scholarly journals The kinetic properties and reaction mechanism of histamine methyltransferase from human skin

1980 ◽  
Vol 191 (3) ◽  
pp. 899-899
Keyword(s):  
Reaction Mechanism ◽  
Human Skin ◽  
Kinetic Properties ◽  
Histamine Methyltransferase

scholarly journals The kinetic properties and reaction mechanism of histamine methyltransferase from human skin

1980 ◽  
Vol 187 (3) ◽  
pp. 819-828 ◽  
Author(s):  
D M Francis ◽  
M F Thompson ◽  
M W Greaves
Keyword(s):  
Reaction Mechanism ◽  
Human Skin ◽  
Substrate Inhibition ◽  
Kinetic Properties ◽  
Sequential Reaction ◽  
Methyl Group ◽  
Michaelis Constants ◽  
Histamine Methyltransferase ◽  
The Mean ◽  
Inhibited Enzyme

The substrate kinetic properties of histamine methyltransferase from human skin were studied at limiting concentrations of both histamine and S-adenosylmethionine. Substrate inhibition by histamine was observed at concentrations above 10 microM. Primary plots showed evidence of a sequential reaction mechanism. The Michaelis constants were derived from secondary plots of slopes from the primary plots ([S]/v versus [S]) versus reciprocal of the second substrate concentration. The mean Km values for histamine and S-adenosylmethionine were 4.2 and 1.8 microM respectively. Histamine in concentrations of 25-100 microM inhibited enzyme activity uncompetitively with respect to S-adenosylmethionine. No substrate inhibition was observed with S-adenosylmethionine. To elucidate the reaction mechanism further, inhibition by the two products, S-adenosylhomocysteine and 1-methylhistamine, was studied. S-Adenosylhomocysteine inhibited non-competitively with respect to histamine and competitively with respect to S-adenosylmethionine. 1-Methylhistamine inhibited non-competitively with respect to histamine and to S-adenosylmethionine. These results are interpreted as providing evidence for an ordered sequential Bi Bi reaction mechanism, with the methyl-group donor S-adenosylmethionine as the first substrate that adds to the enzyme and histamine as the second substrate. 1-Methylhistamine is the first product to leave the enzyme and S-adenosylhomocysteine is the second. The results are discussed in terms of the possible role that this enzyme could play in the modulation of histamine-mediated reactions in skin.

scholarly journals Pyrolysis Kinetic Properties of Thermal Insulation Waste Extruded Polystyrene by Multiple Thermal Analysis Methods

Materials ◽  
2020 ◽  
Vol 13 (24) ◽  
pp. 5595
Author(s):  
Ang Li ◽  
Wenlong Zhang ◽  
Juan Zhang ◽  
Yanming Ding ◽  
Ru Zhou
Keyword(s):  
Reaction Mechanism ◽  
Thermal Insulation ◽  
Conversion Rate ◽  
Model Fitting ◽  
Pso Algorithm ◽  
Energy Utilization ◽  
Kinetic Properties ◽  
Insulation Material ◽  
Heating Rates ◽  
Model Free

Extruded polystyrene (XPS) is a thermal insulation material extensively applied in building systems. It has attracted much attention because of outstanding thermal insulation performance, obvious flammability shortcoming and potential energy utilization. To establish the reaction mechanism of XPS’s pyrolysis, thermogravimetric experiments were performed at different heating rates in nitrogen, and multiple methods were employed to analyze the major kinetics of pyrolysis. More accurate kinetic parameters of XPS were estimated by four common model-free methods. Then, three model-fitting methods (including the Coats-Redfern, the iterative procedure and masterplots method) were used to establish the kinetic model. Since the kinetic models established by the above three model-fitting methods were not completely consistent based on different approximations, considering the effect of different approximates on the model, the reaction mechanism was further established by comparing the conversion rate based on the model-fitting methods corresponding to the possible reaction mechanisms. Finally, the accuracy of the above model-fitting methods and Particle Swarm Optimization (PSO) algorithm were compared. Results showed that the reaction function g(α) = (1 − α)−1 − 1 might be the most suitable to characterize the pyrolysis of XPS. The conversion rate calculated by masterplots and PSO methods could provide the best agreement with the experimental data.

Human skin androgen metabolism and preliminary evidence for its control by two forms of 17β-hydroxysteroid oxidoreductase

1982 ◽  
Vol 93 (3) ◽  
pp. 403-413 ◽  
Author(s):  
M. B. Hodgins ◽  
J. B. Hay ◽  
J. B. Donnelly
Keyword(s):  
Human Skin ◽  
Preliminary Evidence ◽  
The Other ◽  
Kinetic Properties ◽  
Oxidoreductase Activity ◽  
Androgen Metabolism ◽  
Steroid Substrate ◽  
Number Of Patients ◽  
Membrane Bound

