Role of enzyme interactions in the regulation of glycolysis and gluconeogenesis. Purification and properties of the phospho- and dephospho-forms of glycogen phosphorylase from swine kidney

Kinetic modeling of glucose central metabolism in hepatocytes and hepatoma cells

BACKGROUND

Kinetic modeling and control analysis of a metabolic pathway may identify the steps with the highest control in tumor cells, and low control in normal cells, which can be proposed as the best therapeutic targets.

METHODS

Enzyme kinetic characterization, pathway kinetic modeling and control analysis of the glucose central metabolism were carried out in rat (hepatoma AS-30D) and human (cervix HeLa) cancer cells and normal rat hepatocytes.

RESULTS

The glycogen metabolism enzymes in AS-30D, HeLa cells and hepatocytes showed similar kinetic properties, except for higher AS-30D glycogen phosphorylase (GP) sensitivity to AMP. Pathway modeling indicated that fluxes of glycogen degradation and PPP were mainly controlled by GP and NADPH consumption, respectively, in both hepatocytes and cancer cells. Likewise, hexose-6-phosphate isomerase (HPI) and phosphoglucomutase (PGM) exerted significant control on glycolysis and glycogen synthesis fluxes in cancer cells but not in hepatocytes. Modeling also indicated that glycolytic and glycogen synthesis fluxes could be strongly decreased when HPI and PGM were simultaneously inhibited in AS-30D cells but not in hepatocytes. Experimental assessment of these predictions showed that both the glycolytic and glycogen synthesis fluxes of AS-30D cells, but not of hepatocytes, were inhibited by oxamate, by inducing increased Fru1,6BP levels, a competitive inhibitor of HPI and PGM.

CONCLUSION

HPI and PGM seem suitable targets for decreasing glycolytic and glycogen synthesis fluxes in AS-30D cells but not in hepatocytes.

GENERAL SIGNIFICANCE

The present study identified new therapeutic targets within glucose central metabolism in the analyzed cancer cells, with no effects on non-cancer cells.

Thermostable alpha-glucan phosphorylases: characteristics and industrial applications

α-Glucan phosphorylases (α-GPs) catalyze the reversible phosphorolysis of α-1,4-linked polysaccharides such as glycogen, starch, and maltodextrins, therefore playing a central role in the usage of storage polysaccharides. The discovery of these enzymes and their role in the course of catalytic conversion of glycogen was rewarded with the Nobel Prize in Physiology or Medicine in 1947. Nowadays, however, thermostable representatives attract special attention due to their vast potential in the enzymatic production of diverse carbohydrates and derivatives such as (functional) oligo- and (non-natural) polysaccharides, artificial starch, glycosides, and nucleotide sugars. One of the most recently explored utilizations of α-GPs is their role in the multi-enzymatic process of energy production stored in carbohydrate biobatteries. Regardless of their use, thermostable α-GPs offer significant advantages and facilitated bioprocess design due to their high operational temperatures. Here, we present an overview and comparison of up-to-date characterized thermostable α-GPs with a special focus on their reported biotechnological applications.

Isolation and partial characterization of two forms of rat heart glycogen phosphorylase

Rat heart glycogen phosphorylase a has been purified to apparent homogeneity by a procedure involving precipitation with ammonium sulfate, DEAE-Sephacel chromatography, and AMP-Sepharose affinity chromatography. In contrast to the skeletal muscle enzyme which appeared as a single peak upon ion-exchange chromatography, heart phosphorylase was separated into two distinct peaks (I and II), both only active in the presence of AMP. The isoelectric points of skeletal muscle phosphorylase b (dephosphorylated form) and heart phosphorylase Ib were both at 6.2, that of heart phosphorylase IIb was 5.2. The Km of phosphorylase IIb for AMP was threefold lower (7 microM) than that of Ib (22 microM). The dissociation constant K8 of phosphorylase Ia (phosphorylated form) was 0.37 mM for glycogen and 5.6 mM for Pi. Increasing the levels of glycogen decreased the apparent Km for Pi and vice versa. No such interaction between substrate binding was observed with phosphorylase IIa, since the Ks values for glycogen (0.27 mM) and Pi (3.8 mM) were not significantly influenced by increasing concentrations of the other substrate. The specific activity of both isoenzymes was 46 units/mg of protein at pH 6.8 and 30 degrees C. The subunit Mr of both forms was 92,500. By incubation with purified phosphorylase kinase and [gamma-32P]ATP both forms were converted to their corresponding forms by 32P incorporation of 1 mol/mol of subunit. This suggests the existence of two native forms of phosphorylase b in rat heart.

Characteristics of the dephosphorylated form of phosphorylase purified from rat liver and measurement of its activity in crude liver preparations

The phosphorylated form of liver glycogen phosphorylase (alpha-1,4-glucan : orthophosphate alpha-glucosyl-transferase, EC 2.4.1.1) (phosphorylase a) is active and easily measured while the dephosphorylated form (phosphorylase b), in contrast to the muscle enzyme, has been reported to be essentially inactive even in the presence of AMP. We have purified both forms of phosphorylase from rat liver and studied the characteristics of each. Phosphorylase b activity can be measured with our assay conditions. The phosphorylase b we obtained was stimulated by high concentrations of sulfate, and was a substrate for muscle phosphorylase kinase whereas phosphorylase a was inhibited by sulfate, and was a substrate for liver phosphorylase phosphatase. Substrate binding to phosphorylase b was poor (KM glycogen = 2.5 mM, glucose-1-P = 250 mM) compared to phosphorylase a (KM glycogen = 1.8 mM, KM glucose-1-P = 0.7 mM). Liver phosphorylase b was active in the absence of AMP. However, AMP lowered the KM for glucose-1-P to 80 mM for purified phosphorylase b and to 60 mM for the enzyme in crude extract (Ka = 0.5 mM). Using appropriate substrate, buffer and AMP concentrations, assay conditions have been developed which allow determination of phosphorylase a and 90% of the phosphorylase b activity in liver extracts. Interconversion of the two forms can be demonstrated in vivo (under acute stimulation) and in vitro with little change in total activity. A decrease in total phosphorylase activity has been observed after prolonged starvation and in diabetes.