Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: EC:2.7.1.1 (hexokinase)
5,274 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Detailed time courses of uptake of labeled 3-O-methyl-D-glucose and 2-deoxy-D-glycose by untreated and ATP-depleted Novikoff rat hepatoma cells were determined as function of concentration (0.2-10 mM) by a rapid mixing/sampling technique which allows uptake measurements in time intervals as short as 1.5 seconds. Intracellular accumulation of 3-O-methylglucose in untreated and ATP-depleted cells and of deoxyglucose in ATP-depleted cells to equilibrium followed pseudo-first order kinetics and initial velocities were computed from overall time courses of substrate accumulation. Initial velocity was a Michaelis-Menten function of exogenous substrate concentration. The estimated kinetic constants for zero-trans transport of 3-O-methylglucose were about the same for untreated and ATP-depleted cells (Kztm = 1.73 +/- 0.24 mM; Vztmax = 28.8 +/- 3.6 pmoles/microliter cell H2O. sec) and were similar to those for deoxyglucose transport in ATP-depleted cells (Kztm = 0.65 +/- 0.1 mM; Vztmax = 19.6 +/- 1.6 pmoles/microliter cell H2O. sec). Similar kinetic parameters were obtained for the transport of D-glucose and D-galactose in ATP-depleted cells. The transport of 3-O-methylglucose and deoxyglucose were inhibited by each other in a simple competitive manner with apparent Ki's similar to their transport Km's. In untreated cells, in which deoxyglucose was phosphorylated, intracellular steady-state levels of free deoxyglucose accumulated within 10 to 20 seconds of incubation regardless of its concentration in the medium. Thereafter, the rate of deoxyglucose incorporation into total cell material reflected the rate of phosphorylation rather than the transport rate. The rate of deoxyglucose transport exceeded the initial rate of its phosphorylation by 20-40 %. The intracellular steady-state-levels observed during the first 2 minutes of incubation decreased from about 40% of equilibrium level at 0.2 mM deoxyglucose to about 8% at 10 mM. Computer fits of a kinetic equation describing transport and phosphorylation as independent processes operating in tandem to these data are consistent with the observed kinetic constants for hexose transport and hexokinase activity with deoxyglucose as substrate. Upon longer incubation (2-10 minutes) the rate of deoxyglucose uptake by the phosphorylating cells decreased progressively, concomitant with a decrease in intracellular ATP and an increase in intracellular deoxyglucose to equilibrium levels. It is demonstrated that the rate of deoxyglucose uptake, measured at two or more minutes, seriously underestimates the hexose transport rate and yields misleading conclusions regarding the extent and type of inhibition by transport inhibitors, such as persantin or cytochalasin B. Persantin inhibited hexose transport in a simple non-competitive manner (Ki = 20 muM) indicating that the drug affects the function of the hexose carrier.
...
PMID:Deoxyglucose and 3-O-methylglucose transport in untreated and ATP-depleted Novikoff rat hepatoma cells. Analysis by a rapid kinetic technique, relationship to phosphorylation and effects of inhibitors. 67 Mar 3

The association of hexokinase with particulate fractions of liver has been studied during the development of the rat. In the fetal rat liver about 50% of the total hexokinase is bound to the mitochondria. This proportion decreases gradually after birth. No association of hexokinase with mitochondria could be demonstrated in the adult rat liver. It is suggested that the binding of hexokinase to the mitochondria is a consequence of a property which is lost during development of the mitochondrial membranes. At least part of the hexokinase is rather tightly bound to the mitochondria. Glucose-6-phosphate or ATP do not solubilize the enzyme. The proportion of hexokinase bound to the mitochondria is independent of the metabolic state of the hepatocyte as exemplified by studies with inhibitors of oxidative phosphorylation, using the digitonin technique for the separation of particulate and cytosol fractions of isolated fetal rat hepatocytes.
...
PMID:Particulate and free hexokinase in fetal rat liver. 67 2

A prolonged glucose load was administered to four patients with hypokalaemic periodic paralysis and four healthy control sujbects. Muscle ATP and CP concentrations as well as lactate dehydrogenase, hexokinase and phosphorylase activities were similar in those two groups, but succinate dehydrogenase was approximately 50% higher in the control muscles. Muscles fibre composition was almost identical in the two groups, whereas patients had a higher degree of capillarization. Complete muscle weakness was produced in all patients, accompanied by hypokalaemia. Glucose loading resulted in elevated insulin levels and a minor rise in blood glucose level was seen in the patients compared to the control subjects. Glucose loading decreased hexokinase activity in controls, but increased this in the patients. At similar times, muscle and blood lactate levels and blood pyruvate values were generally higher in the patients over the course of the experiment. Initial glycogen concentrations were higher in patients, but glucose loading did not result in greatly increased glycogen values. These data suggest that patients with hypokalaemic periodic paralysis have an enhanced metabolism of carbohydrates and that insulin seems to be an important factor leading to the onset of muscle weakness.
...
PMID:Skeletal muscle characteristics and carbohydrate metabolism after glucose loading in hypokalaemic periodic paralysis. 70 37