Human forehead skin incubated in vitro is known to metabolize testosterone to 17-oxosteroids faster than the reverse reaction, while axillary skin rapidly metabolizes androstenedione to 17β-hydroxysteroids, such as testosterone and 5α-dihydrotestosterone, While this has been confirmed using a larger number of patients, some indication has been found that 17β-hydroxysteroid oxidoreductase activity declines with age in the axilla. The relative rates of 17β-oxidation and reduction (direction of operation of skin 17β-hydroxysteroid oxidoreductase activity) were not altered by a variety of incubation conditions. Large amounts of a membrane-bound 17β-hydroxysteroid oxidoreductase, showing preference for NAD as coenzyme and testosterone (rather than androstenedione) as steroid substrate, were found in forehead skin from one patient. On the other hand, the main axillary skin enzyme in skin from another patient was soluble and showed preference for NADP and androstenedione. It is postulated that 17β-oxidation and reduction in skin is controlled by the relative amount, the coenzyme preferences and the kinetic properties of these two enzymes.

scholarly journals Gating reaction mechanism of neuronal NMDA receptors

2012 ◽  
Vol 108 (11) ◽  
pp. 3105-3115 ◽  
Author(s):  
William F. Borschel ◽  
Jason M. Myers ◽  
Eileen M. Kasperek ◽  
Thomas P. Smith ◽  
Nicholas M. Graziane ◽  
...  
Keyword(s):  
Reaction Mechanism ◽  
Single Channel ◽  
Receptor Expression ◽  
Activation Mechanism ◽  
Neuronal Cultures ◽  
Kinetic Properties ◽  
Recombinant Receptors ◽  
Sufficient Detail ◽  
Activation Mechanisms

The activation mechanisms of recombinant N-methyl-d-aspartate receptors (NRs) have been established in sufficient detail to account for their single channel and macroscopic responses; however, the reaction mechanism of native NRs remains uncertain due to indetermination of the isoforms expressed and possible neuron-specific factors. To delineate the activation mechanism of native NRs, we examined the kinetic properties of currents generated by individual channels located at the soma of cultured rat neurons. Cells were dissociated from the embryonic cerebral cortex or hippocampus, and on-cell single channel recordings were done between 4 and 50 days in vitro (DIV). We observed two types of kinetics that correlated with the age of the culture. When we segregated recordings by culture age, we found that receptors recorded from early (4–33 DIV) and late (25–50 DIV) cultures had smaller unitary conductances but had kinetic profiles that matched closely those of recombinant 2B- or 2A-containing receptors, respectively. In addition, we examined the effects of cotransfection with postsynaptic density protein 95 or neuropilin tolloid-like protein 1 on recombinant receptors expressed in human embryonic kidney-293 cells. Our results add support to the view that neuronal cultures recapitulate the developmental patterns of receptor expression observed in the intact animal and demonstrate that the activation mechanism of somatic neuronal NRs is similar to that described for recombinant receptors of defined subunit composition.

scholarly journals Cerebral-cortex hexokinases. Elucidation of reaction mechanisms by substrate and dead-end inhibitor kinetic analysis

1971 ◽  
Vol 123 (5) ◽  
pp. 707-715 ◽  
Author(s):  
H. S. Bachelard ◽  
A. G. Clark ◽  
M. F. Thompson
Keyword(s):  
Reaction Mechanism ◽  
Enzymic Activity ◽  
Significant Inhibition ◽  
Kinetic Constants ◽  
Mitochondrial Activity ◽  
Kinetic Properties ◽  
Extraction And Purification ◽  
Dead End ◽  
Ping Pong ◽  
Atp Analogues

1. The substrate kinetic properties of cerebral hexokinases (mitochondrial and cytoplasmic) were studied at limiting concentrations of both glucose and MgATP2−. Primary plots of the enzymic activity gave no evidence of a Ping Pong mechanism in three types of mitochondrial preparation tested (intact and osmotically disrupted mitochondria, and the purified mitochondrial enzyme), nor in the purified cytoplasmic preparation. 2. Secondary plots of intercepts from the primary plots (1/v versus 1/s) versus reciprocal of second substrate of the mitochondrial activity gave kinetic constants which differed from those obtained directly from the plots of 1/v versus 1/s or of s/v versus s, although the ratios of the derived constants were consistent. The kinetic constants obtained with the cytoplasmic enzyme from primary and secondary plots were consistent. 3. Deoxyglucose, as alternative substrate, inhibited cytoplasmic hexokinase by competition with glucose, but did not compete when MgATP2− was the substrate varied. The Ki for deoxyglucose when glucose concentrations were varied was 0.25mm. 4. A range of ATP analogues was tested as potential substrates and inhibitors of hexokinase activity. GTP, ITP, CTP, UTP and βγ-methylene-ATP did not act as substrates, nor did they cause significant inhibition. Deoxy-ATP proved to be almost as effective a substrate as ATP. AMP inhibited but did not act as substrate. 5. N-Acetyl-glucosamine inhibited all preparations competitively when glucose was varied and non-competitively when MgATP2− was varied. AMP inhibition was competitive when MgATP2− was the substrate varied and non-competitive when glucose was varied. 6. The results are interpreted as providing evidence for a random reaction mechanism in all preparations of brain hexokinase, cytoplasmic and mitochondrial. The kinetic properties and reaction mechanism do not change on extraction and purification of the particulate enzyme. 7. The results are discussed in terms of the participation of hexokinase in regulation of cerebral glycolysis.

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