A continuous spectrophotometric assay for the enzymatic marker glucose 6-phosphatase

Lemon Balm and Corn Silk Mixture Alleviates Metabolic Disorders Caused by a High-Fat Diet

We recently reported that varying combination ratios of lemon balm (Mellissa officinalis L.) and corn silk extracts (Stigma of Zea mays L. fruit) could reduce the obesity caused by a high-fat diet (HFD). The present study investigated the dose-dependent effect of a 1:1 (w:w) mixture of lemon balm and corn silk extracts (M-LB/CS) on HFD-mediated metabolic disorders and compared the effect with metformin. Oral administration of 50–200 mg/kg of M-LB/CS for 84 days significantly inhibited HFD-induced body weight gain, adipocyte hypertrophy, and lipogenic gene induction without affecting food consumption in mice. Biochemical analyses showed that M-LB/CS blocked abnormal lipid accumulation in the blood by escalating fecal lipid excretion. In addition, M-LB/CS prevented HFD-mediated pancreatic atrophy, decreased the number of insulin- and glucagon-immunoreactive cells, and inhibited increases in glycated hemoglobin, glucose, and insulin. Moreover, M-LB/CS also reduced hepatic injury, lipid accumulation, gluconeogenesis, and lipid peroxidation in parallel with the induction of AMP-activated protein kinase and antioxidant enzymes. Furthermore, M-LB/CS protected the kidney by inhibiting tubular vacuolation and reducing serum creatinine and blood urea nitrogen levels. The prophylactic effect of 100 mg/kg M-LB/CS-administration was comparable to that of metformin. Therefore, M-LB/CS may be an alternative option for managing obesity and its related metabolic disorders.

Cortisol-sensitive urea transport across the gill basolateral membrane of the gulf toadfish (Opsanus beta)

Gulf toadfish ( Opsanus beta) use a unique pulsatile urea excretion mechanism that allows urea to be voided in large pulses via the periodic insertion or activation of a branchial urea transporter. The precise cellular and subcellular location of the facilitated diffusion mechanism(s) remains unclear. An in vitro basolateral membrane vesicle (BLMV) preparation was used to test the hypothesis that urea movement across the gill basolateral membrane occurs through a cortisol-sensitive carrier-mediated mechanism. Toadfish BLMVs demonstrated two components of urea uptake: a linear element at high external urea concentrations, and a phloretin-sensitive saturable constituent ( Km = 0.24 mmol/l; Vmax = 6.95 μmol·mg protein−1·h−1) at low urea concentrations (<1 mmol/l). BLMV urea transport in toadfish was unaffected by in vitro treatment with ouabain, N-ethylmaleimide, or the absence of sodium, conditions that are known to inhibit sodium-coupled and proton-coupled urea transport in vertebrates. Transport kinetics were temperature sensitive with a Q10 > 2, further suggestive of carrier-mediated processes. Our data provide evidence that a basolateral urea facilitated transporter accelerates the movement of urea between the plasma and gills to enable the pulsatile excretion of urea. Furthermore, in vivo infusion of cortisol caused a significant 4.3-fold reduction in BLMV urea transport capacity in lab-crowded fish, suggesting that cortisol inhibits the recruitment of urea transporters to the basolateral membrane, which may ultimately affect the size of the urea pulse event in gulf toadfish.

Urea transport in kidney brush-border membrane vesicles from an elasmobranch, Raja erinacea

Marine elasmobranch fishes maintain high urea concentrations and therefore must minimize urea loss to the environment in order to reduce the energetic costs of urea production. Previous studies have identified a facilitated urea transporter in the kidney of the dogfish. We examined mechanisms of urea transport in the kidney of the little skate Raja erinacea using an isolated brush-border membrane vesicle preparation. Urea uptake by brush-border membrane vesicles is by a phloretin-sensitive, non-saturable uniporter in the dorsal section and a phloretin-sensitive, sodium-linked urea transporter (Km = 0.70 mmol l(-1), Vmax = 1.18 micromol h(-1) mg(-1) protein) in the ventral section of the kidney. This provides evidence for two separate urea transporters in the dorsal versus ventral sections of the kidney. We propose that these two mechanisms of urea transport are critical for renal urea reabsorption in the little skate.

Active urea transport and an unusual basolateral membrane composition in the gills of a marine elasmobranch

In elasmobranch fishes, urea occurs at high concentrations (350-600 mM) in the body fluids and tissues, where it plays an important role in osmoregulation. Retention of urea by the gill against this huge blood-to-water diffusion gradient requires specialized adaptations to the epithelial cell membranes. Experiments were performed to determine the mechanisms and structural features that facilitate urea retention by the gill of the spiny dogfish Squalus acanthias. Analysis of urea uptake by gill basolateral membrane vesicles revealed the presence of a phloretin-sensitive (half inhibition 0.09 mM), sodium-coupled, secondary active urea transporter (Michaelis constant = 10.1 mM, maximal velocity = 0.34 micromol. h(-1). mg protein(-1)). We propose that this system actively transports urea out of the gill epithelial cells back into the blood against the urea concentration gradient. Lipid analyses of the basolateral membrane revealed high levels of cholesterol contributing to the highest reported cholesterol-to-phospholipid molar ratio (3.68). This unique combination of active urea transport and modification of the phospholipid bilayer membrane is responsible for decreasing the gill permeability to urea and facilitating urea retention by the gill of Squalus acanthias.

The association of metalloendopeptidase EC 3.4.24.15 at the extracellular surface of the AtT-20 cell plasma membrane

Endopeptidase EC 3.4.24.15 (EP24.15) is a soluble, neuropeptide-degrading metalloenzyme, widely expressed in the brain, pituitary and gonads. For the physiological metabolism of neuropeptides, the enzyme should be located extracellularly, either associated with the plasma membrane or in the extracellular milieu. Western immunoblot analyses of crude cytosolic and post-nuclear membrane fractions prepared by differential centrifugation revealed a slightly smaller molecular mass ( approximately 2 kDa) for EP24.15 in the post-nuclear membrane fraction. This smaller EP24.15 species was also present in an enriched fraction of plasma membrane prepared by Percoll gradient centrifugation. To ascertain whether EP24.15 is associated with the extracellular surface of plasma membrane, two sets of experiments were carried out. First, Western immunoblot analysis of AtT-20 cells treated with the membrane-impermeable, thiol-cleavable cross-linker, 3, 3'-dithio-bis(sulpho-succinimidyl-propionate) (DTSSP), indicated an extracellular membrane association. After cross-linking and thiol-reduction, a distinct band corresponding to EP24.15 was significantly diminished under non-reducing conditions. Second, immunocytochemical studies performed at 4 degrees C on non-permeabilized AtT-20 cells (i.e., non-fixed to prevent antibody internalization), indicated that EP24.15 was expressed on the surface of the AtT-20 cells. We furthermore determined that EP24.15 enzymatic activity is present on the extracellular surface of the cell discernable from the secreted enzyme. These results suggest that the EP24.15 is associated with the extracellular surface of the AtT-20 cell plasma membrane and is enzymatically active. Taken together, the results are consistent with a putative role in the degradation of neuropeptides acting at the external cell surface.

An assay for glucose 6-phosphatase based on the formation of glucose

A rapid, convenient method for the assay of glucose 6-phosphatase dependent on the removal of radioactive substrate from radioactive product by Dowex 2 fluoride is described. The enzymatic reaction is stopped by the addition of an ethanolic slurry of the resin. After the tubes are shaken, the radioactivity of glucose in the clarified supernatant layer is measured. The release of glucose is directly proportional to time and enzyme concentration. Detergents do not interfere with the method.