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Biomedical subjects

D C Watts

Publications and source records attributed to D C Watts.

At least 163 records · Page 9Linked to original sources
PubMed1980

Capillary rheology of two composite resin systems.

The rheological properties of two commercial resin systems, Isopast and Cosmic, were studied in the shear rate range 0.65-65 s-1 by capillary techniques. Isopast represents a low inorganic filled composite, whereas Cosmic is a conventionally filled system whose resin matrix resembles that of Isopast. All components of both systems obeyed power law dependence of shear stress as a function of shear rate according to the relation 1n tau = 1n K + n 1n gamma; values of n for Cosmic catalyst and paste and Isopast paste were of the order of 0.6 whereas Isopast catalyst was of the order of 0.3. Setting systems obeyed a power law relationship during the working time with an exponent n congruent to 0.7-1.0. Accordingly the systems appear pseudoplastic in component form and slightly pseudoplastic to near Newtonian after mixing.

Chemical Phenomena↗
PubMed1979

13C NMR spectroscopic analysis of poly(electrolyte) cement liquids.

13C NMR spectroscopy has been applied to the analysis of carboxylic poly-acid cement liquids. Monomer incorporation, composition ratio, sequence statistics, and stereochemical configuration have been considered theoretically, and determined experimentally, from the spectra. Conventionally polymerized poly(acrylic acid) has an approximately random configuration, but other varieties may be synthesized. Two commercial glass-ionomer cement liquids both contain tartaric acid as a chelating additive but the composition of their poly-acids are different. Itaconic acid units, distributed randomly, constitute 21% of the repeating units in one of these polyelectrolytes.

Acrylic Resins↗
PubMed1976

Effects of arginine and some analogues of the partial adenosine triphosphate-adenosine diphosphate exchange reaction catalysed by arginine kinase. Evolutionary divergence in the mechanism of action of a monomer and a dimer arginine kinase.

1. Both the monomer arginine kinase from lobster muscle and the dimer arginine kinase from Holothuria forskali catalyse the ATP-ADP partial exchange reaction at rates equal to 3 and 0.6% of the normal rate of transphosphorylation respectively. The Mg2+-nucleotide complex is the substrate for this as it is for the kinase reaction. 2. Analogues of arginine inhibit the exchange reaction of the lobster enzyme but enhance that of the Holothuria enzyme. 3. With the lobster enzyme NO3- has no effect on the exchange reaction alone and inhibit only slightly the apparent enhancement of the exchange reaction produced by the addition of arginine. This is compatible with previous findings for this enzyme that formation of the anion-stabilized dead-end complex, enzyme-arginine-MgADP-NO3-, does not occur to any marked degree. 4. About 80% of the ADP-ATP exchange reaction of the lobster enzyme remains after inhibition with iodoacetamide. This is further decreased to 65% by the addition of L-arginine, indicating that this substrate does bind to the thiolmodified enzyme. 5. It is concluded that the partial exchange reaction is a genuine phenomenon not mediated by trace amounts of arginine. From the effects of arginine and related compounds it would appear that during the normal kinase reaction the partial ATP-ADP exchange reaction is suppressed in the lobster enzyme but enhanced in the Holothuria enzyme. This reflects a remarkable evolutionary divergence of two homologous enzymes.

Adenosine Diphosphate↗
PubMed1975

Kinetic studies and effects of anions on creatine phosphokinase from skeletal muscle of rhesus monkey (Macaca mulatta).

A purification procedure for creatine kinase (EC 2.7.3.2) from muscle of the monke35--170 muequiv H+/mg protein per min at 30 degrees C and a yield of approx. 0.5 g/kg muscle. Assuming equilibrium kinetics, synergistic binding of substrates at one catalytic site is found for both the forward and back reactions. Kinetic constants for the binding of each substrate to the free enzyme and the enzyme-second substrate complex are determined and are compared with those for the enzyme from other species. Inhibition by small anions is determined in the presence of different combinations of substrates and products. SO4(2-) inhibits by simple competitive inhibition and probably binds at the site of the transferrable phosphoryl group. Inhibition by NO3-, NO2-, SCN- and Cl- is more complex and these ions are suggested to mimic the transferrable phosphoryl group in a planar transition-state complex. These anions stabilize the dead-end complex, enzyme-creatine-MgADP, which lacks the transferable phosphoryl group. The effects of these anions on the dissociation constants of the enzyme-substrate complexes is reported and is in accord with the above hypothesis. The dead-end complex in the absence of anion does not protect the essential thiol group against inhibition by iodoacetamide. Addition of NO3- or Cl- to the dead-end complex or a substrate equilibrium mixture without anion confers protection. The essential thiol group is inhibited by iodoacetamide at a rate which is essentially independent of pH over the normal stability range of the enzyme. Contrary to our previous report this pH independence is not altered by the presence of dead-end complex, creatine plus MgADP, in the presence or absence of anion or in the presence of a substrate equilibrium mixture. It is inferred that the 'essential' thiol group of the monkey enzyme has essentially the same properties as that of the rabbit enzyme. In consequence, the inferences made about the role of this group based on our previous work on the monkey enzyme are no longer valid. The present findings are compatible with the essential thiol group playing a conformational role in the catalytic process.

