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D G Walker

Publications and source records attributed to D G Walker.

At least 91 records · Page 5Linked to original sources
PubMed1985

Comparison of glucokinase in C3H/He and C58 mice that differ in their hepatic activity.

Partially purified preparations of the hepatic glucokinase from C3H/He and C58 inbred mice have been used to explore the molecular basis for the observed twofold difference in activity between the strains. The single codominant gene that appears to regulate activity, the alleles of which are designated Gka and Gkb, respectively, for the two strains, could represent a structural gene change. This now seems unlikely because the mouse enzyme, although showing small differences from rat glucokinase, appeared to be identical in the two strains with respect to thermal stability, electrophoretic mobility in agarose gels, and kinetic properties such as the apparent Km values for MgATP2- and glucose and the unique cooperative interaction with the latter substrate. The enzymes also reacted identically in a range of immunological tests (double-diffusion, immunoelectrophoresis, immune precipitation and immune inhibition assays) and ELISA immune inhibition assays indicated that the twofold difference in activity was due to a similar difference in antigenically active enzyme. Genetic control over the physiologically significant regulation of enzyme amount is therefore probable.

Animals↗
PubMed1985

A comparison of glucose metabolism and related hormonal parameters in two strains of mice having differing hepatic glucokinase activities.

Parameters of glucose metabolism in the livers of two inbred strains of mice, C3H/He and C58, which have high- and low-glucokinase activities respectively, have been determined. Unlike insulin concentrations, the plasma glucagon concentrations are similar in the two strains. Certain of the numbers of insulin receptors per hepatocyte cell surface area were higher in starved than fed animals of the same strain but affinities were the same, while only small differences in receptor numbers were found between the strains in starved animals. The difference in glucokinase activity, determined spectrophotometrically and confirmed by measurements of detritiation of [2-3H]glucose by hepatocytes incubated in vitro, does not apparently influence the minimal rate of glucose recycling as measured by the relative loss of 3H and 14C from [2-3H, U-14C]glucose. The development profiles for the two strains show a marked developmental difference arising around 20 days after birth.

Aging↗
PubMed1984

Histochemical and immunohistochemical localization of hexokinase isoenzymes in normal rat liver.

Histochemical and immunohistochemical procedures have been used to examine the localization of three of the four hexokinase isoenzymes present in the liver of fed female Wistar rats. Distinctive distribution patterns were found for hexokinase type I and glucokinase but hexokinase type II was not detectable. Hexokinase type I was identified in sinusoidal cells and in bile duct epithelia, nerves and arteries in the portal triad. Glucokinase, the major isoenzyme, was confined to parenchymal cells where it was present in much higher amounts in perivenous compared with periportal hepatocytes. Staining within these two zones was not homogeneous and each had a mosaic appearance caused by the presence of a few hepatocytes containing little or no glucokinase amongst the majority of darkly stained cells in perivenous areas and a few darkly stained cells amongst the majority of unstained cells in periportal areas. Hence, hepatocytes in situ are a strikingly heterogeneous population of cells. Their metabolic status cannot be controlled simply by the differential supply of oxygen, substrates and hormones to different regions of the liver acini as proposed in the metabolic zonation model. Phenotypic differences may exist between cells within a given metabolic zone which influence their ability to respond to different environmental conditions.

Animals↗
PubMed1984

The ubiquitous localization of type I hexokinase in rat peripheral nerves, smooth muscle cells and epithelial cells.

An indirect immunoperoxidase technique has been used to determine the localization of type I hexokinase in a wide variety of Carnoy-fixed, paraffin-embedded rat tissues. The results suggest that the widespread tissue distribution of the isoenzyme is due to its ubiquitous localization in the nervous, smooth muscle and epithelial components of each tissue. The majority of the immunostaining was confined to cells with substantial energy requirements which are probably mainly satisfied through the breakdown of glucose. This observation is consistent with the known predominance of type I hexokinase in the central nervous system and with the regulatory role allotted to it in this tissue.

Animals↗
PubMed1983

Antigenic cross-reactivities between mammalian hexokinases.

Immunoelectrophoresis and gel diffusion techniques have demonstrated that, contrary to previous reports, antigenic cross-reactivity does exist between the three major rat hexokinase isoenzymes. Immune inhibition and immune precipitation assays performed in parallel generally failed to detect this cross-reactivity and possible reasons for this failure are discussed. The inability of the latter assay systems to detect cross-reactivity would seem to account for the discrepancies between the present results and those obtained in earlier studies.

Animals↗
PubMed1983

Hepatic glucokinase activity and circulating insulin concentrations in two inbred mouse strains.

