scholarly journals Calcium diffusion in uterine smooth muscle sheets.

1982 ◽  
Vol 80 (2) ◽  
pp. 257-277 ◽  
Author(s):  
S Kato ◽  
T Ogasawara ◽  
T Osa
Keyword(s):  
Potassium Depolarization ◽  
High Potassium ◽  
Tension Development ◽  
Uterine Smooth Muscle ◽  
Rate Limiting Step ◽  
Ca Addition ◽  
Calcium Diffusion ◽  
Apparent Diffusion ◽  
The Difference ◽  
Rate Limiting

The potassium contracture in the longitudinal muscle of estrogen-treated rat uterus was kinetically investigated. The rates of tension development after Ca addition and relaxation after Ca removal were measured under the high-potassium depolarization. Both rates decreased with an increase in preparation thickness. The relaxation rate had only a slight dependence on temperature. On the contrary, both relaxation and contraction rates in a contraction induced by an electrical stimulation strongly depended on temperature, but not on preparation size. These results suggest that the Ca diffusion process in the extracellular space is the rate-limiting step in relaxation of Ca-dependent contracture under potassium depolarization. The diffusion model, in which the effect of the unstirred layer was considered, could quantitatively explain the experimental results. The apparent diffusion coefficient in the muscle sheet was estimated to be approximately 3 x 10(-7) cm2/s. The difference from that in aqueous solution is discussed.

THE ALTERATION OF INTRACELLULAR ENZYMES: IV. KINETICS OF CATALASE ALTERATION INDUCED BY CHEMICAL AGENTS

1956 ◽  
Vol 34 (1) ◽  
pp. 25-38
Author(s):  
J. Gordin Kaplan ◽  
Woon-Ki Paik
Keyword(s):  
Molecular Level ◽  
Narrow Range ◽  
Intact Cell ◽  
Cellular Level ◽  
Causal Connection ◽  
Rate Limiting Step ◽  
Chemical Agents ◽  
The Difference ◽  
Rate Limiting ◽  
Kinetics Of

The rate with which n-butanol alters the properties of yeast catalase has been studied as a function of temperature and concentration of altering agent. Activation energies for catalase alteration lay within the rather narrow range of 20–23 kcal./mole, thus confirming a prediction made previously on the basis of the difference in energies of activation for heat destruction of altered and unaltered catalases. Alteration by optimal concentration of butanol was a reaction of zero order. Chloroform also altered yeast catalase with an activation energy within this range of μ values. The close agreement in μ values leads us to conclude that the action of these two altering agents, at all concentrations, is characterized by the same rate-limiting step, even though their action differs in other respects. It was concluded that catalase alteration is probably all-or-none on the molecular level, rather than on the cellular level. Alteration was invariably accompanied by a decrease in the size of the treated cells; alteration was sometimes accompanied by changes in the cytochrome spectrum, but there was no causal connection between these two events. These data are consistent with the interfacial hypothesis, which, in its present crude form, pictures alteration as consisting essentially in the desorption of catalase from some intracellular interface at which it is normally bound in the intact cell.

scholarly journals Mechanism of the Iridium-Catalyzed Silylation of Aromatic C–H Bonds

2020 ◽  
Author(s):  
Caleb Karmel ◽  
John Hartwig
Keyword(s):  
Computational Studies ◽  
Iridium Complexes ◽  
Phen Ligand ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
The Difference ◽  
Silyl Complex ◽  
Rate Limiting ◽  
Insight Into ◽  
Hydride Ligands

<p>The iridium-catalyzed silylation of aromatic C–H bonds has become a synthetically valuable reaction because it forms aryl silanes with high sterically derived regioselectivity with silane reagents that are produced and consumed on large scales. Many groups, including our own, have reported iridium complexes of phenanthroline or bipyridine ligands as catalysts for this reaction. Yet, little is known about the mechanism by which the iridium-catalyzed silylation of arenes occurs. Indeed, no iridium-silyl complexes have been prepared that react with C-H bonds to form C-Si bonds in a fashion that is chemically and kinetically competent to be part of the catalytic cycle. </p><p><br></p> <p>In this manuscript, we report the synthesis and reactivity of iridium-silyl compelexes of the 2,9-Me<sub>2</sub>Phen ligand that generates the most active known catalyst for the silylation of aromatic C-H bonds. We show by experiment and computation that the most stable and most reactive silyl complex of this ligand contains two silyl and one hydride ligands and by kinetic analysis of the catalytic reaction determine the rate-limiting step for arenes with varying electronic properties. Computational studies indicate that the steric encumberance of the phenanthroline ligand controls the number of silyl ligands bound to iridium and that the difference in the number of silyl ligands leads to large differences to the rates of the reaction. These studies provide insight into the origins of the high activity of the catalyst containing the 2,9-Me<sub>2</sub>Phen ligand.</p>

