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Biomedical subjects

A G Clark

Publications and source records attributed to A G Clark.

At least 217 records · Page 12Linked to original sources
PubMed1982

Production of the Kanagawa hemolysin by Vibrio parahaemolyticus in a synthetic medium.

A synthetic medium capable of supporting growth of Vibrio parahaemolyticus is described. Growth yields and generation times were comparable to growth in a complex medium, although Kanagawa hemolysin was undetectable in the synthetic medium. Upon the addition of single amino acids to this synthetic medium, only D-tryptophan induced production of the hemolysin. L-Tryptophan was found to inhibit the action of the D-stereoisomer. The response to D-tryptophan was pH dependent; the greatest hemolysin expression occurred at pH values below 6.5. The addition of 100 micrometers D-tryptophan to early-logarithmic-phase cultures caused an inhibition of growth and of substrate utilization, both of which lasted 7 h. During this time, hemolysin was produced only intracellularly. The hemolysin appeared in the supernatant only when growth recommenced. The hemolysins within the cell and in the supernatant were both inactivated by antiserum raised against the standard Kanagawa hemolysin.

Amino Acids↗
PubMed1981

The estimation of epistasis in components of fitness in experimental populations of Drosophila melanogaster I. A two-stage maximum likelihood model.

Laboratory populations of Drosophila melanogaster bearing the Curly and Plum marked second chromosome inversions were observed in selection experiments for ten discrete generations. Maximum likelihood estimates of the relative fitnesses of Curly, Plum, Curly-Plum, and wild phenotypes were obtained from selection trajectories. Using these estimates, measures of multiplicative and additive epistasis were calculated. These were partitioned into pre-sampling and post-sampling components, and both were found to be significant. In several cases the sign of the epistasis of the two components was reversed, and the direction of net epistasis depended on the particular inversion. The significance of partitioning epistasis into components is discussed in the light of two locus population genetic theory.

Animals↗
PubMed1981

The estimation of epistasis in components of fitness in experimental populations of drosophila melanogaster II. Assessment of meiotic drive, viability, fecundity and sexual selection.

An analysis of fitness components was performed on laboratory populations of Drosophila melanogaster bearing Curly and Plum marked second chromosome inversions with two genetic backgrounds. Selective differences between Curly, Plum, Curly-Plum, and wild phenotypes were partitioned into meiotic drive, viability, fecundity and mating success. Although no significant meiotic drive was detected, there were significant differences between genotypes in the other three components. Interlocus interactions in fertility and sexual selection components were measured, and statistics to quantify epistasis at these levels of fitness were devised. Significant fitness interactions between genes on the Curly and Plum inversions in viability, fecundity, and sexual selection were found. These results were qualitatively consistent with the maximum likelihood fitness estimates found previously. The expected allele frequency trajectories computed from the estimated components adequately fitted the observed selection trajectories.

Animals↗
PubMed1981

Disequilibrium between linked inversions: an alternative hypothesis.

Data are presented which demonstrate a difference in the rate of recombination between cis and trans inversion double heterozygotes in Drosophila melanogaster. The corresponding two-locus theoretical model in which the double heterozygotes have different rates of recombination and no selection operates is analysed. We show that stable linkage disequilibrium can be maintained in this neutral situation. These results bear on the general observation in wild populations of several insect species that linked inversions manifest linkage disequilibrium.

Animals↗
PubMed1981

Effect of pH on the production of the Kanagawa hemolysin by Vibrio parahaemolyticus.

Production of the Kanagawa hemolysin by patient strains of Vibrio parahaemolyticus was found to respond to the pH rather than to the type of carbohydrate present in the growth medium. Regardless of the carbohydrate present, hemolysin production in peptone broth cultures occurred only when the pH was between 6.5 and 5.5. Mannitol, the sugar used in the Wagatsuma agar, lowered the pH to within this range, thus providing optimal conditions for hemolysin production. Glucose and mannose, although readily metabolized, lowered the pH below this range, inhibiting growth and hemolysin production. Alkaline cultures either without carbohydrates or containing non-metabolizable sugars showed little hemolytic activity because the pH always remained alkaline. In pH-stat cultures maintained at pH 6.2, higher hemolysin yields were produced irrespective of the presence or absence of mannitol. We conclude that the production of the Kanagawa hemolysin is under pH control. Marine strains of V. parahaemolyticus, which are Kanagawa negative, did not express detectable amounts of hemolysin under those conditions shown to stimulate hemolysin production by Kanagawa-positive strains.

Galactose↗
PubMed1980

Rate measurements in cases of substrate inhibition.

Analysis of a simple model for substrate inhibition of an enzyme shows that the methods of Lee and Wilson (Lee, H.-J. and Wilson, I.B. (1971) Biochim. Biophys. Acta 242, 519--522), and Yun and Suelter (Yun, S.-L. and Suelter, C.H. (1977) Biochim. Biophys. Acta 480, 1--13), may be used to derive initial rate data from the sigmoidal-reaction progess curves that may be generated in such cases and in a number of instances, may be used to derive the kinetic parameters defining the rate equation from single-reaction progress curves.

Enzyme Inhibitors↗
PubMed1975

Dependence upon bile volume of the biliary excretion of bromocresol green and amaranth in the anaesthetized rat.

The kinetics of the biliary excretion of both bromocresol green and amaranth are better described in terms of rate equations that are functions of the cumulative volume of bile excreted rather than of time. The rate of disappearance of bromocresol green from the liver also appears to depend on the volume of bile excreted rather than on time. It is proposed that bromocresol green, and probably also amaranth, rapidly equilibrates between the hepatic and biliary compartments as a result of reabsorption from the biliary tree and that the rate-limiting factor in the biliary excretion of these dyes is the removal of dye from the biliary tree by bulk flow. Six methods for the graphical presentation of excretion data are examined and their use in the characterization of the kinetics of an excretion system is discussed.

Amaranth Dye↗
PubMed1973

Cross specificity in some vertebrate and insect glutathione-transferases with methyl parathion (dimethyl p-nitrophenyl phosphorothionate), 1-chloro-2,4-dinitro-benzene and s-crotonyl-N-acetylcysteamine as substrates.

1. Enzymes catalysing the reaction between GSH and methylparathion (dimethyl p-nitrophenyl phosphorothionate), 1-chloro-2,4-dinitrobenzene and S-crotonyl-N-acetylcysteamine were separated by (NH(4))(2)SO(4) precipitation from homogenates of sheep, rat and mouse livers and from homogenates of cockroaches, houseflies and grass grubs. 2. Electrofocusing of the preparations from each of these species separated a number of zones, each of which catalysed the reaction of GSH with all three substrates. 3. Ion-exchange chromatography on CM-cellulose also separated a number of fractions in which activity towards the three substrates coincided. 4. In both separation methods patterns of the activities were consistent with the presence in all species of several GSH transferases each having a degree of cross specificity towards the three substrates.

Animals↗