The application of flow microcalorimetry to the study of enzyme kinetics

The use of flow microcalorimetry to measure velocities for enzyme-catalyzed reactions is described. Studies are presented involving the enzymes chymotrypsin, trypsin, and ribonuclease A. In determining the Michaelis-Menten parameters, V_m and K_m, for these systems complications arise due to substrate depletion and product inhibition. By employing an integrated Michaelis-Menten equation, V_m and K_m can be determined.     

Modeling the kinetics of enzymic reactions in mainly solid reaction mixtures

There is currently considerable interest in using mainly solid reaction mixtures for enzymic catalysis. In these reactions starting materials dissolve into, and product materials crystalize out of, a small amount of liquid phase in which the catalytic reaction occurs. An initial mathematical model for mass transfer effects in such systems is constructed using some physically reasonable approximations. The model equations are solved numerically to determine how the reactant concentrations vary with time and position. To evaluate the extent to which mass transfer limits the overall rate of product formation, an effectiveness factor is defined as the ratio of the observed total reaction rate to the total reaction rate in the reaction limited limit. As expected, the value of the effectiveness factor in steady state is strongly dependent on the Thiele modulus. However, it is also observed that the effectiveness factor can vary widely as a result of changes in the other dimensionless groups characterizing the system. For example, there are situations with Thiele modulus equal to unity in which the value of the effectiveness factor varies between approximately 0.1 and 0.8 as the other parameters are varied in physically reasonable ranges. Analytical asymptotic solutions that provide good approximations to the numerically calculated results in various physically important limiting cases are also presented.     

Enzyme reaction rates and the stochastic theory of kinetics

The kinetics of an elementary reaction step are discussed from the viewpoint of the stochastic theory of chemical kinetics. The general form of the rate constant found in the stochastic approach is described, and compared with the expression from transition state theory. Whereas the stochastic theory predicts a rate enhancement in cases which are not adiabatic (in the quantum mechanical sense), transition state theory,which is essentially an adiabatic theory of reaction rates, does not permit inclusion of the effect. This effect can be expected to be of greater importance in cases of catalysis by structures, such as enzymes, containing large numbers of vibrational degrees of freedom (particularly low frequency ones) than in cases lacking such structures. The stochastic theory is more general than the transition state theory, the rate constant expression given by the latter being obtainable from the former when restrictive assumptions, including that of adiabaticity, are made. Interpretations of enzyme catalysis based on the transition state theory must thus be viewed as speculative.     

Role of microorganisms in the evolution of animals and plants: the hologenome theory of evolution

We present here the hologenome theory of evolution, which considers the holobiont (the animal or plant with all of its associated microorganisms) as a unit of selection in evolution. The hologenome is defined as the sum of the genetic information of the host and its microbiota. The theory is based on four generalizations: (1) All animals and plants establish symbiotic relationships with microorganisms. (2) Symbiotic microorganisms are transmitted between generations. (3) The association between host and symbionts affects the fitness of the holobiont within its environment. (4) Variation in the hologenome can be brought about by changes in either the host or the microbiota genomes; under environmental stress, the symbiotic microbial community can change rapidly. These points taken together suggest that the genetic wealth of diverse microbial symbionts can play an important role both in adaptation and in evolution of higher organisms. During periods of rapid changes in the environment, the diverse microbial symbiont community can aid the holobiont in surviving, multiplying and buying the time necessary for the host genome to evolve. The distinguishing feature of the hologenome theory is that it considers all of the diverse microbiota associated with the animal or the plant as part of the evolving holobiont. Thus, the hologenome theory fits within the framework of the 'superorganism' proposed by Wilson and Sober.     

The kinetics of an enzyme catalyzed reaction in the presence of an unstable,irreversible modifier

• 1.1. The chemical modifications of enzymes is a widely used technique to gain information about the number, type and location of aminoacid residues which are essential for activity. A number of these modifiers are unstable in aqueous solutions and act irreversibly.
• 2.2. The kinetics of such systems have previously been studied under assumptions, some of which are either unnecessary or too restrictive.
• 3.3. We replace these assumptions by others which are more realistic and less stringent.
• 4.4. These assumptions allow us to derive analytical expressions for the evolution of all the species involved in the reaction.
• 5.5. From the analytical expression for the time course of the product formation an experimental design and a kinetic data analysis allowing the easy characterization of these systems are suggested.

     

Kinetics of light emission by photosynthetic systems: Second order light decay kinetics means elovich kinetics in a solid state reaction; 1.5 order light decay kinetics means second order reaction kinetics

Kinetics of biological light emission processes do not mean what they seem to mean, because measured light intensity is not proportional to reactant concentration but to reaction rate. Therefore, the differential equation for light decay is usually different from that of concentration decay, so that mass action interpretations cannot be applied directly to light intensity decay. An observed second order light decay for Chlorella at 6.5°C, implies Elovich solid state reaction kinetics, which agrees with other evidence for solid state processes in photosynthesis. An observed 1.5 order light decay for Cholorella at 28°C implies second order liquid or solid state reaction kinetics. First ordere light decay implies first order reaction kinetics.     

