Štefan Baláž

A Model‐based Dependence of the Human Tissue/Blood Partition Coefficients of Chemicals on Lipophilicity and Tissue Composition

Partitioning of up to thirty-six nonionizable chemicals between seven tissues (fat, liver, brain, kidney, muscle, lung, heart) and blood in humans was modeled using membrane accumulation, protein binding, and distribution in the aqueous phases as relevant processes. The extent of membrane accumulation and protein binding of individual compounds was described as the Collander-type function of their lipophilicity expressed by the 1-octanol/water partition coefficients. The resulting model-based expression described satisfactorily the tissue/blood partition coefficients as a nonlinear function of lipophilicity and tissue composition (a standard content of lipids, proteins, and water). The calibrated model is suitable for prediction of the tissue/blood partition coefficients for non-amphiphilic nonionizable chemicals with the 1-octanol/water partition coefficients varying between 0.01 and 100,000, i.e. practically in the whole range for which equilibrium tissue/blood distribution can be used in physiologically based pharmacokinetic models.

Lipophilicity in trans-bilayer transport and subcellular pharmacokinetics

The aim of subcellular pharmacokinetics in drug design is to model drug disposition and response as a function of the properties of drugs and biosystems involved and the observation time. Biosystems are represented by systems of alternating membranes and aqueous phases that differ in acidity and contain low-molecular cell constituents, enzymes and other proteins. The resulting disposition models are combined with linear free-energy assumptions, drug/receptor binding kinetics and relationships between receptor binding and response to produce model-based quantitativestructure–(time–)activity relationships, QS(T)AR. This review summarizes the present status of subcellular pharmacokinetics with emphasis on passive trans-bilayer transport. In particular, mechanisms of transport are analyzed with respect to amphiphilicity and lipophilicity. The overall rate of transport is strongly governed by amphiphilicity, the tendency of drug molecules to adsorb to the bilayer/water interface. Depending on amphiphilicity, the time needed for a drug to cross a single bilayer ranges from seconds to days. The main advantage of the subcellular pharmacokinetic approach is that the resulting models, once calibrated for a given biosystem, provide a detailed recipe for tailoring the drug properties to ensure optimum disposition.

Effect of chlorobenzoates on the degradation of polychlorinated biphenyls (PCB) by Pseudomonas stutzeri

Chlorobenzoic acids (CBA) are frequently dead-end products of partial aerobic biodegradation of polychlorinated biphenyls (PCB). When CBA produced from PCB accumulate in the growth medium, they can inhibit the bacterial growth and consequently, slow down PCB biodegradation. In this study, the effects of seven mono- and dichlorinated CBA on growth of Pseudomonas stutzeri on different substrates and on the PCB degradation by this strain in a liquid mineral medium were tested. 3-CBA was the strongest growth inhibitor for P. stutzeri growing on glucose, benzoate and biphenyl. It was found to inhibit heavily the elimination of some di- and trichlorinated biphenyls. In contrast, its influence on the elimination of more chlorinated congeners was much less significant.

Biodegradation and evaporation of polychlorinated biphenyls (PCBs) in liquid media

During microbial degradation of PCBs in a liquid medium, two processes influence the PCB concentration in the medium simultaneously: biodegradation and evaporation. The physical loss of PCB due to evaporation frequently causes false positive results in biodegradation experiments. Therefore, if only PCBs are monitored, the determination of the PCB concentration in both liquid and gaseous phases is necessary for a correct appraisal of biodegradation. The kinetics of PCB evaporation and biodegradation were monitored and described by a simple mathematical model. The evaporation and biodegradation rate constants for individual PCB congeners were determined for PCB degradation in liquid medium byPseudomonas stutzeri andAlcaligenes xylosoxidans, both isolated from a longterm PCB-contaminated soil.

