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H G Hers

Publications and source records attributed to H G Hers.

At least 19 recordsLinked to original sources
PubMed1979

Enzyme defect in primary gout.

The rate-limiting step in the degradation of adenine nucleotides in the liver is the conversion of adenosine monophosphate (A.M.P.) to inosine monophosphate by A.M.P. deaminase, which is normally 95% inhibited. When the inhibition is released, uric acid is formed in large excess, and the biosynthesis of purines is increased. We therefore propose that congenital hyperuricaemia is caused by the presence of an abnormal A.M.P. deaminase, which is less sensitive to its physiological inhibitors. Verification of the hypothesis depends upon the availability of liver tissue from patients with congenital hyperuricaemia for kinetic analysis of A.M.P. deaminase. A call for collaboration is addressed to the medical community.

AMP Deaminase↗
PubMed1979

Mechanism of activation of glycogen phosphorylase by fructose in the liver. Stimulation of phosphorylase kinase related to the consumption of adenosine triphosphate.

1. A dose-dependent activation of phosphorylase and consumption of ATP was observed in isolated hepatocytes incubated in the presence of fructose; histone kinase and phosphorylase kinase activities were unchanged at doses of this sugar that were fully effective on phosphorylase. The activation of phosphorylase by fructose was also observed in cells incubated in a Ca2+-free medium as well as in the livers of rats in vivo. 2. In a liver high-speed supernatant, fructose, tagatose and sorbose stimulated the activity of phosphorylase kinase; this effect was dependent on the presence of K+ ions, which are required for the activity of fructokinase; it was accompanied by the transformation of ATP into ADP. In the presence of hexokinase, glucose also stimulated phosphorylase kinase, both in an Na+ or a K+ medium. 3. The activities of partially purified muscle or liver phosphorylase kinase were unchanged in the presence of fructose. 4. Some properties of liver phosphorylase kinase are described, including a high molecular weight and an inhibition at ATP/Mg ratios above 0.5, as well as an effect of ATP concentration on the hysteretic behaviour of this enzyme. 5. The effect of fructose on the activation of phosphorylase is discussed in relation to the comsumption of ATP.

Adenosine Triphosphate↗
PubMed1978

Control of gluconeogenesis and of enzymes of glycogen metabolism in isolated rat hepatocytes. A parallel study of the effect of phenylephrine and of glucagon.

Hepatocytes isolated from the livers of fed rats were used for a comparative study of the effects of phenylephrine, vasopressin and glucagon on gluconeogenesis and on enzymes of glycogen metabolism. When hepatocytes were incubated in the presence of Ca(2+), phenylephrine stimulated gluconeogenesis from pyruvate less than did glucagon, but, in contrast with this hormone, it did not affect the activities of protein kinase and pyruvate kinase, nor the concentration of phosphoenolpyruvate, and it did not decrease the release of (3)H(2)O from [6-(3)H]glucose. The effects of vasopressin were similar to those of phenylephrine. Gluconeogenesis from fructose was also stimulated by phenylephrine and, more markedly, by glucagon at the expense of the conversion of fructose into lactate. Insulin was able to antagonize the stimulatory effect of phenylephrine on gluconeogenesis from pyruvate. When Ca(2+) was removed from the incubation medium, phenylephrine still stimulated gluconeogenesis from pyruvate, but it also caused an activation of protein kinase and an inactivation of pyruvate kinase; accordingly, the concentration of phosphoenolpyruvate was increased, and, in contrast, vasopressin had no effect on all these parameters. The property of phenylephrine to cause the activation of glycogen phosphorylase was decreased by glucose or by the absence of Ca(2+); it was abolished when these two conditions were combined. Glycogen synthase was inactivated by phenylephrine in the presence or the absence of Ca(2+), although presumably by different mechanisms.

Animals↗
PubMed1978

Native and latent forms of liver phosphorylase phosphatase. The non-identity of native phosphorylase phosphatase and synthase phosphatase.

