced appreciable hydrolysis of other nucleoside triphosphates by the membranes. Some known anthelmintics, e.g. niclosamide, praziquantel and mebendazole, had no effect on ATPase activities. In addition to other compounds including respiratory inhibitors and uncouplers of phosphorylation, ruthenium red, which blocks Ca2+ influx into the cestode mitochondria, had no influence on the rate of ATP hydrolysis induced by the cations. Triton X-100 was found most suitable for solubilization of both activities. The differences between cestode ATPase and its mammalian counterpart have been discussed.

Differential time course of liver and kidney glucose-6 phosphatase activity during fasting in rats

We have studied the time course of hepatic and renal microsomal glucose-6 phosphatase (Glc-6Pase) during long-term fasting in the rat. Liver microsomal Glc-6Pase increases up to 48 hr and significantly decreases after 48 hr of fasting. The following activities were determined at 0, 24, 48, 72 and 96 hr: 0.31 +/- 0.02; 0.50 +/- 0.02; 0.54 +/- 0.03; 0.44 +/- 0.03; 0.44 +/- 0.01 mumol min-1 mg protein-1, respectively (all values are means +/- SEM, n = 6). Concomitantly, kidney microsomal Glc-6Pase progressively increases throughout the fast (Vm = 0.21 +/- 0.01; 0.26 +/- 0.004; 0.30 +/- 0.01; 0.37 +/- 0.02; 0.40 +/- 0.01 mumol min-1 mg protein-1, from 0 to 96 hr, respectively). These data suggest that the differential expression of Glc-6Pase activity in the liver and the kidney during long-term fasting could have an important role in the shift from a principally hepatic gluconeogenesis to a hepatic and renal gluconeogenesis.

Effects of glucose and amino acid infusion on glucose turnover in insulin-resistant obese and type II diabetic patients

Glucose turnover was assessed from [6,6-2H]glucose and [U-13C]glucose dilution analysis in six lean nondiabetic subjects, six obese patients with normal glucose tolerance, and six obese patients with non-insulin-dependent diabetes mellitus (NIDDM) during sequential infusions of glucose (13.9 mumol/kg fat-free mass [FFM]/min) and glucose+amino acid (4.2 mg/kg FFM/min). Cori cycle activity was assessed from the difference between glucose turnover obtained from [6,6-2H]glucose and [U-13C]glucose. During infusion of glucose alone, total glucose turnover was increased by 70% in obese NIDDM patients. Amino acid infusion decreased glucose concentrations by 0.8, 0.5, and 1.8 mmol/L in controls, obese patients, and NIDDM patients, respectively. This decrease in glycemia occurred despite an increase in glucose turnover in lean and obese nondiabetic subjects, and was due to an increased metabolic clearance rate (MCR) of glucose. In NIDDM patients the MCR of glucose was unchanged, and the decrease in glycemia was explained by a diminution in hepatic glucose output. Glucose turnover obtained by [6.6-2H] dilution analysis exceeded significantly the values obtained by dilution analysis in obese subjects and obese NIDDM patients, but not in controls. This indicates an increased Cori cycle activity in these patients.

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.

The molecular basis of the type 1 glycogen storage diseases

Microsomal glucose-6-phosphatase catalyses the last step in liver glucose production. Glucose-6-phosphatase deficiency, now termed type 1 glycogen storage disease, was first described almost 40 years ago but until recently very little was known about the molecular basis of the various type 1 glycogen storage diseases. Recently we have shown that at least six different proteins are needed for normal glucose-6-phosphatase activity in liver. Four of the proteins have been purified and three cloned. Study of the type 1 glycogen storage diseases has stimulated investigations of the mechanisms of small molecule transport across the endoplasmic reticulum membrane and demonstrated the existence of novel endoplasmic reticulum transport proteins for glucose and phosphate.