In a theoretical study the influence of hemoglobin and Mg-ions as binding partners of red cell 2,3-diphosphoglycerate and ATP was investigated. Free hemoglobin may be an efficient competitor of Mg2+ for the ligand ATP. At conditions which favour hemoglobin as binding partner (i.e. desoxygenation, low medium pH and incubation temperature, as in blood preservation) up to 95% of the whole cellular ATP (ca. 2mM in cell water) may be bound to hemoglobin (ca. 7 mM). This binding is largely prevented in the presence of physiological amounts of diphosphoglycerate (ca. 7 mM) which is in excess and has a higher binding affinity to hemoglobin. Therefore, diphosphoglycerate keeps ATP (MgATP) in cell water solution at conditions in which Hb would trop it in the presence of Mg2+ (ca. 3mM). It can be calculated that, by lack of free MgATP, the activity of hexokinase within the cell drops by a factor of greater than 10 when diphosphoglycerate is metabolized. This indirect activation by diphosphoglycerate of hexokinase is operative at free concentrations of DPG far below those which exert the well known excess inhibitory effect on hexokinase and phosphofructokinase. In a model study, the activation by diphosphoglycerate of the initial two-kinase stage was introduced into a simplified kinetic model of glycolysis. A pronounced hysteresis loop of the stationary concentrations of ATP and diphosphoglycerate was produced indicating the existence of several stationary states, one with high ATP and high diphosphoglycerate, the other one with low values. It is demonstrated that diphosphoglycerate, being a protector of glycolysis at physiological concentrations, triggers an autocatalytic breakdown of the energy state when permitted to drop to low values.
...
PMID:[Importance of binding of 2,3-diphosphoglycerate and ATP to hemoglobin for erythrocyte glycolysis: activation by 2,3-diphosphoglycerate of hexokinase at intracellular conditions]. 70 29

Activities of four enzymes of the glycolytic pathway, hexokinase, glyceraldehyde 3-phosphate dehydrogenase, pyruvate kinase and lactate dehydrogenase, were determined in a vesicular brush-border preparation from rabbit kidneys. The specific activities of the enzymes were decreased several-hundredfold in the brush-border preparation compared with a kidney homogenate, but the enzymes were not totally absent. Density-gradient centrifugation of the brush-border preparation yielded brush border of even higher purity and also a characteristic pattern of distribution for each of the contaminating intracellular membranes. The presence of hexokinase in the brush-border preparation could be traced to contaminating mitochondria, and that of glyceraldehyde 3-phosphate dehydrogenase, pyruvate kinase and lactate dehydrogenase to contaminating vesicles derived from the endoplasmic reticulum. The brush-border vesicles contained some ATP. An intravesicular concentration of 0.1mm was estimated, indicating that the vesicles had retained at least a part of their original content. Experiments in which fluorescein isothiocyanate-dextran (mol.wt. 20000) was present during cell lysis revealed that much, but not all, of the brush-border contents had been exchanged with the medium. The complete absence of glycolytic enzymes from brush-border vesicles, which had retained part of their original content, indicates that the brush border does not contain glycolytic enzymes in vivo and can be thought of as a compartment of its own, somehow separated from the cytoplasm.
...
PMID:The brush border of rabbit kidney, a cellular compartment free of glycolytic enzymes. 70 4

It is established that purified nuclear and mitochondrial fractions of the rat brain possess a noticeable AMP-deaminase activity. ATP is an effective activator of AMP-deaminase in the both fractions, but this enzyme is also stimulated by hexokinase in the mitochondrial fraction. The ammonia production from ADP in the mitochondrial fraction is connected with the formation on ATP and AMP under the influence of myokinase and subsequent deamination of AMP by AMP-deaminase.
...
PMID:[AMP-deaminase activity of rat brain nuclear and mitochondrial fractions]. 72 89

