Effects of External Mass Transfer and Product Inhibition on a Simulated Immobilized Enzyme‐Catalyzed Reactor for Lactose Hydrolysis

A mathematical model has been developed for predicting the performance and simulation of a packed bed immobilized enzyme reactor performing lactose hydrolysis, which follows Michaelis‐Menten kinetics with competitive product (galactose) inhibition. The performance characteristics of a packed bed immobilized enzyme reactor have been analyzed taking into account the effects of various diffusional phenomena like axial dispersion and external mass transfer limitations. The model design equations are then solved by Galerkin's method and orthogonal collocation on finite elements. The effects of external mass transfer and axial dispersion have been studied and their effects were shown to reduce the external effectiveness factor. The effects of product inhibition have been investigated at different operating conditions correlated at different regimes using dimensionless moduli (St, γ, θ, Da)¹⁾. The product inhibition was shown to reduce the substrate conversion, and, additionally, to decrease the effectiveness factor when Da > Da_xo, however, it increases the effectiveness factor when Da < Da_xo. The effectiveness factor is found to be independent of the product inhibition at a crossover point at which Da_xo is defined. Effects of St and Pe have been investigated at different kinetic regimes and the results show that their effects have a strong dependency on the kinetic parameters θ, γ (i.e., K_m/K_p), and Da_xo.     

THE CONFORMATION OF CLATHRIN LIGHT-CHAINS ON TRISKELIONS PROBED BY ANTIBODY INHIBITION

The conformation of clathrin light-chains along the proximal arm of the clathrin triskelion was studied by using rabbit anti-(light-chain peptides) to inhibit the binding of a mouse monoclonal antibody against an epitope in the amino-terminal region. Prior incubation of triskelions with rabbit antisera raised against the extreme carboxyl-terminal of the light-chains partially inhibited binding. The inhibition was largely removed when tested on light-chains that had been freed from triskelions. This suggests that when the light-chains bind the heavy-chain, they adopt a conformation in which the amino and carboxyl-terminal domains are not fully extended, but fold such that these two domains face each other.     

Endogenous Ammonium Generation in Maize Roots and its Relationship to other Ammonium Fluxes

An investigation to determine the magnitude of the back reactionswhich occur during net ammonium uptake by roots and during netammonium assimilation within roots was undertaken with maize(Zea mays L.). Ten-day-old seedlings, which had been grown on250 mmol m^–3 ammonium at pH 4 or 6, were pretreated for3 h in the absence or presence of 500 mmol m ^–3 MSX (methionine-DL-sulphoximine),an inhibitor of the glutamine synthetase-catalysed pathway ofammonium assimilation. They were then exposed for 2 h to 99A% ¹⁵N-ammonium ± MSX. Substantial ammonium cycling occurredduring net ammonium uptake. Efflux was enhanced by MSX treatment,reflecting a 2- to 3-fold accumulation of ammonium in the roottissue. Influx of ammonium was also increased by treatment withMSX, indicating that influx was enhanced when products of ammoniumassimilation were dissipated. The decline in root ¹⁴N-ammoniumaccounted for only a small fraction of the ¹⁴N-ammonium recoveredin the ambient ¹⁵N-ammonium solution, revealing a substantialgeneration of endogenous ¹⁴N-ammonium during the 2 h exposure.The net quantity of ammonium generated was increased appreciablywhen assimilation of ammonium was restricted by MSX and it wasestimated to occur at least 50% faster than net ammonium uptake.Presence of MSX severely decreased translocation of ¹⁵N to shootsbut had a smaller influence on incorporation of ¹⁵N into macromoleculesof the root tissue. The various ammonium flux rates were notgreatly affected by growth at pH 4.0, implying a considerableresistance of ammonium assimilation processes in these maizeroots to the high ambient acidity commonly induced by exposureto ammonium Key words: Ammonium generation, uptake, assimilation     

Cardiovascular Function in Turtles During Anoxia and Acidosis: In vivo and in vitro Studies

During prolonged experimental submergence, freshwater turtlesbecome anoxic and develop a combined respiratory and non-respiratoryacidosis. Anoxia and acidosis are known to depress cardiac functionin turtles and other vertebrate species. In vitro studies ofventricular and atrial tissue of turtles indicate that increasedextracellular Ca⁺⁺ concentration can reverse these depressantactions. Intact turtlesutilize this compensatory mechanism duringanoxia and other conditions leading to acidosis byincreasingplasma Ca⁺⁺ concentration. In addition, cardiac cells may releaseCa⁺⁺ from cell organelles to compensate for induced respiratoryacidosis. Both mechanisms presumably improve cardiac contractilityby elevating the level of sarcoplasmicCa⁺⁺.     