Renal mitochondrial glutamine metabolism during K+ depletion

1986 ◽  
Vol 250 (4) ◽  
pp. F667-F673 ◽  
Author(s):  
S. Sastrasinh ◽  
M. Sastrasinh
Keyword(s):  
Glutamine Metabolism ◽  
Base Line ◽  
Maximal Level ◽  
Isolated Mitochondria ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
The Difference ◽  
Rate Limiting ◽  
Cortical Slices ◽  
K Depletion

We studied changes in renal mitochondrial glutamine metabolism during the development of and recovery from K+ depletion in rats. Significant increase in mitochondrial NH3 production was noted after 3 days of K+-free diet. Ammoniagenesis in K+-depleted animals reached maximal level within 2 wk of K+ deprivation when there was 64% increase in NH3 production. In contrast to the pattern of changes in mitochondrial ammoniagenesis, phosphate-dependent glutaminase (PDG) activity increased within the first 48 h of K+ deprivation, before there was any increase in NH3 production, and did not plateau even after 2 wk of K+-free diet. The disparity between NH3 production and PDG activity cannot be explained by the difference in matrix glutamate level, thus raising the possibility that mitochondrial glutamine entry may be a rate-limiting step for ammoniagenesis during K+ depletion. Recovery from K+ depletion was studied in animals prefed with K+-free diet for 2 wk prior to the initiation of K+-supplemented diet. Muscle K+ content of K+-depleted animals returned to normal after 1 wk of K+ replacement. Mitochondrial NH3 production decreased concomitantly with the attenuation in K+ deficit but did not reach the base-line value even after K+ deficit was completely corrected. Additional experiments with isolated cortical tubules also showed persistent increase in NH3 production after the correction of K+ deficit. Thus, unlike earlier studies in rats during the recovery from metabolic acidosis, which showed only increased ammoniagenesis in isolated mitochondria but not in cortical slices, animals recovered from K+ depletion demonstrated augmented NH3 production both in isolated mitochondria and intact renal tissues.

Nickel-Catalyzed Anti-Markovnikov Hydroarylation of Unactivated Alkenes with Unactivated Arenes Facilitated by non-Covalent Interactions

2019 ◽  
Author(s):  
Noam Saper ◽  
Akito Ohgi ◽  
David Small ◽  
Kazuhiko Semba ◽  
Yoshiaki Nakao ◽  
...  
Keyword(s):  
Decomposition Analysis ◽  
Energy Decomposition Analysis ◽  
Ni Catalyst ◽  
Rate Limiting Step ◽  
Secondary Coordination Sphere ◽  
Non Covalent Interactions ◽  
The Difference ◽  
Markovnikov Addition ◽  
Rate Limiting ◽  
Covalent Interactions

<div><div><div><p>Anti-Markovnikov additions to alkenes have been a longstanding goal of catalysis, and anti-Markovnikov addition of arenes to alkenes would produce alkylarenes that are distinct from those formed by acid-catalyzed processes. Existing hydroarylations are either directed or occur with low reactivity and low regioselectivities for the linear alkylarene. Herein, we report the first undirected hydroarylation of unactivated alkenes with unactivated arenes that occurs with high regioselectivity for the anti-Markovnikov product. The reaction occurs with a Ni catalyst ligated by a highly sterically hindered N-heterocyclic carbene (NHC, L4 or L5). Catalytically relevant arene- and alkene-bound Ni complexes have been characterized, and the rate-limiting step was shown to be reductive elimination to form the C-C bond. DFT calculations, combined with energy decomposition analysis (EDA), suggest that the difference in activity between catalysts containing large and small carbenes results more from stabilizing intramolecular, non-covalent interactions in the secondary coordination sphere than from steric hindrance.</p></div></div></div>