1. The specific activity of yeast hexokinase A depends on the concentration of the protein in the solution being assayed. When a solution containing 13.5 mg of hexokinase A/ml is diluted 10--100-fold at various values of pH and temperature, there is a gradual decline in the specific activity of the enzyme until an equilibrium value is reached, which varies with the chosen experimental conditions. 2. The catalytic activity lost when hexokinase A (1 mg/ml) is incubated at 30degreesC is recovered by lowering the temperature to 25degreesC. 3. These concentration- and temperature-dependent phenomena are consistent with the existence of a monomer-dimer equilibrium in which the dimer alone is the catalytic form of the enzyme. 4. Glucose alone prevents the decline in specific activity of hexokinase A after dilution, but it does not re-activate dilute solutions solutions of the enzyme. It is concluded that glucose binds to both the dimer and the monomer and prevents both association and dissociation. 5. The progress curve describing the phosphorylation of glucose catalysed by hexokinase A does not attain a steady state. It is possible that dissociation of catalytically active dimers in a ternary complex with glucose and ATP (or glucose 6-phosphate and ADP) could explain the non-linearity of this progress curve.
...
PMID:Dissociation and catalysis in yeast hexokinase A. 78 48

(1) Ca2+ efflux from rabbit skeletal muscle sarcoplasmic reticulum vesicles pre-loaded with 45Ca2+ was studied in the presence and in the absence of external Ca2+. (2) In the absence of Ca2+ in the assay media, ADP activates the Ca2+ efflux. The increment of Ca2+ efflux requires Pi, is coupled to ATP synthesis, and is inhibited by external Ca2+ (Ki 0.1-0.2 muM). (3) When Ca2+ is added to the assay media, ADP alone activates the Ca2+ efflux, but this is coupled to a Ca2+ influx of the same magnitude. It is therefore an exchange of internal for external Ca2+ in a 1:1 ratio. (4) The ADP-activated Ca2+ exchange requires external Ca2+ with an apparent Km of 0.1-0.2 muM, does not require the addition of Pi or Mg2+, although 3-10 mM MgCl2 activates it. It is not inhibited by the removal of contaminating ATP with hexokinase plus glucose. (5) It seems likely that Ca2+ can be translocated across sarcoplasmic reticulum membrane without the formation of a phosphorylated intermediate.
...
PMID:ADP-activated calcium ion exchange in sarcoplasmic reticulum vesicles. 81 33

The compounds P1-(adenosine-5')-P3-(glucose-6) triphosphate (Ap3 glucose) and P1-(adenosine-5')-P4-(glucose-6)tetraphosphate (Ap4 glucose) were synthesized as possible transition-state analogs for hexokinase (ATP: D-hexose 6- phosphotransferase, EC 2.7.1.1). Both compounds were inhibitors of this enzyme, competitive against ATP and apparently uncompetitive against glucose. The inhibition constants for Ap3 glucose and Ap4 glucose were 0.43 mM and 0.37 mM, respectively. These results indicate that the inhibitors do not appreciably bind to hexokinase until the glucose binding site is filled. The sugar portion of the inhibitors therefore does not contribute to binding, and the compounds are acting as ATP analogs, Ap3 glucose and Ap4 glucose are also slow substrates for glucose-6-phosphate dehydrogenase.
...
PMID:Inhibition of hexokinase by multisubstrate analogs. 83 47

Three glucose-phosphorylating enzymes having different specificities for glucose and fructose were separated from the cell-free extract of Candida tropicalis by means of ammonium sulfate fractionation and chromatography on DEAE-cellulose and Sephadex G-100. Two of them, which phosphorylated fructose 1.5 times faster than glucose, were designated as hexokinase I and II (ATP : D-hexose 6-phosphotransferase, EC 2.7.1.1.), and the other with very low or no fructose-phosphorylating activity, as glucokinase (ATP : D-glucose 6-phosphotransferase, EC 2.7.1.2). Km values for glucose with both hexokinase I and glucokinase were 0.3 mM, and that for fructose with hexokinase I was 2.2 mM. Time-course changes in the levels of these enzymes in C. tropicalis growing on glucose and on n-alkane revealed that hexokinase was induced specifically by the sugars, while glucokinase was a constitutive enzyme. Addition of cycloheximide to the culture medium prevented the increase in the hexose-phosphorylating activity and in the Fru/Glu ratio (the ratio of enzymatic phosphorylation of fructose to that of glucose) in the cells. Although Candida lipolytica also contained hexokinase and glucokinase, both enzymes seemed to be constitutive.
...
PMID:Glucose-phosphorylating enzymes of Candida yeasts and their regulation in vivo. 83 48


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>

---