Degradation kinetics of pentachlorophenol by Phanerochaete chrysosporium

The extracellular enzymes and cell mass from the pregrown Phanerochaete chrysosporium cultures were used for the degradation of PCP. The use of both extracellular enzymes and cell mass resulted in extensive mineralization of PCP, while the action of only the crude extracellular enzymes led to the formation of a degradation intermediate (TCHD). A kinetic model, which describes the relationship among PCP degradation, initial PCP concentration, dosage of extracellular enzymes, and cell mass concentration, was developed. Based on this model, various effects of initial PCP concentration, dosage of extracellular enzymes, and cell mass concentration were evaluated experimentally. It was found that when initial PCP concentration is lower than 12 mumol/L, the model of a parallel-series first-order reaction is sufficient to describe the degradation process. PCP disappearance and mineralization were enhanced by increasing either the extracellular enzyme concentration or the cell mass concentration. As high as 70% of PCP mineralization could be obtained by using a higher dosage of extracellular enzymes and cell mass. Various parameters of the kinetic model were determined and the model was verified experimentally. Simulation using this model provided the criteria needed to choose rational dosages of extracellular enzymes and cell mass for the degradation of PCP. Data reported allow some insight into the function of the extracellular enzymes and cell mass of P. chrysosporium in degradation processes of toxic pollutants and assist in the design and evaluation of practical bioremediation methods.     

Degradation of mono-, di-, and trihalogenated benzoic acids by Pseudomonas aeruginosa JB2.

Pseudomonas aeruginosa JB2 was isolated from a polychlorinated biphenyl-contaminated soil by enrichment culture containing 2-chlorobenzoate as the sole carbon source. Strain JB2 was subsequently found also to grow on 3-chlorobenzoate, 2,3- and 2,5-dichlorobenzoates, 2,3,5-trichlorobenzoate, and a wide range of other mono- and dihalogenated benzoic acids. Cometabolism of 2,4-dichlorobenzoate was also observed. Chlorocatechols were the central intermediates of all chlorobenzoate catabolic pathways. Degradation of 2-chlorobenzoate was routed through 3-chlorocatechol, whereas 4-chlorocatechol was identified from the metabolism of both 2,3- and 2,5-dichlorobenzoate. The initial attack on chlorobenzoates was oxygen dependent and most likely mediated by dioxygenases. Although plasmids were not detected in strain JB2, spontaneous mutants were detected in 70% of glycerol-grown colonies. The mutants were all of the following phenotype: benzoate+, 3-chlorobenzoate+, 2-chlorobenzoate-, 2,3-dichlorobenzoate-, 2,5-dichlorobenzoate-. While chlorocatechols were oxidized by the mutants at wild-type levels, oxidation of 2-chloro- and 2,3- and 2,5-dichlorobenzoates was substantially diminished. These findings suggested that strain JB2 possessed, in addition to the benzoate dioxygenase, a halobenzoate dioxygenase that was necessary for the degradation of chlorobenzoates substituted in the ortho position.     

Selfish DNA: A Sexually-Transmitted Nuclear Parasite

A quantitative population genetics model for the evolution of transposable genetic elements is developed. This model shows that "selfish" DNA sequences do not have to be selectively neutral at the organismic level; indeed, such DNA can produce major deleterious effects in the host organism and still spread through the population. The model can be used to explain the evolution of introns within eukaryotic genes; this explanation does not invoke a long-term evolutionary advantage for introns, nor does it depend on the hypothesis that eukaryotic gene structure may be an evolutionary relic. Transposable genes that carried information specifying sexual reproduction in the host organism would favor their own spread. Consequently, it is tempting to speculate that some of the genes controlling sex were originally selected as transposable elements.     

Differential effects of GTP on the coupling of beta-adrenergic receptors to adenylate cyclase from frog and turkey erythrocytes. Application of new methods for the analysis of receptor-effector coupling.

