Cyclic GMP directly regulates a cation conductance in membranes of bovine rods by a cooperative mechanism

Cyclic GMP causes the release of endogenous Ca2+ from rod outer segments, whose plasma membrane has been made permeable, or from isolated discs. Approximately 11,000 Ca2+ ions are released per disc at saturating concentrations of cyclic GMP. The velocity and the amplitude of the release of Ca2+ are dependent on the concentration of cyclic GMP. The maximal rate of the Ca2+ efflux is approximately 7 X 10(4) Ca2+ ions s-1 rod-1. The Ca2+ release by cyclic GMP is independent of light. The activation of the efflux occurred within a narrow range of the cyclic GMP concentration (30-80 microM) and does not obey a simple Michaelis-Menten scheme. Instead, the kinetic analysis of the Ca2+ efflux suggests that a minimum number of 2 molecules of cyclic GMP activates the ion conductance in a cooperative fashion. The release of Ca2+ by cyclic GMP requires a gradient of Ca2+ ions across the disc membrane. If the endogenous Ca2+ gradient is dissipated by means of the ionophore A23187, the release of Ca2+ by cyclic GMP is abolished. Ca2+ is released by analogues of cyclic GMP which are either modified at the 8-carbon position of the imidazole ring or by the deaza-analogue of cyclic GMP. Congeners of cyclic GMP which are modified at the ribose, phosphodiester, or pyrimidine portion of the molecule are ineffective. The hydrolysis of cyclic GMP by the light-regulated phosphodiesterase of rod outer segments is not a necessary condition for the Ca2+ release because 8-bromo-cyclic GMP, a congener resistant to hydrolysis, is a more powerful activator of the release than cyclic GMP itself. Ca2+ release by cyclic GMP is inhibited by organic and inorganic blockers of Ca2+ channels. The l-stereoisomer of cis-diltiazem blocks the release of Ca2+ at micromolar concentrations, whereas the d-form is much less effective. These results suggest that disc membranes contain a cationic conductance which is permeable to Ca2+ ions and which is regulated through the cooperative binding of at least 2 molecules of cyclic GMP to regulatory sites of the transport protein. By this mechanism, subtle changes in the concentration of cyclic GMP could promote large changes in the flux of Ca2+ ions across the disc membrane.     

Halothane inhibits the neurotoxin stimulated [14C]guanidinium influx through 'silent' sodium channels in rat glioma C6 cells

We have investigated the effect of pharmacological agents on [14C]guanidinium ion influx through sodium channels in C6 rat glioma and N18 mouse neuroblastoma cells. The sodium channels of the N18 cells can be activated by aconitine alone, indicating that they are voltage-dependent channels. In contrast, sodium channels in the C6 cells require the synergistic action of aconitine and scorpion toxin for activation and are therefore characterized as so-called silent channels. The general anesthetic halothane used at clinical concentrations, specifically inhibited the ion flux through the silent sodium channel of C6 rat glioma cells. The voltage-dependent channels of the N18 cells were insensitive to halothane at the concentrations tested.     

Thermodynamic linkages in rabbit muscle pyruvate kinase: kinetic, equilibrium, and structural studies

The mechanism of allosteric regulation of rabbit muscle pyruvate kinase (PK) was examined in the presence of the allosteric inhibitor phenylalanine (Phe). Steady-state kinetic, equilibrium binding, and structural studies were conducted to provide a broad data base to establish a reasonable model for the interactions. Phe was shown to induce apparent cooperativity in the steady-state kinetic measurements at pH 7.5 and 23 degrees C. The apparent Km for phosphoenolpyruvate was shown to increase with increasing Phe concentrations. These results imply that Phe reduces the affinity of PK for phosphoenolpyruvate. This conclusion was substantiated by equilibrium binding studies which yielded association constants of phosphoenolpyruvate as a function of Phe concentration. The binding constant of Phe was also determined at pH 7.0 and 23 degrees C. The effect of ligands on the hydrodynamic properties of PK was monitored by difference sedimentation velocity, sedimentation velocity, and equilibrium experiments. The results showed that PK remains tetrameric both in the presence and in the absence of Phe. However, Phe induces a small decrease in the sedimentation coefficient of the enzyme; hence, it suggests a loosening of the protein structure. The accessibility of the sulfhydryl residues of the enzyme also increases in the presence of Phe. Furthermore, the Phe-induced conformational change was approximately 90% complete when only 25% of the binding sites were saturated. This result suggested that the regulatory behavior of PK might satisfactorily be described by the two-state model of Monod-Wyman-Changeux [Monod, J., Wyman, J., & Changeux, J.-P. (1965) J. Mol. Biol. 12, 88-118].     

Cytochrome P-450-catalyzed desaturation of valproic acid in vitro. Species differences, induction effects, and mechanistic studies

The cytochrome P-450-mediated desaturation of valproic acid (VPA) to its hepatotoxic metabolite, 2-n-propyl-4-pentenoic acid (4-ene-VPA), was examined in liver microsomes from rats, mice, rabbits and humans. The highest substrate turnover was found with microsomes from rabbits (44.2 +/- 2.7 pmol of product/nmol P-450/15 min), while lower activities were observed in preparations from human, mouse, and rat liver, in that order. Pretreatment of animals with phenobarbital led to enhanced rates of formation of 4-ene-VPA in vitro and yielded induction ratios for desaturation ranging from 2.5 to 8.4, depending upon the species. Comparative studies in the rat showed that phenobarbital is a more potent inducer of olefin formation than either phenytoin or carbamazepine. The mechanism of the desaturation reaction was studied by inter- and intramolecular deuterium isotope effect experiments, which demonstrated that removal of a hydrogen atom from the subterminal C-4 position of VPA is rate limiting in the formation of both 4-ene- and 4-hydroxy-VPA. Hydroxylation at the neighboring C-5 position, on the other hand, was highly sensitive to deuterium substitution at that site, but not to deuteration at C-4. Based on these findings, it is proposed that 4-ene- and 4-hydroxy-VPA are products of a common P-450-dependent metabolic pathway, in which a carbon-centered free radical at C-4 serves as the key intermediate. 5-Hydroxy-VPA, in contrast, derives from an independent hydroxylation reaction.     

