Newer developments in enzymic determination of D-glucose and its anomers

Purification, Characterization, and cDNA Cloning of a Prominent β-Glucosidase from the Gut of the Xylophagous Cockroach Panesthia angustipennis spadica

In this study, a β-glucosidase (PaBG1b) with high specific activity was purified from gut extracts of the wood-feeding cockroach Panesthia angustipennis spadica using Superdex 75 gel filtration chromatography and High-Trap phenyl hydrophobic chromatography. The protein was purified 14-fold to a single band identified by sodium dodecyl sulfate polyacrylamide gel electrophoresis, with an apparent molecular mass of 56.7 kDa. The specific activity of the purified enzyme was 708 μmol/min/mg protein using cellobiose as substrate. To the best of our knowledge, this is the highest specific activity reported among β-glucosidases to date. The purified PaBG1b showed optimal activity at pH 5.0 and retained more than 65 % of the activity between pH 4.0 and 6.5. The activity was stable up to 50 °C for 30 min. Kinetic studies on cellobiose revealed that the K _m was 5.3 mM, and the V _max was 1,020 μmol/min/mg. The internal amino acid sequence of PaBG1b was analyzed, and two continuous sequences (a total of 39 amino acids) of the C-terminal region were elucidated. Based on these amino acid sequences, a full-length cDNA (1,552 bp) encoding 502 amino acids was isolated. The encoded protein showed high similarity to β-glucosidases from glycoside hydrolase family 1. Thus, the current study demonstrated the potential of PaBG1b for application in enzymatic biomass-conversion as a donor gene for heterologous recombination of cellulase-producing agents (fungi or bacteria) or an additive enzyme for cellulase products based on the high-performance of PaBG1b as a digestive enzyme in cockroaches.

Physiological regulation of the derepressible phosphate transporter in Saccharomyces cerevisiae

The extracellular phosphate concentration permissive for the expression of different amounts of the active high-affinity Pho84 phosphate transporter in the plasma membrane as well as the PHO84 messenger RNA levels in low-phosphate-grown Saccharomyces cerevisiae cells is very narrow and essential for a tight regulation of the transporter. The Pho84 transporter undergoes a rapid degradation once the supply of phosphate and/or carbon source is exhausted.

Non-enzymatic protein glycosylation: back-titration assay

The glycosylation of albumin in vitro, as judged from the incorporation of D-[U-14C]glucose into trichloroacetic acid (TCA)-precipitable material represents a time-, temperature- and concentration-related process. It is markedly increased by NaCNBH3. A close correlation is observed between the radioactive data and the percentage of glycosylated albumin, as measured by affinity chromatography. The latter method, however, does not give information on the precise stoichiometry of the protein glycosylation. The relative extent of protein glycosylation can also be estimated by a back-titration procedure, the condensation of labelled D-glucose with the protein being inversely related to its prior degree of glycosylation. The back-titration assay was applied to plasma samples from normal and diabetic rats or human subjects and used to compare the glycosylation of albumin by distinct hexoses and hexose-phosphates. L-Glucose was found as efficient as D-glucose in causing albumin glycosylation and, hence, could conceivably be used to investigate in vivo changes in the intrinsic properties of extracellular proteins secondary to their glycosylation.

Anomeric specificity of liver glycogenolysis

In rat liver slices incubated in the absence of exogenous D-glucose, both the basal and glucagon-stimulated output of D-glucose resulted in the production of a greater relative amount of alpha-D-glucose than that found at anomeric equilibrium. Comparable results were obtained in isolated hepatocytes. In these experiments, the rate of glycogenolysis largely exceeded that of glycogen synthesis. These findings indicate that liver glycogenolysis represents an alpha-stereospecific process.

Anomeric preference in uptake of D-glucose and of D-galactose by rat lenses

To determine the anomeric preference of uptake of D-glucose and of D-galactose by rat lenses, we crystallized alpha-, beta-D-[U-14C]glucose (720 microCi/mmol) and alpha-, beta-D-[U-14C]galactose (180 microCi/mmol) by our method and incubated them separately with rat lenses for 1 min, because of the short half-life of mutarotation of alpha-D-glucose (9.6 min) and of alpha-D-galactose (4.6 min) in HEPES medium at 30 degrees C. During aerobic incubation of rat lenses in HEPES medium containing radioactive alpha or beta anomer of D-glucose, there was no significant difference in the rate of uptake between alpha and beta anomers of D-glucose by rat lenses. However, 1.59 times greater incorporation of alpha-D-galactose was observed over that of beta-D-galactose under the same conditions.

Anomeric specificity of hexokinase and glucokinase activities in liver and insulin-producing cells

Conflicting data have been reported concerning the anomeric specificity of glucokinase. In the present study, liver hexokinase (Km for D-glucose 0.4 mM) displayed a higher affinity for but lower Vmax. with alpha- than with beta-D-glucose. The velocity of the reaction catalysed by liver glucokinase was higher with with beta- than with alpha-D-glucose, whatever the glucose concentration. The apparent Km of glucokinase was somewhat lower, however, with alpha- than with beta-D-glucose. Comparable results were obtained for the high-Km glucokinase-like enzymic activity present in normal pancreatic islets or insulin-producing tumoral cells. These results suggest that the anomeric specificity of glucokinase cannot account for the higher rate of glycolysis found in islets exposed to alpha- as distinct from beta-D-glucose.

Rapid and sensitive, colorimetric determination of the anomers of D-glucose with D-glucose oxidase, peroxidase, and mutarotase

A modification, utilising mutarotase, of an enzymic, colorimetric system for determining D-glucose with D-glucose oxidase, peroxidase, and ABTS was satisfactory for the assay of the anomers of D-glucose in aqueous solution. The time required for a single assay is approximately 10 min, and the lower limit is 0.4 microgram of D-glucose. The method is applicable to the anomer analysis of D-glucose released by enzymic hydrolysis of D-glucosides.

Uptake of D-glucose anomers by rat retina

Uptake of D-glucose anomers by isolated rat retina was studied. After 3 min incubation at 37 degrees C in the presence of alpha or beta anomer (750 mug/ml), a significantly greater uptake (1.32 mg/g wet tissue) of beta-anomer was observed compared with that of alpha-D-glucose (1.11 mg/g wet tissue). This result and other data suggest that the carrier for D-glucose transport in the retina prefers the beta-anomer stereospecifically.