Role of optimal control theory in cancer chemotherapy

This paper presents a review of the ways in which optimal control theory interacts with cancer chemotherapy. There are three broad areas of investigation. One involves miscellaneous growth kinetic models, the second involves cell cycle models, and the third is a classification of "other models." Both normal and tumor cell populations are included in a number of the models. The concepts of deterministic optimal control theory are applied to each model in such a way as to present a cohesive picture. There are applications to both experimental and clinical tumors. Suggestions for designing better chemotherapy strategies are presented.     

Long-bone geometry in columnar-limbed animals: allometry of the proboscidean appendicular skeleton

Limb-bone allometry was investigated for 19 species of proboscideans, spanning almost the entire phylogenetic spectrum. More archaic proboscideans ('gompthotheres') have substantially thicker long-bone diaphyses relative to length than elephantids, as has been suggested previously, but contrary to previous suggestions it could not be confirmed that Mammuthus had more massive long-bone diaphyses on average than extant Elephas and Loxodonta . When correcting for phylogeny, the circumference of the limb bones to their length in proboscideans as a group generally scale with negative allometry, becoming stouter with increased length, as would be expected from limb mechanics. Few slopes were, however, statistically significantly negatively allometric. Correcting for phylogeny produced better correlations than traditional regression analyses, in contrast to most other studies where the reverse is the case. Intraspecific analyses of extant Elephas and Loxodonta , in addition to Mammuthus primigenius , Mammut americanum, and Gomphotherium productum , also resulted in negatively allometric regression slopes, frequently conforming to the theory of elastic similarity, as could be expected from the columnar posture of proboscideans. At present the reasons for the more massive limbs of gomphotheres s.l. are not fully understood. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 149 , 423–436.     

Some contributions of signal detection theory to the analysis of stimulus control in animals

This paper reviews some applications of Signal Detection Theory (SDT) to the quantitative analysis of non-human animal discrimination. The basic detection model is briefly outlined and the separation of sensitivity and bias is illustrated. Several other applications and ideas are reviewed, including the rating method, some implications of signal and criterion variance, and the measurement of 'guesses' not encompassed by standard SDT. The conceptual framework and analytic tools of SDT help to clarify processes underlying stimulus control and provide direction to more complete process models.     

Role of leptin in the control of postprandial pancreatic enzyme secretion.

Leptin released by adipocytes has been implicated in the control of food intake but recent detection of specific leptin receptors in the pancreas suggests that this peptide may also play some role in the modulation of pancreatic function. This study was undertaken to examine the effect of exogenous leptin on pancreatic enzyme secretion in vitro using isolated pancreatic acini, or in vivo in conscious rats with chronic pancreatic fistulae. Leptin plasma level was measured by radioimmunoassay following leptin administration to the animals. Intraperitoneal (i.p.) administration of leptin (0.1, 1, 5, 10, 20 or 50 microg/kg), failed to affect significantly basal secretion of pancreatic protein, but markedly reduced that stimulated by feeding. The strongest inhibition has been observed at dose of 10 microg/kg of leptin. Under basal conditions plasma leptin level averaged about 0.15 +/- 0.04 ng/ml and was increased by feeding up to 1.8 +/- 0.4 ng/ml. Administration of leptin dose-dependently augmented this plasma leptin level, reaching about 0.65 +/- 0.04 ng/ml at dose of 10 microg/kg of leptin. This dose of leptin completely abolished increase of pancreatic protein output produced by ordinary feeding, sham feeding or by diversion of pancreatic juice to the exterior. Leptin (10(-10)-10(-7) M) also dose-dependently attenuated caerulein-induced amylase release from isolated pancreatic acini, whereas basal enzyme secretion was unaffected. We conclude that leptin could take a part in the inhibition of postprandial pancreatic secretion and this effect could be related, at least in part, to the direct action of this peptide on pancreatic acini.     

Transient phase kinetics of enzyme reactions where more than one species of enzyme is present at the start of the reaction

This paper presents an extension of the analysis given by Darvey (1968) of the transient phase kinetics of a general enzyme mechanism to situations where more than one species of enzyme are present at the start of the reaction. Solutions are presented for the time course of the changes in concentration of all the enzyme and ligand (substrate, product, activator or inhibitor) species participating in the reaction during the transient phase. The conclusions which were reached for the restrictive initial condition (only one species of enzyme is present at the start of the reaction) are shown to also hold for the more general initial condition considered in this paper.     

动物长期能量收支理论及研究进展

阐述了持续代谢率和持续代谢范围的概念,并介绍有关制约因子的４种假说：食物可利用性假说、外周制约假说、中心制约假说以及同形态椹制约假说。此外,还就持续代谢率的进化原因、实验验证的方法和结论,以及与最小维持消耗（ＢＭＲ）的关系和机制进行了分析。持续代谢率对动物生活具有影响,具有生态学意义。最后,分析了本领域未来的４个发展方向。