Electrogenic properties of the cloned Na+/glucose cotransporter: I. Voltage-clamp studies

Summary Cystic fibrosis (CF) is characterized by abnormal epithelial Cl^– conductance (G_Cl). In vitro studies that have shown that cAMP regulation is an intrinsic property of the CF-affected G_Cl(CF-G_Cl) have been carried out previously on cultured secretory cells and on nonepithelial cells. Even though G_Cl in absorption is defective in CF, a clear demonstration of cAMP regulation of CF-G_Cl in a purely absorptive tissue is lacking. We studied the cAMP regulation of CF-G_Cl in the microperfused intact human reabsorptive sweat duct. About 40% of the ducts responded to cAMP (responsive) while the remainder of the ducts did not. In responsive ducts, cAMP-elevating agents: -adrenergic agonist isoproterenol (IPR), CPT-cAMP, forskolin, theophylline or IBMX increased G _tby about 2.3-fold (n = no. of ducts = 8). Removal of media Cl^–, but not amiloride pretreatment (in the lumen), abolished the cAMP response, indicating exclusive activation of G_Cl. cAMP activated both apical and basolateral G_Cl. cAMP hyperpolarized gluconate: Cl^– (lumen: bath) transepithelial bionic potentials (V _t=–20.3±5.2 mV, mean ±se, n=9) and transepithelial 3 1 luminal NaCl dilution diffusion potentials (V _t=–8.8±2.9 mV, n=5). cAMP activated basolateral G_Cl as indicated by increased bi-ionic (gluconate: Cl^–, bath: lumen) diffusion potentials (by about 12 mV). The voltage divider ratio in symmetric NaCl solutions increased by 60%. Compared to responsive ducts, nonresponsive ducts were characterized by smaller spontaneous transepithelial potentials in symmetrical Ringer's solution (V _t=–6.9±0.8 mV, n=24, nonresponsive vs. –19.4±1.8 mV, n=22, responsive ducts) but larger bi-ionic potentials (–94±6 mV, n=35, nonresponsive vs. –65±5 mV, n=17, responsive ducts) and dilution diffusion potentials (–40±5 mV, n=11, nonresponsive vs. –29±3 mV, n=7, responsive ducts). These results are consistent with an inherently (prestimulus) maximal activation of G_Cl in nonresponsive ducts and submaximal activation of G_Cl in responsive ducts. We conclude that cAMP activates CF-G _Cl which is expressed and abnormal in both apical and basal membranes of this absorptive epithelium in CF.Abbreviations CF cystic fibrosis - G _t transepithelial conductance - V _b electrical potential across the basolateral membrane - V _a electrical potential across the apical membrane - V _t transepithelial potential - V _b transepithelial currentinduced voltage deflections across the basolateral membrane - V _a transepithelial current-induced voltage deflections across the apical membrane - V _t transepithelial current-induced voltage deflection across the epithelium - VDR voltage divider ratio - G_Cl transepithelial Cl^– conductance - CF-G_Cl cystic fibrosis-affected Cl^– conductance - EMF electromotive force - IPR isoproterenol - IBMX 3-isobutyl-1-methylxanthine - CPT-cAMP chlorophenylthio-adenosine 3-5 cyclic monophosphate - PGE₂ prostaglandin E₂     

Cycloheximide increases the thermostability of proteins in Chinese hamster ovary cells

Protein denaturation resulting from temperatures between 42.0 degrees C and 50 degrees C has been observed and implicated as the lethal lesion for hyperthermic cell killing. A logical corollary is that protection against hyperthermic killing requires stabilization of cellular proteins against thermal denaturation. To test this, Chinese hamster ovary cells were treated with the heat protector cycloheximide and then subjected to differential scanning calorimetry to measure protein denaturation. Cycloheximide stabilized proteins that denatured between 42 degrees C and 52 degrees C in control cells by increasing their transition (denaturation) temperature by an average of 1.3 degrees C. In addition, cycloheximide reduced the cytotoxicity of actinomycin D and adriamycin, suggesting that protein stabilization protects cells against stresses other than hyperthermia.     

The substrate reactivity of mu-monothiopyrophosphate with pyrophosphate-dependent phosphofructokinase: evidence for a dissociative transition state in enzymatic phosphoryl group transfer.