RED CELL METABOLISM IN THE PREMATURE INFANT

PEDIATRICS ◽  
1968 ◽  
Vol 41 (2) ◽  
pp. 473-482
Author(s):  
Frank A. Oski ◽  
Charles Smith ◽  
Ernestine Brigandi
Keyword(s):  
Premature Infant ◽  
Cell Metabolism ◽  
Glucose Consumption ◽  
Red Cell ◽  
Rate Limiting Step ◽  
Cell Age ◽  
The Difference ◽  
Rate Limiting ◽  
Glycolytic Rate ◽  
In Cells

Erythrocyte glucose consumption was measured in the cells from premature and term in fants, adults, and subjects with reticulocytosis. Glucose consumption was related to red cell age as reflected in red cell G-6-PD and GOT activities. The erythrocytes of the premature infant appeared to consume less glucose than would be expected from their mean cell age. Studies in reconstituted hemolysates indicated that the hexokinase reaction was the rate limiting step in cells from all groups and that glucose consumption could be markedly increased in newborn hemolysates by the addition of adenosine triphosphate. The presence of a different isoenzymatic form of hexokinase in cells of the neonate is discussed as one possible factor contributing to the difference in glycolytic rate.

Nickel-Catalyzed Anti-Markovnikov Hydroarylation of Unactivated Alkenes with Unactivated Arenes Facilitated by non-Covalent Interactions

2019 ◽  
Author(s):  
Noam Saper ◽  
Akito Ohgi ◽  
David Small ◽  
Kazuhiko Semba ◽  
Yoshiaki Nakao ◽  
...  
Keyword(s):  
Decomposition Analysis ◽  
Energy Decomposition Analysis ◽  
Ni Catalyst ◽  
Rate Limiting Step ◽  
Secondary Coordination Sphere ◽  
Non Covalent Interactions ◽  
The Difference ◽  
Markovnikov Addition ◽  
Rate Limiting ◽  
Covalent Interactions

<div><div><div><p>Anti-Markovnikov additions to alkenes have been a longstanding goal of catalysis, and anti-Markovnikov addition of arenes to alkenes would produce alkylarenes that are distinct from those formed by acid-catalyzed processes. Existing hydroarylations are either directed or occur with low reactivity and low regioselectivities for the linear alkylarene. Herein, we report the first undirected hydroarylation of unactivated alkenes with unactivated arenes that occurs with high regioselectivity for the anti-Markovnikov product. The reaction occurs with a Ni catalyst ligated by a highly sterically hindered N-heterocyclic carbene (NHC, L4 or L5). Catalytically relevant arene- and alkene-bound Ni complexes have been characterized, and the rate-limiting step was shown to be reductive elimination to form the C-C bond. DFT calculations, combined with energy decomposition analysis (EDA), suggest that the difference in activity between catalysts containing large and small carbenes results more from stabilizing intramolecular, non-covalent interactions in the secondary coordination sphere than from steric hindrance.</p></div></div></div>

Conjugation is rate limiting in hepatic transport of ursodeoxycholate in the rat

1982 ◽  
Vol 243 (3) ◽  
pp. G208-G213 ◽  
Author(s):  
I. Zouboulis-Vafiadis ◽  
M. Dumont ◽  
S. Erlinger
Keyword(s):  
Bile Acid ◽  
Bile Acids ◽  
Liver Toxicity ◽  
Hepatic Transport ◽  
Secretory Rate ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
The Difference ◽  
Conjugation Process ◽  
Rate Limiting

It has been reported that biliary secretion is the limiting step in the hepatic transport of bile acids by the hepatocyte from plasma to canalicular bile. The aim of the present study was to examine the role of conjugation in the transport process using ursodeoxycholate (UDCA) and tauroursodeoxycholate (TUDCA), two bile acid with low liver toxicity. Rats were given constant intravenous infusions of cholate (C), taurocholate (TC), UDCA, or TUDCA at progressively increasing rates. The biliary maximum secretory rate (SRm), in nmol . min-1 . 100 g body wt-1, for TC (1,835.2 +/- 135.5, mean +/- SE) was not significantly different from that of C (1,749.4 +/- 85.6). In contrast, the SRm for TUDCA (5,909.4 +/- 304.4) was approximately sevenfold that of UDCA (802.1 +/- 134.2), the difference being statistically significant (P less than 0.001). The SRm of UDCA in the presence of a taurine infusion (1,367 +/- 84.4) was higher than that of UDCA infused alone but still much lower than that of TUDCA. Phenobarbital sodium pretreatment did not increase SRm of UDCA alone or in the presence of a taurine infusion. These results suggest that in the rat 1) conjugation is the rate-limiting step in the overall transport of UDCA (and perhaps other bile acids) by the liver, and 2) the conjugation process itself is limiting, rather than the availability of taurine. They support the view that, although not mandatory for secretion into bile, conjugation of bile acids confers a biological advantage, possibly by increasing the solubility of the bile acid.

scholarly journals Rate-limiting steps in the tension development of freeze-glycerinated vascular smooth muscle.