Hepatic glucokinase activity in the C3H/He strain of mice is about twice that in the C58 strain. Genetic analysis of hybrids and back-crosses indicates control of activity by a single codominant gene. The adaptations of activity that occur when animals of the two strains are starved, fed a carbohydrate-free diet or made streptozotocin-diabetic are similar in the two strains. In almost all the situations tested, plasma insulin concentrations were higher in the C3H/He strain than in the C58 strain. On the assumption that insulin plays some role in the synthesis of glucokinase, the possible association between the plasma insulin concentration and hepatic glucokinase activity in mice in which plasma glucose concentrations are similar is discussed. The twofold difference in glucokinase activity between the two strains is associated with only minor differences in glucose tolerance and insulin response to a glucose load.

Animals↗
PubMed1981

The separate roles of glucose and insulin in the induction of glucokinase in hepatocytes isolated from neonatal rats.

1. The specificity of the effect of glucose on the induction of glucokinase activity that occurs when hepatocytes freshly isolated from 13-day-old rats are incubated in Medium 199 together with insulin [Wakelam & Walker (1980) FEBS Lett. 111, 115-119] was examined. A pattern that is different from other known effects of glucose is found, and metabolism of this compound is not necessarily to account for this particular effect. 2. The effects of a raised glucose concentration and of insulin on the induction can be separated. The hexose initiates the process in the absence of insulin in a manner that is sensitive to actinomycin D but not to cycloheximide. The subsequent effect of insulin is dependent on the prior effect of glucose or other positive analogue, does not require the presence of glucose and is inhibited by cycloheximide but not by actinomycin D. 3. Induction of glucokinase in vitro in hepatocytes from neonatal animals is inhibited by adrenaline, glucagon and dibutyryl cyclic AMP, but not by vasopressin or angiotensin II. The inhibition by cyclic AMP is on the stage requiring insulin and is comparatively specific, because total protein synthesis is not apparently diminished. 4. The implications of these results are discussed with reference to possible mechanisms of induction and to the situation in vivo.

Animals↗
PubMed1981

The hematogenous origin of osteoclasts: experimental evidence from osteopetrotic (microphthalmic) mice treated with spleen cells from beige mouse donors.

The excessive skeletal mass and reduced bone resorption characteristic of osteopetrosis in microphthalmic (mi) mice can be corrected by irradiation and transfer of spleen cells from a normal littermate. Osteoclasts in beige (bg) mice, a mutation without osteopetrosis, have giant lysosomal granules. These two facts were exploited to trace osteoclast lineage. Microphthalmic mice treated with whole-body irradiation and spleen cells from a beige donor resorbed the excessive skeletal mass and recovered from osteopetrosis. Furthermore, osteoclasts in treated mi mice had giant lysosomal granules and resembled those found in bg donors when examined by light and transmission electron microscopy. These data provide direct evidence for a hematogenous origin of osteoclasts in mammals.

Animals↗
PubMed1981

Cationic ferritin binding sites and surface charge densities of transformed cells.

Bioelectric surface properties of the high and low tumor-producing cell lines, NCTC 2472 and NCTC 2555, respectively, were determined by cationic ferritin binding and the electrophoretic mobility of intact cells. Measurements of anionic sites were bases on the number of cationic ferritin particles per 0.01 mu 2 that were electronically tagged and counted by an image analyzer. The average particle count was 45 for the control "high" cells and 34 for the control "low" cells. The surface charge densities, expressed as electrostatic units per cm-2 x 10(-13) were 2.34 and 1.18 at 50 V (2 mA) for the "high" and "low" control cells, respectively. Enzymic cleavage of sialic acid and other carbohydrate moieties resulted in up to an 81% reduction in the charge densities and a 57% reduction of the anionic sites of the "high" cells. The electrophoretic mobility of cells with bound cationic ferritin showed that up to 50% of the exposed anionic sites fail to bind cationic ferritin. Preliminary findings on the particle size/distribution by image analysis showed wide ranges in both particle size and interparticle distances that may limit cationic ferritin binding.

Animals↗
PubMed1980

Apparent 'glucokinase' activity in non-hepatic tissues due to N-acetyl-D-glucosamine kinase.

1. Electrophoretic examination of tissue extracts from rat intestinal mucosa, kidney, lung, spleen, mammary gland, adipose tissue, heart muscle and placenta in agarose gels did not reveal the presence of any glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.2) activity corresponding to that present in rat liver. 2. All these tissues do contain an enzyme that possesses very high-Km glucose-phosphorylating activity but which has a slightly lower electrophoretic mobility than glucokinase and can be separated from it by various means. 3. This phosphotransferase activity is due to N-acetyl-D-glucosamine kinase (ATP:2-acetamido-2-deoxy-D-glucose 6-phosphotransferase, EC 2.7.1.59), which has been partialyy purified from intestinal mucosa tissue and shown to have similar kinetic properties to the same enzyme previously purified more extensively from liver and kidney. 4. It is suggested that many of the effects reported in the literature of 'glucokinase' activity in non-hepatic tissues are probably due to N-acetyl-D-glucosamine kinase.

