scholarly journals Effects of magnesium on cyclic GMP hydrolysis by the bovine retinal rod cyclic GMP phosphodiesterase

1995 ◽  
Vol 308 (2) ◽  
pp. 653-658 ◽  
Author(s):  
D Srivastava ◽  
D A Fox ◽  
R L Hurwitz
Keyword(s):  
Cyclic Gmp ◽  
Ternary Complex ◽  
Light Response ◽  
Competitive Inhibitor ◽  
Velocity Versus ◽  
Retinal Rod ◽  
The Press ◽  
Rapid Equilibrium ◽  
Kinetics Of ◽  
Dissociation Constant Kd

Knowledge of the kinetics of the rod cyclic GMP phosphodiesterase is essential for understanding the kinetics and gain of the light response. Therefore, the interactions between Mg2+, cyclic GMP, and purified, trypsin-activated bovine rod cyclic GMP phosphodiesterase (EC 3.1.4.17) were examined. The effects of Mg2+ and of cyclic GMP on the rod phosphodiesterase activity were mutually concentration-dependent. Formation of a free Mg-cyclic GMP complex is unlikely due to its high dissociation constant (Kd = 19 mM). Plots of 1/velocity versus 1/[cyclic GMP] as a function of [Mg2+] and 1/velocity versus 1/[Mg2+] as a function of [cyclic GMP] intersected to the left of the 1/velocity axis. This is consistent with the formation of a ternary complex between the phosphodiesterase, Mg2+, and cyclic GMP. A competitive inhibitor of the phosphodiesterase relative to cyclic GMP, 3-isobutyl-1-methylxanthine, non-competitively inhibited the enzyme relative to Mg2+, Pb2+, a competitive inhibitor of the phosphodiesterase relative to Mg2+ [D. Srivastava, R.L. Hurwitz and D. A. Fox (1995) Toxicol. Appl. Pharmacol, in the press] non-competitively inhibited the enzyme relative to cyclic GMP. Collectively these results are suggestive of a rapid equilibrium random binding order of Mg2+ and cyclic GMP to the rod phosphodiesterase.

scholarly journals Physiological evidence that light-mediated decrease in cyclic GMP is an intermediary process in retinal rod transduction.

1982 ◽  
Vol 80 (1) ◽  
pp. 103-123 ◽  
Author(s):  
W H Miller
Keyword(s):  
Cyclic Gmp ◽  
Initial Phase ◽  
Light Response ◽  
Receptor Potential ◽  
Rod Outer Segments ◽  
Retinal Rod ◽  
Biochemical Studies ◽  
Kinetics Of ◽  
Hydrolysis Of ◽  
Retinal Rod Outer Segments

Brief, intracellularly injected pulses of cyclic GMP transiently depolarized toad retinal rod outer segments (ROS). The depolarization is antagonized by light, perhaps by the activation of phosphodiesterase (PDE), as shown in the biochemical studies of others. As measured by the antagonism of cyclic GMP pulses by light, PDE activity peaks after the peak of the receptor potential and has approximately the same recovery time as the membrane voltage after weak illumination, but recovers more slowly than the membrane potential after strong illumination, as sensitivity does in other preparations. A cyclic GMP pulse delivered just after the hyperpolarizing phase of the receptor potential tends to turn off the light response. The kinetics of recovery from this turnoff are similar to those of the initial phase of the receptor potential. This similarity suggests that the initial phase of the receptor potential is controlled by light-activated PDE. Both EGTA and saturating doses of cyclic GMP block the light response, but only cyclic GMP increases response latency, which suggests that if calcium is involved in transduction, it is controlled by the hydrolysis of cyclic GMP. After brief pulses of cyclic AMP, a new steady state of increased depolarization occasionally develops. The effects described above also occur under these conditions. The results are consistent with the hypothesis that light-activated hydrolysis of cGMP is an intermediary process in transduction.

Kinetic Aspects of the Interaction of Blood Clotting Enzymes

1968 ◽  
Vol 19 (03/04) ◽  
pp. 364-367 ◽  
Author(s):  
H. C Hemker ◽  
P. W Hemker
Keyword(s):  
Enzyme Kinetics ◽  
Blood Clotting ◽  
Competitive Inhibition ◽  
Competitive Inhibitor ◽  
Kinetics Of

SummaryThe enzyme kinetics of competitive inhibition under conditions prevailing in clotting tests are developed and a method is given to measure relative amounts of a competitive inhibitor by means of the t — D plot.

scholarly journals C-terminal phosphorylation regulates the kinetics of a subset of melanopsin-mediated behaviors in mice