mu-Monothiopyrophosphate (MTP), an analogue of pyrophosphate (PPi) with sulfur in place of oxygen in the bridge position, is a substrate for the enzyme pyrophosphate-dependent phosphofructokinase. At pH 9.4 and 6 degrees C, the maximal velocity for the phosphorylation of fructose 6-phosphate (F6P) by MgMTP is about 2.8% of that with MgPPi as the phosphoryl donor. The kinetic mechanism is equilibrium random with rate-limiting transformation of the substrate ternary complex to the product when either MgMTP or MgPPi is the phosphoryl donor. This is known from independent studies to be kinetic mechanism at pH 8.0 and 25 degrees C [Bertagnolli, B. L., & Cook, P. F. (1984) Biochemistry 23, 4101-4108]. The dissociation constant of MgPPi is 14 microM, that of MgMTP is 64 microM, and that of F6P from the enzyme is about 5 mM. The Km values for MgPPi and MgMTP are 14.5 and 173 microM, respectively. MgMTP competes with MgPPi for binding to the enzyme. The values of kcat are 3.4 s-1 and 140 s-1 for MgMTP and MgPPi, respectively, at pH 9.4 and 6 degrees C. The estimated rate enhancement factors are 3.6 x 10(5) and 1.4 x 10(14) for the reactions of MgMTP and MgPPi, respectively. Therefore, MgMTP is a reasonably good substrate for PPi-dependent PKF, on the basis of comparisons of kcat. However, the rate enhancement factors show that the enzyme is a poor catalyst for the reaction of MgMTP. Lesser enzymatic catalysis in the reaction of MgMTP compared with MgPPi is largely compensated for by the greater intrinsic reactivity of MgMTP. Thus, the larger substrate MgMTP is well accommodated in the active site, and the dissociative reaction of MgMTP is well accommodated in the transition state. The results are interpreted to indicate a dissociative transition state for phosphoryl group transfer by PPi-dependent PFK. A modified synthesis and purification of MTP are described, in which (trimethylsilyl)trifluoromethanesulfonate and tetra-N-butylammonium iodide are used in place of iodotrimethylsilane to dealkylate tetramethyl-MTP.     

Catecholamine-Sensitive Guanylate Cyclase from Human Caudate Nucleus

Abstract: Partial purification of soluble guanylate cyclase on DEAE-Sephacel yields two separate peaks of guanylate cyclase activity. After 10-fold purification of the soluble enzyme, guanylate cyclase is markedly inhibited by micromolar concentrations of dopamine (I₅₀= 0.2 μm). Dopamine inhibition is observed whether the reaction is conducted with Mn²¹ or with Mg²⁺, under atmosphere or N₂(g), and using enzyme from either peak from the DEAESephacel column. Other catecholamines also inhibit partially purified guanylate cyclase with an order of potency at 1 μm of: dopamine =l -DOPA > norepinephrine = isoproterenol = adrenochrome > epinephrine. The structural requirements for inhibition are two free hydroxyl groups on the phenyl ring and an ethylamine side chain. Dopamine also inhibits the Triton X-100-solubilized microsomal guanylate cyclase after partial purification on DEAESephacel. Neither chlorpromazine, propranolol, nor phentolamine at 20 μm effectively block the dopamine inhibition of partially purified soluble guanylate cyclase. Micromolar concentrations of the reducing agents dithiothreitol and glutathione also inhibit partially purified guanylate cyclase, but unlike these agents, catecholamines can inhibit whether added in the reduced or the oxidized forms. Inhibition of enzyme activity by micromolar concentrations of dopamine, adrenochrome, or dithiothreitol is rapidly reversed by dilution and the dopamine inhibition is competitive with MgGTP. Inhibition does not appear to involve covalent binding or to result from the ability of catecholamines to reduce the concentrations of oxygen or free radicals in solution.     

Uridine diphosphate galactose 4-epimerase: nucleotide and 8-anilino-1-naphthalenesulfonate binding properties of the substrate binding site.

Escherichia coli UDP-galactose 4-epimerase in its native form (epimerase.NAD) binds 8-anilino-1-naphthalenesulfonate (ANS) at one tight binding site per dimer with a dissociation constant of 25.9 +/- 2.1 micrometer at pH 8.5 and 27 degrees C. This appears to be the substrate binding site, as indicated by the fact that ANS is a kinetically competitive reversible inhibitor with a Ki of 27.5 micrometer and by the fact that ANS competes with UMP for binding to the enzyme. Upon binding at this site the fluorescence quantum yield of ANS is enhanced 185-fold, and its emission spectrum is blue shifted from a lambdamax of 515 to 470.nm, which suggests that the binding site is shielded from water and probably hydrophobic. Competitive binding experiments with nucleosides and nucleotides indicate that nucleotide binding at this site involves coupled hydrophobic and electrostatic interactions. The reduced form of the enzyme (epimerase.NADH) has no detectable binding affinity for ANS. The marked difference in the affinities of the native and reduced enzymes for ANS is interpreted to be a manifestation of a conformational difference between these enzyme forms.     

Clearance of lysosomal hydrolases following intravenous infusion. The role of liver in the clearance of beta-glucuronidase and N-acetyl-beta-D-glucosaminidase.