The directly measurable (native) phosphorylase phosphatase present in a fresh mouse liver extract is bound to particulate glycogen and is not inhibited by heat-stable inhibitors. Treatment of the extract with trypsin or ethanol at room temperature caused a more than 10-fold increase in phosphorylase phosphatase activity. This increased activity stems from the activation of completely inactive (latent) enzyme, the major part of which is present in the high-speed supernatant. The trypsin-revealed activity can be completely blocked by heat-stable inhibitors. Treatment of the animal with glucocorticoids increases, and fasting decreases the activity of the native phosphorylase phosphatase. The level of latent enzyme, however, is unaffected by these treatments. The major portion of synthase phosphatase in the fresh liver extract is bound to glycogen. This enzyme is inhibited by the heat-stable inhibitor-2 and inactivated by trypsin or ethanol as well as by several treatments that have little effect on phosphorylase phosphatase. Upon DEAE-cellulose chromatography at 0 degrees C of a fresh liver extract, phosphorylase phosphatase and synthase phosphatase were resolved as separate, single peaks. If the preparation was not kept at 0 degrees C during the entire procedure, two peaks of each enzyme were observed. Under these conditions the first peak of phosphorylase phosphatase and of synthase phosphatase coincided. From these findings it is concluded that synthase phosphatase and phosphorylase phosphatase, in their native form, are distinct enzymes.

Animals↗
PubMed1978

Phosphorylation of glucose in isolated rat hepatocytes. Sigmoidal kinetics explained by the activity of glucokinase alone.

The conversion of glucose into glucose 6-phosphate in an extract of isolated rat hepatocytes incubated in the presence of MgATP was studied spectrophotometrically at 340nm and also by a radiochemical procedure based on the release of (3)H from [2-(3)H]glucose. Both methods gave similar results. The glucose-saturation curve was sigmoidal and the shape of this curve was not influenced by the ionic composition of the incubation medium. The activity at 0.5mm-glucose was only 1-2% of V(max.), indicating a virtual absence of low-K(m) hexokinase in the preparation. The radiochemical method was also used for the determination of glucose phosphorylation by intact hepatocytes. The glucose-saturation curve was also markedly sigmoidal, but the s(0.5) (substrate concentration at half-maximal velocity) and the Hill coefficient were larger than in extracts of hepatocytes. These two parameters became smaller when cells were incubated in a medium in which Na(+) ions were replaced by K(+) ions. The increased rate of phosphorylation at low glucose concentration in a K(+) medium was accompanied by an increased rate of metabolite recycling between glucose and glucose 6-phosphate and also by an increased uptake of glucose. In both media phosphorylation of glucose was inhibited co-operatively by N-acetylglucosamine. Calculations indicate that this inhibition would reach 100% at saturation of the inhibitor, although at lower concentrations of N-acetylglucosamine it was smaller than expected from the known K(i) of N-acetylglucosamine for glucokinase. The rate of phosphorylation of glucose was proportional to the amount of glucokinase in hepatocytes from newborn rats and in conditions such as starvation and diabetes in which the total amount of glucokinase in the liver is decreased. In the same conditions, glucose 6-phosphatase activity was either normal or increased. It is concluded that the phosphorylation of glucose in isolated hepatocytes follows sigmoidal kinetics, which can be explained by the activity of glucokinase alone with no participation of low-K(m) hexokinase or of glucose 6-phosphatase.

Acetylglucosamine↗
PubMed1977

The mechanism of adenosine triphosphate depletion in the liver after a load of fructose. A kinetic study of liver adenylate deaminase.