1982 ◽  
Vol 79 (3) ◽  
pp. 437-452 ◽  
Author(s):  
J W Peterson
Keyword(s):  
Smooth Muscle ◽  
Protein Content ◽  
Time Course ◽  
Isometric Tension ◽  
Native Protein ◽  
Arterial Smooth Muscle ◽  
Tension Development ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting

A method for "skinning" arterial smooth muscle is presented which yields isometric tension development typically 60-80% of maximum physiological tension in the presence of micromolar Ca++ and millimolar Mg-ATP, while retaining essentially the native protein content. Using the methods of "CA jump," the time-course of Ca++-activated tension development in the skinned artery can be made identical to, but not faster than, the rate of tension development in the intact artery. In the skinned artery, activating free [Ca++] does not substantially alter the rate at which tension development approaches the final steady tension attained at that free [Ca++] (less than 25% decline in speed for a 10-fold decrease in [Ca++]). These observations are taken to mean that the rate-limiting step in isometric tension development in arterial smooth muscle does not depend directly on Ca++.

scholarly journals Kinetic study of methanol oxidation on Pt2Ru3/C catalyst in the alkaline media

2007 ◽  
Vol 72 (11) ◽  
pp. 1095-1101 ◽  
Author(s):  
A.V. Tripkovic ◽  
K.Dj. Popovic ◽  
J.D. Lovic
Keyword(s):  
Solid Solution ◽  
Alkaline Media ◽  
Xrd Pattern ◽  
Bifunctional Mechanism ◽  
Oxidation Of Methanol ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Naoh Solution ◽  
The Difference ◽  
Rate Limiting

The electrochemical oxidation of methanol in NaOH solution was examined on a thin film Pt2Ru3/C electrode. The XRD pattern revealed that the Pt2Ru3 alloy consisted of a solid solution of Ru in Pt and a small amount of Ru or a solid solution of Pt in Ru. It was shown that in alkaline solution, the difference in activity between Pt/C and Pt2Ru3/C is significantly smaller than in acid solution. It is proposed that the reaction follows a quasi bifunctional mechanism. The kinetic parameters indicated that the chemical reaction between adsorbed COad and OHad species could be the rate limiting step.

Nickel-Catalyzed Anti-Markovnikov Hydroarylation of Unactivated Alkenes with Unactivated Arenes Facilitated by non-Covalent Interactions

2019 ◽  
Author(s):  
Noam Saper ◽  
Akito Ohgi ◽  
David Small ◽  
Kazuhiko Semba ◽  
Yoshiaki Nakao ◽  
...  
Keyword(s):  
Decomposition Analysis ◽  
Energy Decomposition Analysis ◽  
Ni Catalyst ◽  
Rate Limiting Step ◽  
Secondary Coordination Sphere ◽  
Non Covalent Interactions ◽  
The Difference ◽  
Markovnikov Addition ◽  
Rate Limiting ◽  
Covalent Interactions

<div><div><div><p>Anti-Markovnikov additions to alkenes have been a longstanding goal of catalysis, and anti-Markovnikov addition of arenes to alkenes would produce alkylarenes that are distinct from those formed by acid-catalyzed processes. Existing hydroarylations are either directed or occur with low reactivity and low regioselectivities for the linear alkylarene. Herein, we report the first undirected hydroarylation of unactivated alkenes with unactivated arenes that occurs with high regioselectivity for the anti-Markovnikov product. The reaction occurs with a Ni catalyst ligated by a highly sterically hindered N-heterocyclic carbene (NHC, L4 or L5). Catalytically relevant arene- and alkene-bound Ni complexes have been characterized, and the rate-limiting step was shown to be reductive elimination to form the C-C bond. DFT calculations, combined with energy decomposition analysis (EDA), suggest that the difference in activity between catalysts containing large and small carbenes results more from stabilizing intramolecular, non-covalent interactions in the secondary coordination sphere than from steric hindrance.</p></div></div></div>

Sign in / Sign up

Export Citation Format

Share Document