An evolutionary fast‐track to biocalcification

The ability to construct mineralized shells, spicules, spines and skeletons is thought to be a key factor that fuelled the expansion of multicellular animal life during the early Cambrian. The genes and molecular mechanisms that control the process of biomineralization in disparate phyla are gradually being revealed, and it is broadly recognized that an insoluble matrix of proteins, carbohydrates and other organic molecules are required for the initiation, regulation and inhibition of crystal growth. Here, we show that Astrosclera willeyana, a living representative of the now largely extinct stromatoporid sponges (a polyphyletic grade of poriferan bauplan), has apparently bypassed the requirement to evolve many of these mineral‐regulating matrix proteins by using the degraded remains of bacteria to seed CaCO₃ crystal growth. Because stromatoporid sponges formed extensive reefs during the Paelozoic and Mesozoic eras (fulfilling the role that stony corals play in modern coral reefs), and fossil evidence suggests that the same process of bacterial skeleton formation occurred in these stromatoporid ancestors, we infer that some ancient reef ecosystems might have been founded on this microbial–metazoan relationship.     

Description of Paratetrahymena parawassi n. sp. using Morphological and Molecular Evidence and a Phylogenetic Analysis of Paratetrahymena and Other Taxonomically Ambiguous Genera in the Order Loxocephalida (Ciliophora,Oligohymenophorea)

ABSTRACT. The marine scuticociliate Paratetrahymena parawassi n. sp. is described on the basis of morphology, especially infraciliature, and the sequence of its small subunit (SSU) rRNA gene to become the second known member of its genus. Paratetrahymena and other ciliates in the order Loxocephalida possess a mixture of morphological and morphogenetic features characteristic of the subclasses Hymenostomatia and Scuticociliatia. Accordingly, we used SSU rRNA sequences to analyze the phylogeny of Paratetrahymena and three other loxocephalid genera. Paratetrahymena and Cardiostomatella vermiformis formed a moderately well‐supported clade that diverged at a deep level from all other scuticociliates, supporting separation of loxocephalids from other scuticociliates as a suprafamilial taxon. Sathrophilus holtae was a sister taxon to Paratetrahymena and Cardiostomatella in a poorly supported, unresolved relationship; nevertheless, association of all three genera into a single clade was supported by an approximately unbiased (AU) test. Any association of these genera singly or as a group with the Hymenostomatia was rejected decisively by AU tests and by a complete absence in the loxocephalids of the unique nucleotide identities that distinguish hymenostomes. Therefore, the morphological and morphogenetic similarities of loxocephalids to hymenostomes may be plesiomorphies, and the conflicting mix of scuticociliate and hymenostome characteristics seen in loxocephalids may result from differing rates of character evolution. Dexiotrichides pangi and Urocentrum, which is currently classified as a peniculid, formed a small clade that associated with hymenostomes and peritrichs. Monophyly of the Loxocephalida with Dexiotrichides and/or Urocentrum included was not rejected by AU; however, inclusion of Urocentrum in the Peniculia was rejected by AU tests. A hypothesis is offered to explain the lack of resolution of loxocephalid ciliates and Urocentrum in phylogenetic trees, namely that their phylogenetic positions are influenced by a combination of heterogeneous data and long‐branch attraction caused by poor representation of taxa in analyses. The well‐known genus Cyclidium, a member of the order Pleuronematida, was revealed to be polyphyletic as a byproduct of our analyses of loxocephalids. In particular, Cyclidium porcatum appears to fall outside the clade containing typical members of the subclass Scuticociliatia and thus invites investigation as a possible member of the order Loxocephalida.     

Surface-feeding on cephalopods by procellariiform seabirds in the southern Benguela region, South Africa

Cephalopod beaks recovered from stomach samples taken from 14 seabird species in the southern Benguela region off Southern Africa and from one species at Sub-Antarctic Marion Island, were identified as far as possible, counted and the lower rostral lengths (LRLs) measured. Dorsal mantle lengths (DMLs) and body masses of the cephalopods eaten were estimated. The results of analyses by percentage frequency of occurrence and numerical abundance are discussed with reference to present knowledge of the distribution of cephalopods eaten by seabirds in the areas studied. Division of the cephalopod component of seabird diets into species which float, and species which sink, after death indicates that the birds forage on dead or moribund cephalopods on the surface, rather than catching live bioluminescent cephalopods at night.     

Xylem cavitation in two mature Scots pine forests growing in a wet and a dry area of Britain

The xylem cavitation rate, stem water content, stomatal conductance and leaf water potential of mature Scots pine (Pinus sylvestris L.) were compared over a period of 18 months at two sites in Britain where trees were planted from the same seed source. The sites were at Thetford in south-east England, where the climate is relatively warm and dry (average rainfall of 600mm per year), and Aberfoyle in central Scotland, where it is relatively cool and wet (average rainfall of 1500 mm year^?1). In the first year of study (1992) the natural difference between the sites was amplified by a severe drought affecting south-east England. Acoustic emissions (as a result of cavitation) were detected at both sites, but were not an everyday occurrence, and rates depended on short-term meteorological variation. The relative water content (RWC) of the trunk at breast height at Thetford was significantly lower than at Aberfoyle, and declined in parallel with increasing severity of drought. Following the release from drought a gradual increase in RWC was found, but the pre-drought RWC was not attained. The same xylem water potential gradient and stomatal conductance was found at the two sites; but on a diurnal basis, as water potential declined, cavitation rates increased. There was no difference in vulnerability to cavitation or in hydraulic conductance between the sites.