1. The hepatic concentration of several nucleotides and metabolites was measured during the first few minutes after an intravenous load of fructose to mice. The first changes, observed at 30s, were a decrease in the concentration of Pi and a simultaneous accumulation of fructose 1-phosphate. The decrease in the concentrations of ATP and GTP proceeded more slowly. An increase in the concentration of IMP was detected only after 1 min and could therefore not be considered to be the cause of the accumulation of fructose 1-phosphate. 2. To explain the temporary burst of adenine nucleotide breakdown that occurs after a load of fructose, the kinetics of AMP deaminase (EC 3.5.4.6) from rat liver were reinvestigated at physiological (0.2 mM) concentration of substrate. For this purpose, a new radiochemical-assay procedure was developed. At 0.2mM-AMP a low activity could be measured, which was more than 90% inhibited by 5mM-Pi. ATP (3MM) increased the enzyme activity over 200-fold. Pi alone did not influence the ATP-activated enzyme, but 0.5mM-GTP caused a 60% inhibition. The combined effect of both inhibitors at their physiological concentrations reached 95%. 3. It is proposed that the rapid degradation of adenine nucleotides that occurs after a load of fructose is caused by a decrease in the concentration of both inhibitors, Pi and GTP, soon counteracted by the decrease in the concentration of ATP. 4. Some of the kinetic parameters of liver AMP deaminase were computed in terms of the concerted transition theory of Monod, Wyman & Changeux (1965) (J. Mol. Biol. 12, 88-118).

AMP Deaminase↗
PubMed1977

A kinetic study of the soluble 5'-nucleotidase of rat liver.

1. The kinetic properties of the 5'-nucleotidase (EC 3.1.3.5) present in the cytosol of rat liver were investigated in relation to the conversion of adenine nucleotides into uric acid, with particular reference to the stimulation of this process by fructose. The enzyme was assayed by the release of Pi and by a new and more sensitive radiochemical procedure. 2. When IMP was used as substrate, the partially purified enzyme displayed almost hyperbolic kinetics (h = 1.1) with S0.5 = 1.2 mM. Similar kinetics were observed with GMP and other nucleoside 5'-monophosphates, except AMP. 3. Vmax. of the enzyme for AMP was about the same as for IMP, but the kinetics were sigmoidal (h = 1.6) with S 0.5 = 10 mM. 4. The hydrolysis of IMP was inhibited competitively by GMP. IMP, at concentrations up to 0.5 mM, had a paradoxical stimulatory action on the hydrolysis of 2-5 mM-AMP and was inhibitory at higher concentrations. 5. The activity of the enzyme towards AMP and IMP was stimulated by ATP and GTP, and inhibited by Pi. Activators and inhibitor approximately cancelled each others' effects. At pH 7.4, the enzymic activity with 0.2 mM-AMP was undetectable under physiological conditions. 6. It is concluded that, in the liver cell, AMP is not hydrolysed by the soluble 5'-nucleotidase, but that its degradation requires prior deamination to IMP.

Adenosine Monophosphate↗
PubMed1977

Hormonal and ionic control of the glycogenolytic cascade in rat liver.

1. A parallel dose-dependent activation of histone kinase, phosphorylase kinase and phosphorylase was observed in isolated hepatocytes incubated in the presence of glucagon; the effect of suboptimal concentrations of glucagon was antagonized by insulin. 2. An activation of phosphorylase which was not accompanied by a stable change in the activity of phosphorylase kinase was observed in hepatocytes incubated with phenylephrine, isoproterenol or vasopressin as well as on decapitation of unanesthetized animals. A dissociation of the two enzymic activities was also observed in hepatocytes incubated in the presence of a high concentration of glucose, in which phosphorylase was strongly inactivated with no change in the activity of phosphorylase kinase. 3. The activation of phosphorylase by phenylephrine in isolated hepatocytes was counteracted by insulin, greatly decreased by the absence of Ca2+ from the incubation medium, and completely suppressed by the replacement of Na+ by K+. 4. In a liver extract, phosphorylase kinase could also be activated by trypsin. Control, glucagon-activated or trypsin-activated phosphorylase kinase was inhibited by about 70% by EGTA and the activity was restored by the addition of Ca2+. 5. The mechanisms that control the activity of phosphorylase kinase and of phosphorylase are discussed.