2017 ◽  
Vol 114 (10) ◽  
pp. 2741-2746 ◽  
Author(s):  
Preethi Somasundaram ◽  
Glenn R. Wyrick ◽  
Diego Carlos Fernandez ◽  
Alireza Ghahari ◽  
Cindy M. Pinhal ◽  
...  
Keyword(s):  
Ganglion Cells ◽  
Light Response ◽  
Circadian Phase ◽  
Light Intensities ◽  
C Terminus ◽  
Phase Alignment ◽  
Electrophysiological Measurements ◽  
Virally Encoded ◽  
Phototransduction Pathway ◽  
Kinetics Of

Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin and mediate several non–image-forming visual functions, including circadian photoentrainment and the pupillary light reflex (PLR). ipRGCs act as autonomous photoreceptors via the intrinsic melanopsin-based phototransduction pathway and as a relay for rod/cone input via synaptically driven responses. Under low light intensities, where only synaptically driven rod/cone input activates ipRGCs, the duration of the ipRGC response will be determined by the termination kinetics of the rod/cone circuits. Little is known, however, about the termination kinetics of the intrinsic melanopsin-based phototransduction pathway and its contribution to several melanopsin-mediated behaviors. Here, we show that C-terminal phosphorylation of melanopsin determines the recovery kinetics of the intrinsic melanopsin-based photoresponse in ipRGCs, the duration of the PLR, and the speed of reentrainment. In contrast, circadian phase alignment and direct effects of light on activity (masking) are not influenced by C-terminal phosphorylation of melanopsin. Electrophysiological measurements demonstrate that expression of a virally encoded melanopsin lacking all C-terminal phosphorylation sites (C terminus phosphonull) leads to a prolonged intrinsic light response. In addition, mice expressing the C terminus phosphonull in ipRGCs reentrain faster to a delayed light/dark cycle compared with mice expressing virally encoded WT melanopsin; however, the phase angle of entrainment and masking were indistinguishable. Importantly, a sustained PLR in the phosphonull animals is only observed at brighter light intensities that activate melanopsin phototransduction, but not at dimmer light intensities that activate only the rod/cone pathway. Taken together, our results highlight how the kinetics of the melanopsin photoresponse differentially regulate distinct light-mediated behaviors.

scholarly journals Quantitative effects of environmental variation on stomatal anatomy and gas exchange in a grass model

2021 ◽  
Author(s):  
Tiago DG Nunes ◽  
Magdalena W Slawinska ◽  
Heike Lindner ◽  
Michael T Raissig
Keyword(s):  
Gas Exchange ◽  
Anatomical Variation ◽  
Brachypodium Distachyon ◽  
Carbon Assimilation ◽  
Light Response ◽  
Gas Analysis ◽  
Environmental Signals ◽  
Grass Model ◽  
Kinetics Of

Stomata are cellular pores on the leaf epidermis that allow plants to regulate carbon assimilation and water loss. Stomata integrate environmental signals to regulate pore apertures and optimize gas exchange to fluctuating conditions. Here, we quantified intraspecific plasticity of stomatal gas exchange and anatomy in response to seasonal variation in Brachypodium distachyon. Over the course of two years we (i) used infrared gas analysis to assess light response kinetics of 120 Bd21-3 wild-type individuals in an environmentally fluctuating greenhouse and (ii) microscopically determined the seasonal variability of stomatal anatomy in a subset of these plants. We observed systemic environmental effects on gas exchange measurements and remarkable intraspecific plasticity of stomatal anatomical traits. To reliably link anatomical variation to gas exchange, we adjusted anatomical gsmax calculations for grass stomatal morphology. We propose that systemic effects and variability in stomatal anatomy should be accounted for in long-term gas exchange studies.

scholarly journals Mitochondrial adenosine triphosphatase from human placenta--inhibition by free magnesium ions of ITP hydrolysis.

1994 ◽  
Vol 41 (1) ◽  
pp. 39-44 ◽  
Author(s):  
Z Aleksandrowicz
Keyword(s):  
Competitive Inhibition ◽  
Competitive Inhibitor ◽  
Hill Coefficient ◽  
Negative Cooperativity ◽  
Magnesium Ions ◽  
Beef Liver ◽  
Bicarbonate Ions ◽  
The Hill ◽  
Kinetics Of ◽  
Free Magnesium

