Graphical determination of mean activation energy and standard deviation in a microheterogeneity model of enzyme deactivation

Traditionally, enzyme populations have been treated as if they were either homogenous, or heterogeneous with distinct and separable subpopulations. The microheterogeneity model, however, assumes that there is a continuous distribution of properties in the population. In the area of enzyme deactivation kinetics, this model describes the heterogeneous population as having a continuous distribution of activation energy of deactivation. This distribution is characterized by mean activation energy, and a standard deviation of activation energy. The microheterogeneity model contains two parameters, (0) and sigma. Parameter (0) is the mean value of for a heterogeneous enzyme population; is the activation energy divided by absolute temperature and the ideal gas constant. Parameter sigma is the standard deviation of the Gaussian distribution of values in the population. If the population is homogeneous, then = (0) for all enzyme molecules and sigma = 0. There are certain ratios which are independent of (0) and dependent upon sigma. Two important ratios are t(1/4)/t(1/2) and t(1/2)/t(1/2) ('), where t(1/2) (') represents t(1/2) for a homogeneous enzyme population with the same mean ((0)), as the heterogeneous population. If there is experimental deactivation data for the heterogeneous population which is well behaved, the first ratio, t(1/4)/t(1/2), can be determined by estimating the time in minutes at which the enzyme has lost 25% of its activity (t(1/4)), and the time in minutes at which the enzyme has lost 50% of its activity (t(1/2)), and then taking the ratio t(1/4)/t(1/2). The corresponding value of sigma can be estimated from a graph. The ratio t(1/2)/t(1/2) (') can be found directly as a function of t(1/4)/t(1/2), and can be estimated from another graph. The value of (0) can then be calculated from the formulasgiven in the article.     

ALTERED ZYGOSPORE WALL ULTRASTRUCTURE CORRELATES WITH REDUCED ABIOTIC STRESS RESISTANCE IN A MUTANT STRAIN OF CHLAMYDOMONAS MONOICA (CHLOROPHYTA)1

The zygospore of Chlamydomonas is a diploid resting stage that provides protection from environmental extremes. The remarkable abiotic stress resistance of the zygospore can be explained, in part, by the presence of a massive wall that includes a sporopollenin‐containing surface layer ( Van Winkle‐Swift and Rickoll 1997 ). A Chlamydomonas monoica Strehlow zygospore‐specific mutant strain (D19) was obtained previously by screening for loss of chloroform resistance in zygospore populations derived from self‐mating of post‐mutagenesis clones. Exposure of D19 zygospores to solar UV radiation or germicidal radiation also resulted in a pronounced decrease in survival of D19 zygospores relative to wildtype zygospore survival. Similarly, resistance to NaCl‐induced osmotic shock was reduced in D19 zygospores, especially when exposed to very high (e.g., 20% w/v) salt concentrations. Mature zygospores of C. monoica exhibit a UV‐induced blue surface autofluorescence that may indicate the presence of phenolic wall components. The intensity of zygospore autofluorescence was significantly reduced in D19 zygospores. As revealed by TEM, the surface layer of mature homozygous D19 zygospores was disrupted, suggesting a defect in wall assembly. Zygospore‐specific chloroform sensitivity, UV sensitivity, and reduced autofluorescence cosegregated in tetrads derived from D19 heterozygotes (i.e., if a progeny clone from a cross involving D19 and a normal strain was found to be chloroform sensitive, it was always also UV sensitive and showed reduced autofluorescence), indicating that all three characteristics were the consequence of the same Mendelian mutation.     

Comparison of Solubilised Kainate and α-Amino-3-Hydroxy-5-Methylisoxazolepropionate Binding Sites in Chick Cerebellum

Abstract: [³H]Kainate bound to chick cerebellar membranes with a K _D of 0.6 μ M and with an exceptionally high B max of 165 pmol/mg of protein. In octylglucoside-solubilised extracts, the affinity of [³H]kainate was reduced ( K _D= 2.7 μ M ), but the B _max was relatively unchanged (130 pmol/mg of protein). The rank potency of competitive ligands was domoate > kainate > 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) > glutamate. Binding sites for α-[³H]amino-3-hydroxy-5-methylisoxazolepropionate ([³H]AMPA) were much less abundant, with K _D and B _max values in membranes of 86 n M and I pmol/mg of protein, respectively. The affinity of [³H]AMPA binding was also reduced on solubilisation ( K _D= 465 n M ), but there was an increase in the B _max (1.7 pmol/mg of protein). Quisqualate and CNQX were the most effective displacers of [³H]AMPA binding, but kainate was also a relatively potent inhibitor. However, in contrast to the displacement profile for [³H]kainate, domoate was markedly less potent than kainate at displacing [³H]AMPA. These results suggest that [³H]AMPA binds to a small subset of the kainate sites that, unlike the majority of the [³H]kainate binding protein, which has been reported to be located in the Bergmann glia, may represent neuronal unitary non- N -methyl-D-aspartate receptors.     