Animals↗
PubMed1977

The effect of insulin on the glycogenolytic cascade and on the activity of glycogen synthase in the liver of anaesthetized rabbits.

1. The administration of insulin to anaesthetized rabbits caused the inactivation of liver phosphorylase and phosphorylase kinase, but did not change either the hepatic concentration of cyclic AMP or the activity of cyclic AMP-dependent histone kinase. All measured parameters were increased by the subsequent administration of glucagon. 2. Activation of glycogen synthase by insulin was only observed when phosphorylase had been strongly inactivated.

Animals↗
PubMed1976

Hormonal control of pyruvate kinase activity and of gluconeogenesis in isolated hepatocytes.

Treatment of isolated rat hepatocytes with saturating concentrations of glucagon caused several modifications properties of pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40): S0.5 (substrate concentration at half maximum velocity) for phosphoenolpyruvate was about doubled, whereas Vmax was not changed; the activity measured at 0.15 mM phosphoenolpyruvate (physiological concentration) was reduced 65-80%; and there was also an increase in the Hill coefficient and in the affinity of the enzyme for the inhibitors Mg-ATP and alanine. Glucagon, 3':5'-cyclic AMP, and epinephrine caused an inactivation of pyruvate kinase together with a sitmulation of gluconeogenesis. Insulin (10 nM) antagonized the effect of suboptimal doses of glucagon or cyclic AMP and of even maximal doses of epinephrine, on both pyruvate kinase activity and on gluconeogenesis. These observations can be explained by a phosphorylation of pyruvate kinase by cyclic-AMP-dependent protein kinase, as described by Ljungström et al. [(1974) Biochim. Biophys. Acta 358, 289-298] in a reconstructed system. They offer a molecular explanation for the hormonal control of gluconeogenesis. Glucose caused an inhibition of gluconeogenesis with no corresponding change in pyruvate kinase activity.

Animals↗
PubMed1976

The control of glycogen metabolism in the liver.

The major factor that controls glycogen metabolism in the liver is the concentration of phorphorylase alpha. Indeed, this enzyme catalyzes the limiting step of glycogen breakdown and, by controlling the activity of synthetase phosphatase, also regulates glycogen synthesis. The formation of phosphorylase alpha is stimulated by cAMP, by glycogen, and presumably also by some still ill-defined ionic changes. The ininactivation of phosphorylase is greatly stimulated by glucose and inhibited by AMP and glycogen. Glycogen synthesis is proportional to the concentration of synthetase alpha, which in normally fed animals is formed only when most of the phosphorylase is in the beta form. The inactivation of glycogen synthetase is stimulated by cAMP, an elevated concentration of which puts a double lock on glycogen synthetase by activating phosphorylase alpha (and thereby preventing synthetase activation) and by inactivating glycogen synthetase. The effect of cAMP, 5'-AMP, glucose, and glycogen can presently be explained in molecular terms. The main missing link is in the ionic effect whose elucidation might lead to the understanding of the mode of action of insulin.

1,4-alpha-Glucan Branching Enzyme↗
PubMed1975

The stimulation of liver phosphorylase b by AMP, fluoride and sulfate. A technical note on the specific determination of the a and b forms of liver glycogen phosphorylase.

1. The activity of liver phosphorylase b from several mammalian species has been studied. The enzyme from rat or mouse has a higher activity than the rabbit enzyme, which is itself more active than pig liver phosphorylase b. 2 The activity of liver phosphorylase b is influenced by anions and by AMP, and these effects are influenced by pH. Fluoride, which is currently added to the assay mixture of phosphorylase a in crude preparations, is about as active as sulfate as a stimulator of phosphorylase b. 3. When assayed at pH 6.1 and in the presence of 0.15 M NaF, the activity of rat liver phosphorylase b reaches 25% of that of the a enzyme; if 1 mM AMP is also present, this value rises to 50%. 4. Methods are described that allow the determination of liver phosphorylase a without interference of b, and the determination of total phosphorylase (a+b) in rat liver.

Adenosine Monophosphate↗