Certain highly purified forms of rat lysosomal glycosidases, β-glucuronidase and N-acetyl-β-d-glucosaminidase, are rapidly cleared from the circulation following intravenous infusion. Several lines of evidence are presented which indicate that the primary site of enzyme uptake is the liver. Clearance of the two enzymes was unaffected by nephrectomy, whereas it was abolished by evisceration. Tissue distribution experiments with native and [¹²⁵I]β-glucuronidase indicate the liver as the major, if not exclusive, site of enzyme uptake. Experiments with the isolated perfused liver showed clearance of certain enzyme preparations but not others. Those enzymes cleared by the isolated perfused liver were likewise cleared in vivo. Liver fractionation studies following infusion of large doses of β-glucuronidase revealed a rapid, short-lived increase in microsomal β-glucuronidase and a slower but larger increase in lysosomal β-glucuronidase. The results indicate that β-glucuronidase, N-acetyl-β-d-glucosaminidase, and probably other glycosidases are rapidly incorporated into the lysosomal compartment of liver.     

Freeze-thaw survival of the free-living nematode Caenorhabditis briggsae.

Approximately 75% or more of the L2 and L3 juvenile stages of the free-living nematode Caenorhabditis briggsae survived freezing and thawing without loss of fertility. Optimum survival depended upon a combination of conditions: (1) pretreatment with 5% DMSO at 0 °C for 10 min, (2) 0.2 °C per minute cooling rate from 0 to ?100 °C prior to immersion into liquid nitrogen, and (3) a 27.6 °C per minute warming rate from ?196 °C to ?10 °C. Storage at ?196 °C for more than 100 days was without effect on viability or fertility. Some of the L4 (about 50%) and adult (about 3%) stages survive the routine freeze-thaw treatment. However, there was no recovery of either embryonic stages or embryonated eggs from ?196 °C under these standard conditions. Either very fast cooling (about 545 °C/min) or fast warming (about 858 °C/min) rates diminished survival of the L2 and L3 stages drastically.Scanning electron microscopy revealed that freeze-thaw survivors with aberrant swimming behavior had cuticular defects. In juvenile forms, the altered swimming motion was lost after a molt whereas as abnormal adults grew, sinusoidal movement resumed. In the L4 and adult forms the cuticular abnormalities lowered viability and fertility. It is concluded that survival of nematodes from a freeze-thaw cycle is contingent upon establishing specific cryobiological conditions by varying aspects of the procedure that gave high recoveries of L2 and L3 stages.     

Kinetic and binding studies of Mn (II) and fructose 1,6-bisphosphate with rabbit liver hexosebisphosphatase.

The separate interaction of the substrate fructose 1,6-bisphosphate and a metal ion cofactor Mn2+ with neutral hexosebisphosphatase has been studied under equilibrium conditions at pH 7.5 with gel filtration and electron paramagnetic resonance measurements, respectively. Binding data for both ligands to the enzyme yielded nonlinear Scatchard plots that analyze in terms of four negatively cooperative binding sites per enzyme tetramer. Graphical estimates of the binding constants were refined by a computer searching procedure and nonlinear least squares analysis. These results are qualitatively similar to those obtained from binding studies involving teh alkaline enzyme, a modified form of hexosebisphosphatase whose pH optimum is in the alkaline pH region. Both forms of the enzyme enhance the proton relaxation rate of water protons by a factor of approximately 7 to 8 at 24 MHz, demonstrating similar metal ion environments. Teh activator Co(III)-EDTA did not affect Mn2+ binding to the neutral enzyme. In the presence of (alpha + beta)methyl-D-fructofuranoside 1,6-bisphosphate, however, two sets--each containing four Mn2+ binding sites--were observed per enzyme tetramer with loss of the negatively cooperative interaction. These results are viewed in terms of four noncatalytic and four catalytic Mn2+ binding sites. Parallel kinetic investigations were conducted on the neutral enzyme to determine specific activity as a function of Mn2+ and fructose 1,6-bisphosphate concentration. A pro-equilibrium sequential pathway model involving Mn2+-enzyme and the Mn2+-fructose 1,6-bisphosphate complex both as substrate and as an allosteric inhibitor satisfactorily fit the kinetic observations. All possible enzyme species were computed from the determined binding constants and grouped according to the number of moles of Mn2+-fructose 1,6-bisphosphate complex bound to the Mn2+-enzyme, and individual rate constants were calculated. The testing of other models and their failure to describe the kinetic observations are discussed.     

Semiautomated processing of systolic time intervals.

Systolic time intervals are commonly used to identify changes in ventricular function. The method described facilitates measurement and calculation of many such intervals. This method utilizes a printed polygraph recording of electrocardiogram, phonocardiogram, and carotid pulse contour; a digitizing device for reading the necessary coordinates from the record; and a minicomputer for calculation of the intervals and for data analysis. Intervals related to approximately 100 pulse beats can be read, calculated, printed in chart and graph form, and subjected to some analyses in about an hour.