Structure-mutagenicity relationships of 5-nitro-2-furylethylenes in Salmonella typhimurium TA98

Mutagenicity of three selected series of 2-furylethylene was determined by the Ames test. These included: nine alkylesters and eleven N-alkylamides of 5-nitro-2-furylacrylic acid (NFAA) and ten derivatives differing not only at exocyclic double bond, but also in the position 5 of the furan ring. Mutagenicity of the derivatives depends on the presence of the 5-nitro-furan centre in the molecule; side chains in the position 2 modify the degree of mutagenicity. Among the derivatives of NFAA tested as changing the substituents virtually does not affect chemical properties of the 5-nitrofuran ring. Mutagenicity of the n-alkyl congeners decreases linearly with increasing lipophilicity. Mutagenicity of the derivatives with branched alkyl substituents is lower than expected from the behaviour of the n-alkyl homologues.

A kinetic distribution model of evaporation, biosorption and biodegradation of polychlorinated biphenyls (PCBs) in the suspension of Pseudomonas stutzeri

Kinetics of distribution of PCBs in an active bacterial suspension of Pseudomonas stutzeri was studied by monitoring the evaporated amounts and the concentration remaining in the liquid medium with the biomass. To determine the biodegradation rate constants of the individual congeners of the PCB formulation Delor 103, a model considering biosorption, evaporation, and primary biodegradation constructed previously was used. Rate constants of biodegradation imply that biodegradation of individual congeners is structure-dependent process. Biodegradability decreases with increasing number of chlorine substituents in the molecule, especially if they are in the ortho and para positions. On the other hand, the increasing number of free ortho and meta positions in the biphenyl molecule leads to better biodegradability. For a simple empirical determination of the influence of the chlorine substitution pattern on biodegradability, the di- and trichlorobiphenyl rate constants of biodegradation were analysed.

Evaporation kinetics of polychorinated biphenyls during biodegradation experiments

During microbial degradation of polychlorinated biphenyls (PCB) in liquid media two processes take place simultaneously: elimination of PCB and evaporation of PCB. The physical loss of PCB due to evaporation causes frequently false positive results in long-term biodegradation experiments. Therefore, if only the PCB concentration is to be measured, its determination in both liquid and gaseous phase is essential for a correct appraisal of biodegradation. The kinetics of PCB evaporation have been monitored and the evaporation rate constants for individual PCB congeners have been determined. The values of evaporation rate constants show a good corelation with the values of the 1-octanol/water partition coefficient.

Liposome/saline partition coefficients of low-molecular-weight soluted by gel chromatography

A chromatographic method for the determination of association constants of rapidly dissociable complexes is described and applied to quantification of liposome/saline partition coefficients using gel chromatography. The approach allows for estimation of the free solute concentration in the sample by simple manual processing of the intact right-hand part of the solute peak deformed due to gradual diffusion of the accumulated solute from the liposomes along the separation column. Validity of the procedure was confirmed by both reasonable agreement with equilibrium dialysis data and model-based deconvolution of the distorted peak into its two components corresponding to initially unbound compound and to that escaped from the liposomes during the separation process.

Quantitative structure-activity relationship of carbonylcyanide phenylhydrazones as uncouplers of mitochondrial oxidative phosphorylation

The dependence of the uncoupling activity in the series of 16 carbonylcyanide phenylhydrazones on their physico-chemical properties (partition coefficient, dissociation constant and rate constant for reaction with thiols) is investigated using two physiologically based models, one for protonophoric mechanism of uncoupling and the other assuming the covalent modification of a membrane constituent to be the key step in this process. As indicated by uptake experiments, at the given conditions a lipophilic-hydrophilic equilibrium is attained without any loss of the compounds via chemical reactions. Using this fact to reduce the number of adjustable parameters, a better fit to the data on stimulation of respiration is obtained with the former (protonophoric) model.

Degradation of PCB by Bacteria Isolated from Long-Time Contaminated Soil

Four bacterial isolates belonging to the genera Pseudomonas and Alcaligenes were obtained by the enrichment method, using biphenyl as the sole carbon source, from the soil, which underwent long-time contamination with technical mixtures of PCB. Kinetics of PCB degradation by individual isolates was measured using n-hexane extraction of the cultivation media in proper time intervals and analysed by congener specific gas chromatography with electron capture detection. All isolates exhibit interesting biodegradative potential. Specific degradation of individual congeners with respect to the number and position of chlorine substituents is discussed. The influence of glucose, biphenyl and 3-chlorobenzoic acid on the PCB degradation has been assessed.