Animals↗
PubMed1980

Factors that prevent the premature appearance of glucokinase in neonatal rat liver.

1. The physiological factors that prevent the precocious appearance of glucokinase activity in the 13-day-old rat that can be induced by oral glucose administration were explored. 2. Evidence is presented that the galactose component of milk sugar is inhibitory. In the absence of this inhibitory galactose, the amount of glucose necessary to effect appreciable induction is greater than that present in milk. 3. The induction is prevented both by administration of mannoheptulose, which inhibits insulin release, and by excess insulin; the amount of insulin available therefore seems to be critical. 4. The inhibition of induction by galactose does not appear to be via competition with glucose but by enhancing insulin release and thereby making this excessive. The relative amounts of glucose and insulin appear to be important in regulating glucokinase induction. 5. The precocious induction of glucokinase by glucose is inhibited by simultaneous treatment with approriate amounts of adrenaline, glucagon, dibutyryl cyclic AMP or isoprenaline but not by vasopressin or angiotensin II. 6. No single cause of glucokinase induction in neonatal rat liver can be recognized. The process is subject to regulation by many factors at a time subsequent to when competence to synthesize the enzyme has been established.

Animals↗
PubMed1980

The isolation and preliminary characterization of N-acetyl-D-glucosamine kinase from rat kidney and liver.

1. Procedures for the extensive purification in high yield of N-acetyl-D-glucosamine kinase from rat liver and kidney are described. The separation of this enzyme from hepatic glucokinase depended primarily on their differing behaviour on an affinity column of Sepharose--N-(6-aminohexanoyl)-2-amino-2-deoxy-D-glucopyranose. 2. This N-acetyl-D-glucosamine kinase also catalyses the phosphorylation of N-acetyl-D-mannosamine and, at a lower rate, several other sugar analogues, including D-glucose. 3. A comparison of the behaviour of the enzyme during gel filtration and electrophoresis in sodium dodecyl sulphate/polyacrylamide gels suggests that N-acetyl-D-glucosamine kinase is a symmetrical dimer of mol.wt. 80000.

Acetylglucosamine↗
PubMed1980

Kinetic characterization of N-acetyl-D-glucosamine kinase from rat liver and kidney.

1. Under normal assay conditions the N-acetyl-D-glucosamine kinases from rat liver and kidney show a pH-dependent lag phase before reaching a steady state, which is probably due to reversible dissociation of the dimeric enzyme. 2. The enzyme catalyses the phosphorylation of N-acetyl-D-glucosamine, N-acetyl-D-mannosamine and D-glucose at pH 7.5, with apparent Km values of 0.06, 0.95 and 600 mM respectively for the enzyme from liver and 0.04, 1.0 and 410 mM respectively for the kidney enzyme. It is strongly inhibited by ADP. 3. The interaction between the enzymes and acceptor substrates shows non-Michaelian kinetics with respect to N-acetyl-D-glucosamine but normal behaviour towards N-acetyl-D-mannosamine and D-glucose. 4. Both N-acetyl-D-glucosamine and N-acetyl-D-mannosamine inhibit the phosphorylation of D-glucose; this inhibition appears to be mixed in character. 5. The facts that the enzymes catalyse the phosphorylation of N-acetyl-D-mannosamine and D-glucose do not detract from the designation of the enzymes as N-acetyl-D-glucosamine kinase. Phosphorylation of glucose in vivo by these kinases is unlikely.

Acetylglucosamine↗
PubMed1979

Thyroid hormones and the precocious induction of hepatic glucokinase in the neonatal rat.

1. Oral intubation of glucose is more effective than intraperitoneal injection in inducing the premature appearance of hepatic glucokinase in suckling rats. 2. The inducing effect of glucose is enhanced by treatment of the animals 12 h or more earlier with 1 microgram triiodothyronine/g body weight. 3. Low but significant activities of glucokinase appear at the normal time of development in hypothyroid neonatal rats. Intubation of glucose into 13-day-old and 24-day-old hypothyroid results in the rapid appearance of glucokinase similar to that in normal animals treated likewise. 4. The enhancing effect of thyroid hormones on glucokinase induction by glucose does not necessarily mean that the normal postnatal increase in plasma thyroid hormones is essential for the normal appearance of glucokinase activity at the time of weaning. Other possible explanations are discussed.

Animals↗