Enumeration of denitrifying microbial populations in turf

Summary Denitrifer populations of a silt and silt loam soil under a Kentucky bluegrass turf were enumerated using the most probable number (MPN) procedure. The influence of soil texture, soil depth, soil moisture, and additions of nitrate fertilizer on denitrifier populations were determined. Saturated soil conditions increased denitrifier populations 87-fold in the silt soil and 121-fold in the silt loam soil. Denitrifier populations did not differ significantly between soil depths and additions of fertilizer nitrate did not influence populations.     

Free and polymerized tubulin in cultured bone cells and Chinese hamster ovary cells: the influence of cold and hormones

A low pH method of liposome-membrane fusion (Schneider et al., 1980, Proc. Natl. Acad. Sci. U. S. A. 77:442) was used to enrich the mitochondrial inner membrane lipid bilayer 30-700% with exogenous phospholipid and cholesterol. By varying the phospholipid-to- cholesterol ratio of the liposomes it was possible to incorporate specific amounts of cholesterol (up to 44 mol %) into the inner membrane bilayer in a controlled fashion. The membrane surface area increased proportionally to the increase in total membrane bilayer lipid. Inner membrane enriched with phospholipid only, or with phospholipid plus cholesterol up to 20 mol %, showed randomly distributed intramembrane particles (integral proteins) in the membrane plane, and the average distance between intramembrane particles increased proportionally to the amount of newly incorporated lipid. Membranes containing between 20 and 27 mol % cholesterol exhibited small clusters of intramembrane particles while cholesterol contents above 27 mol % resulted in larger aggregations of intramembrane particles. In phospholipid-enriched membranes with randomly dispersed intramembrane particles, electron transfer activities from NADH- and succinate-dehydrogenase to cytochrome c decreased proportionally to the increase in distance between the particles. In contrast, these electron- transfer activities increased with decreasing distances between intramembrane particles brought about by cholesterol incorporation. These results indicate that (a) catalytically interacting redox components in the mitochondrial inner membrane such as the dehydrogenase complexes, ubiquinone, and heme proteins are independent, laterally diffusible components; (b) the average distance between these redox components is effected by the available surface area of the membrane lipid bilayer; and (c) the distance over which redox components diffuse before collision and electron transfer mediates the rate of such transfer.     

Lysine biosynthesis inMethanobacterium thermoautotrophicum is by the diaminopimelic acid pathway

Methanobacterium thermoautotrophicum, an archaebacterium, possesses the first and last enzymes of the diaminopimelic acid pathway for lysine biosynthesis, dihydrodipicolinate synthase, and diaminopimelate decarboxylase. It does not have saccharopine dehydrogenase, the last enzyme of the aminoadipate pathway for lysine biosynthesis. The dihydrodipicolinate synthase is inhibited but not repressed by lysine. We conclude that this microbe uses the diaminopimelate pathway for synthesis of lysine.Deceased.     

Evidence against an abnormal hepatic microsomal lipid matrix as the primary genetic defect in the jaundiced Gunn rat

The congenitally jaundiced Gunn rat does not conjugate bilirubin but does conjugate bilirubin dimethyl diester. Partial defects in conjugating p-nitrophenol and demethylating aminopyrine are also evident. A proposed mechanism to explain this combination of findings is a defective microsomal membrane. To examine the 'matrix' of Gunn microsomal membranes, hepatic microsomes were isolated from Gunn (jj) and outbred Wistar (JJ) rats and were studied by electron paramagnetic resonance spectroscopy of 7-doxylstearic and 12-doxylstearic acid probes, fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene, glucose-6-phosphatase activity vs. temperature, and lipid analysis. The data indicate several factors related to lipid bilayer order do not differ in microsomes from jj and JJ.     

Interacting role of thyroxine and growth hormone in the hepatic synthesis of alpha 2u-globulin and its messenger RNA

Hypophysectomy completely abolishes and thyroidectomy results in a 90% reduction in the hepatic content of alpha 2u-globulin and its mRNA in the male rat. Thyroid hormone is also known to be required for the synthesis and secretion of pituitary growth hormone. In the hypothyroid rat either thyroxine or growth hormone was found to increase the activity and number of sequences of the mRNA for alpha 2u-globulin (measured by translational assay and hybridizational analysis with a cloned cDNA probe) to the euthyroid level. Treatment of hypophysectomized rats with a hormone combination containing growth hormone but not thyroxine increased the hepatic level of the mRNA for alpha 2u-globulin to that of normal animals. From these results we conclude that thyroxine indirectly influences the hepatic concentration of the mRNA for alpha 2u-globulin through its effect on pituitary growth hormone. Although administration of growth hormone to hypothyroid animals raised the hepatic concentration of alpha 2u-globulin mRNA to the euthyroid level, synthesis of alpha 2u-globulin remained low (50% of the normal). Complete recovery of alpha 2u-globulin synthesis required thyroxine. Therefore, in addition to an indirect effect on the hepatic level of alpha 2u-globulin mRNA, thyroxine also directly influences the synthesis of this protein. This direct effect of thyroxine on alpha 2u-globulin synthesis seems to be exerted at a step distal to the formation of mature mRNA.