Seed dispersal kernel of the largest surviving megaherbivore—the African savanna elephant

Owing to the late Pleistocene extinctions, the megafauna of Europe, Australia and the Americas disappeared, and with them the dispersal service they offered megafaunal fruit. The African savanna elephant, the largest remaining megaherbivore, offers valuable insights into the seed dispersal services provided by extinct megafauna in prehistoric times. Elephant seed dispersal studies have for the most part concentrated on African and Asian forest elephants. African savanna elephants are morphologically distinct from their forest counterparts. Like the forest elephants they consume large quantities of fruit from a large number of tree species. Despite this little is known of the savanna trees that rely on elephants for their dispersal or the spatial scale at which these seeds are dispersed. We combined information from feeding trials conducted on four park elephants with field telemetry data from 38 collared elephants collected over an 8‐year period in APNR/Kruger National Park to assess the seed dispersal service provided by savanna elephants. This study provides the first detailed account of the spatial scale at which African savanna elephants disperse seeds. Our mechanistic model predicts that 50 percent of seeds are carried over 2.5 km, and distances up to 65 km are achievable in maximum gut passage time. These findings suggest the savanna elephant as the longest distance terrestrial vertebrate disperser yet investigated. Maintaining their ecological role as a seed disperser may prove a significant factor in the conservation of large‐fruited tree diversity within the savannas. These results suggest that extinct megafauna offered a functionally unique dispersal service to megafaunal fruit.     

Supplementation with antioxidants and folinic acid for children with Down’s syndrome: randomised controlled trial

Objectives To assess whether supplementation with antioxidants, folinic acid, or both improves the psychomotor and language development of children with Down’s syndrome.Design Randomised controlled trial with two by two factorial design.Setting Children living in the Midlands, Greater London, and the south west of England.Participants 156 infants aged under 7 months with trisomy 21.Intervention Daily oral supplementation with antioxidants (selenium 10 μg, zinc 5 mg, vitamin A 0.9 mg, vitamin E 100 mg, and vitamin C 50 mg), folinic acid (0.1 mg), antioxidants and folinic acid combined, or placebo.Main outcome measures Griffiths developmental quotient and an adapted MacArthur communicative development inventory 18 months after starting supplementation; biochemical markers in blood and urine at age 12 months.Results Children randomised to antioxidant supplements attained similar developmental outcomes to those without antioxidants (mean Griffiths developmental quotient 57.3 v 56.1; adjusted mean difference 1.2 points, 95% confidence interval −2.2 to 4.6). Comparison of children randomised to folinic acid supplements or no folinic acid also showed no significant differences in Griffiths developmental quotient (mean 57.6 v 55.9; adjusted mean difference 1.7, −1.7 to 5.1). No between group differences were seen in the mean numbers of words said or signed: for antioxidants versus none the ratio of means was 0.85 (95% confidence interval 0.6 to 1.2), and for folinic acid versus none it was 1.24 (0.87 to 1.77). No significant differences were found between any of the groups in the biochemical outcomes measured. Adjustment for potential confounders did not appreciably change the results.Conclusions This study provides no evidence to support the use of antioxidant or folinic acid supplements in children with Down’s syndrome.Trial registration Clinical trials {"type":"clinical-trial","attrs":{"text":"NCT00378456","term_id":"NCT00378456"}}NCT00378456.     

Similarities in Restriction Fragment Patterns of Mitochondrial DNAs of PhytophthoraSpecies Strongly Depend on the Restriction Enzyme Used Due to Heterogeneous Base Distribution and Sequence Conservation

Mitochondrial DNAs of six morphologically different Phytophthora species were digested with 15 restriction enzymes. The numbers of restriction fragments obtained differed considerably from those theoretically expected for random base distribution. Enzymes with relatively many G and C in their recognition sequences produced significantly larger numbers of fragments. Moreover, fragments generated by most of these enzymes were more often shared by two or more species than those from enzymes with more A and T in their recognition sequence. It is concluded that base distribution in mitochondrial DNA of Phytophthora is heterogeneous,AT-rich stretches occurring scattered over the mitochondrial genome and GC-rich regions present in conserved sequences, presumably genes. A practical consequence for taxonomic RFLP studies is that optimal enzymes can be selected, depending on the desired level of resolution.     

Estrogen treatment during development alters adult partner preference and reproductive behavior in female laboratory rats

There is broad acceptance for the idea that during development estradiol ‘organizes’ many aspects of reproductive behavior including partner preferences in the laboratory rat. With respect to partner preference, this idea is drawn from studies where estrogen action was in someway blocked, either through aromatase or estrogen receptor inhibition, during development in male rats. The lack of estrogens neonatally results in a decrease in the male rat's preference for females. In this study, the effect of early postnatal estradiol treatment on the partner preferences of female rats was examined as a further test of the hypothesis that male-typical partner preference is dependent upon early exposure to estrogens. Our principal finding was that increased postnatal estradiol exposure during development affected partner preference in the expected direction, and this effect was seen under several adult hormonal and behavioral testing conditions. Female rats that received exogenous estradiol during development spent more time with an estrous female and less time with a sexually active male than did cholesterol treated females. The estradiol treatment also disrupted normal female sexual behavior, receptivity, and proceptivity.     

Cdc20 hypomorphic mice fail to counteract de novo synthesis of cyclin B1 in mitosis

Cdc20 is an activator of the anaphase-promoting complex/cyclosome that initiates anaphase onset by ordering the destruction of cyclin B1 and securin in metaphase. To study the physiological significance of Cdc20 in higher eukaryotes, we generated hypomorphic mice that express small amounts of this essential cell cycle regulator. In this study, we show that these mice are healthy and not prone to cancer despite substantial aneuploidy. Cdc20 hypomorphism causes chromatin bridging and chromosome misalignment, revealing a requirement for Cdc20 in efficient sister chromosome separation and chromosome-microtubule attachment. We find that cyclin B1 is newly synthesized during mitosis via cytoplasmic polyadenylation element-binding protein-dependent translation, causing its rapid accumulation between prometaphase and metaphase of Cdc20 hypomorphic cells. Anaphase onset is significantly delayed in Cdc20 hypomorphic cells but not when translation is inhibited during mitosis. These data reveal that Cdc20 is particularly rate limiting for cyclin B1 destruction because of regulated de novo synthesis of this cyclin after prometaphase onset.