A detailed comparison of the interaction of beta-adrenergic receptors with adenylate cyclase stimulation and modification of this interaction by guanine nucleotides has been made in two model systems, the frog and turkey erythrocyte. Objective analysis of the data was facilitated by the development of new graphical methods which involve the use of logit-logit transformations of percent receptor occupancy versus percent enzyme stimulation plots (coupling curves). Receptor-cyclase coupling in turkey erythrocyte membranes demonstrates a proportional relationship between receptor occupancy and adenylate cyclase activation and is unaffected by exogenous guanine nucleotides. By comparison, the proportional relationship of receptor occupancy and adenylate cyclase activation observed in frog erythrocyte membranes in the absence of guanine nucleotides is modified by the addition of exogenous guanine nucleotides such that a greater fractional enzyme stimulation is elicited by low receptor occupancy. Methodological criteria crucial for valid comparison of receptor occupancy and adenylate cyclase activity are delineated. In addition, the possible molecular mechanisms of receptor-cyclase coupling which might give rise to the coupling curves observed are discussed.     

pH-dependent leaving group effects on hydrolysis reactions of phosphate and phophonate esters catalyzed by wheat germ acid phosphatase.

The substrate specificity of carefully purified wheat germ acid phosphatase was examined and the Michaelis constants for substrates having widely varying leaving groups were determined at pH values 4.6, 8.0, and 9.2. The pH-dependent leaving group effects were consistent with the formation of a covalent phosphoryl histidine intermediate in the reaction process catalyzed by this enzyme. In addition, the enzyme was found to hydrolyze nitrophenyl esters of methyl-, chloromethyl-, and phenylphosphonic acids at rates comparable to those observed for phosphomonoester hydrolysis. The data are most simply interpreted on the basis of a nucleophilic displacement by an active-site histidine residue to form an intermediate N′-phosphonyl histidine species, followed by decomposition of this intermediate by nucleophilic attack by water, analogous to the decomposition process of the N′-phosphoryl enzyme species.     

Housing mice in a caging system with automatic flushing.

Male Har:(ICR)BR mice were housed 8 wk in wire bottom cages over paper, in wire bottom cages over an automatic cascade flushing system, or in solid-bottom plastic cages with bedding material. There were no substantial differences in general health or weight gain. Pentobarbital LD50 values were lower for the mice housed in wire bottom cages than for the mice in solid bottom cages with bedding. This difference was probably related to gastrointestinal content. It appears that the automatic cascade flushing system is suitable for housing mice for periods up to 8 wk.     

New Mechanisms to Explain the Effects of Added Lactose Fines on the Dispersion Performance of Adhesive Mixtures for Inhalation

Fine excipient particles or ‘fines’ have been shown to improve the dispersion performance of carrier-based formulations for dry powder inhalation. Mechanistic formulation studies have focussed mainly on explaining this positive effect. Previous studies have shown that higher drug contents may cause a decrease in dispersion performance, and there is no reason why this should not be true for fines with a similar shape, size and cohesiveness as drug particles. Therefore, the effects on drug detachment of ‘fine lactose fines’ (FLF, X₅₀ = 1.95 µm) with a similar size and shape as micronised budesonide were studied and compared to those of ‘coarse lactose fines’ (CLF, X₅₀ = 3.94 µm). Furthermore, interactions with the inhalation flow rate, the drug content and the mixing order were taken into account. The observed effects of FLF are comparable to drug content effects in that the detached drug fraction was decreased at low drug content and low flow rates but increased at higher flow rates. At high drug content the effects of added FLF were negligible. In contrast, CLF resulted in higher detached drug fractions at all flow rates and drug contents. The results from this study suggest that the effects of fines may be explained by two new mechanisms in addition to those previously proposed. Firstly, fines below a certain size may increase the effectiveness of press-on forces or cause the formation of strongly coherent fine particle networks on the carrier surface containing the drug particles. Secondly, when coarse enough, fines may prevent the formation of, or disrupt such fine particle networks, possibly through a lowering of their tensile strength. It is recommended that future mechanistic studies are based on the recognition that added fines may have any effect on dispersion performance, which is determined by the formulation and dispersion conditions.