The effects of Mg2+ and bicarbonate on the kinetics of ITP hydrolysis by soluble ATPase (F1) from human placental mitochondria were studied. Increasing amounts of Mg2+ at fixed ITP concentration, caused a marked activation of F1 followed by inhibition at higher Mg2+ concentration. The appropriate substrate for the mitochondrial F1 seems to be the MgITP complex as almost no ITP was hydrolysed in the absence of magnesium. Mg2+ behaved as a competitive inhibitor towards the MgITP complex. In this respect the human placental enzyme differ from that from other sources such as yeast, beef liver or rat liver. The linearity of the plot presenting competitive inhibition by free Mg2+ of MgITP hydrolysis (in the presence of activating bicarbonate anion) suggests that both Mg2+ and MgITP bind to the same catalytic site (Km(MgITP) = 0.46 mM, Ki(Mg) = 4 mM). When bicarbonate was absent in the ITPase assay, placental F1 exhibited apparent negative cooperativity in the presence of 5 mM Mg2+, just as it did with MgATP as a substrate under similar conditions. Bicarbonate ions eliminated the negative cooperativity with respect to ITP (as the Hill coefficient of 0.46 was brought to approx. 1), and thus limited inhibition by free Mg2+. The results presented suggest that the concentration of free magnesium ions may be an important regulatory factor of the human placental F1 activity.

scholarly journals The kinetics of transport of lactate and pyruvate into isolated cardiac myocytes from guinea pig. Kinetic evidence for the presence of a carrier distinct from that in erythrocytes and hepatocytes

1989 ◽  
Vol 264 (2) ◽  
pp. 409-418 ◽  
Author(s):  
R C Poole ◽  
A P Halestrap ◽  
S J Price ◽  
A J Levi
Keyword(s):  
Guinea Pig ◽  
Cardiac Myocytes ◽  
Ventricular Myocytes ◽  
Silicone Oil ◽  
Free Acid ◽  
Competitive Inhibitor ◽  
Isolated Cardiac Myocytes ◽  
Lactate And Pyruvate ◽  
Time Courses ◽  
Kinetics Of

1. Time courses for the uptake of L-lactate, D-lactate and pyruvate into isolated cardiac ventricular myocytes from guinea pig were determined at 11 degrees C or 0 degrees C (for pyruvate) in a citrate-based buffer by using a silicone-oil-filtration technique. These conditions enabled initial rates of transport to be measured without interference from metabolism of the substrates. 2. At a concentration of 0.5 mM, transport of all these substrates was inhibited by approx. 90% by 5 mM-alpha-cyano-4-hydroxycinnamate; at 10 mM-L-lactate a considerable portion of transport could not be inhibited. 3. Initial rates of L-lactate and pyruvate uptake in the presence of 5 mM-alpha-cyano-4-hydroxycinnamate were linearly related to the concentration of the monocarboxylate and probably represented diffusion of the free acid. The inhibitor-sensitive component of uptake obeyed Michaelis-Menten kinetics, with Km values for L-lactate and pyruvate of 2.3 and 0.066 mM respectively. 4. Pyruvate and D-lactate inhibited the transport of L-lactate, with Ki values (competitive) of 0.077 and 6.6 mM respectively; the Ki for pyruvate was very similar to its Km for transport. The Ki for alpha-cyano-4-hydroxycinnamate as a non-competitive inhibitor was 0.042 mM. 5. These results indicate that L-lactate, D-lactate and pyruvate share a common carrier in guinea-pig cardiac myocytes; the low stereoselectivity for L-lactate over D-lactate and the high affinity for pyruvate distinguish it from the carrier in erythrocytes and hepatocytes. The metabolic roles for this novel carrier in heart are discussed.

Extracellular ATP selectively modulates a high-voltage-activated calcium conductance in salivary gland cells of the leech Haementeria ghilianii

1993 ◽  
Vol 181 (1) ◽  
pp. 313-319
Author(s):  
WA Wuttke ◽  
MS Berry
Keyword(s):  
Salivary Gland ◽  
High Voltage ◽  
Cyclic Gmp ◽  
Cyclic Nucleotides ◽  
Extracellular Atp ◽  
Salivary Gland Cells ◽  
Selective Modulation ◽  
Voltage Dependent ◽  
Kinetics Of ◽  
Higher Doses

Extracellular ATP appears to have a widespread role as a neurotransmitter or neuromodulator in mammals (Gordon, 1986; Burnstock, 1990), but little is known about any similar functions in invertebrates. During studies of the effects of cyclic nucleotides on electrically excitable salivary cells of the leech, we found that cyclic GMP produced a rapid (less than 1min) reduction of spike duration, suggesting an extracellular effect (Wuttke and Berry, 1991). We now show that micromolar concentrations of ATP (and higher doses of other nucleotides) also reduce spike duration, and that this is caused by depression of a specific voltage-dependent Ca2+ conductance. Selective modulation of Ca2+ current by external ATP has rarely been found, and the effect is also unusual because it changes the kinetics of inactivation rather than those of activation.

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