Acetates↗
PubMed1975

Evolutionary variation between a monomer and a dimer arginine kinase. Purification of the enzyme from Holothuria forskali and a comparison of some properties with that from Homarus vulgaris.

1. A purification procedure for the dimeric arginine kinase of the sea cucumber Holothuria forskali is described. 2. The enzyme has a mean molecular weight of 77250 and is composed of two equal, dissociable subunits. 3. It also shows co-operativity between substrate binding at one catalytic site to a much greater extent than the nomomeric lobster arginine kinase for which such co-operativity could not be detected unambiguously. The constants for substrate binding are reported assuming that the enzyme follows rapid-equilibrium random kinetics. From a comparison with other species, the development of co-operativity between the nucleotide- and guanidine-binding sites on one subunit is suggested to have occurred more than once in the evolution of the phosphagen kinases and is not dependent on subunit aggregation. 4. Both enzymes show similar pH profiles for thermal inactivation at 22 degrees C and have very similar stabilities. Above 40 degrees C the dimeric enzyme is much more stable than the monomer. Rate constants for heat inactivation and Arrhenius activation energies are reported. 5. The dimeric enzyme is also more stable to urea inactivation. Substrates and argininic acid all improve the stability of both enzymes. The effects of individual substrates are more distincitive with the dimeric enzymes and increase its stability to an extent that makes it about as stable as dogfish creatine kinase. In the physiological range dimerization does not seem to confer any particular advantage with respect to stability over the monomer form.

Animals↗
PubMed1975

Effects of anions on a monomeric and a dimeric arginine kinase.

1. Some effects of anions on the rates of phosphoarginine synthesis by monomeric (lobster) and by dimeric (Holothuria forskali) arginine kinases are reported. 2. As with creatine kinase, acetate ions activate both enzymes: Cl- was also found to activate both although this was an inhibitor of creatine kinase. 3. NO3- inhibits the lobster enzyme. Inhibition is of the mixed type with respect to MgATP. Ki greater than Ki' and Ks greater than Ks' indicating that the presence of NO3- promotes the binding of substrate and vice versa. 4. NO3- alone has no effect on the difference spectrum of the lobster enzyme but in the presence of arginine, MgATP, MgADP, MgAMP or MgIDP the difference spectrum is greatly enhanced. A profound effect on the ionization state of tyrosine residues is inferred. 5. With the Holothuria enzyme low concentrations of NO3- activate in a manner that is competitive with arginine. Higher concentrations cause inhibition of the mixed type with respect to arginine in a similar manner to that found with MgATP for the lobster kinase. 6. Of a range of anions tested only NO3- and NO2- enhanced the inhibition of enzyme activity by MgADP, indicating the formation of a pseudo-transition-state dead-end complex, enzyme-arginine-NO3--MgADP. The effect was essentially independent of temperature with the Holothuria enzyme, but with the lobster enzyme was much less marked and temperature dependent. The difference may reflect the different stabilities of the monomer and dimer enzymes, although with neither arginine kinase is the stabilization of the dead-end complex as marked as is found with creatinine kinase.

Adenosine Diphosphate↗
PubMed1973

The modification of cholinesterase activity by 5,5'-dithiobis-(2-nitrobenzoic acid) included in the coupled spectrophotometric assay. Evidence for a non-catalytic substrate-binding site.

1. Compared with the acetylcholinesterase assay carried out in the absence of a dithiol, the presence of 5,5'-dithiobis-(2-nitrobenzoic acid) caused marked activation, 6,6'-dithiodinicotinic acid and 2,2'-dithiobis-(5-nitropyridine) less so and 2,2'-dithiodipyridine (aldrithiol-2) had no effect at all. Measurements are further complicated in that the 5-thio-2-nitrobenzoate ion also appears to interact with the enzyme, resulting in slightly lowered absorbance values. 2. Acetylthiocholine competes for the 5,5'-dithiobis-(2-nitrobenzoic acid)-binding site so that activation is essentially eliminated by saturating concentrations of substrate. The presence of the dithiol decreases the K(m) value of acetylthiocholine. 3. Similar results were obtained with pseudocholinesterase. However, with butyrylthiocholine clear activation was still observed under V(max.) conditions in addition to K(m) being lowered. 4. All the data yielded Hill coefficients of 1 and analysis of the results leads to the conclusion that activation results from the dithiol being bound to a site on the subunit that is actively catalysing ester hydrolysis. 5. The use of aldrithiol-2 is recommended for kinetic work where absolute quantitative measurements are required.

Benzoates↗