The linear free-energy relationship between partition coefficients and the binding and conformational perturbation of macromolecules by small organic compounds

Kinetics of unidirectional transport in multimembrane systems as influenced by binding to macromolecules

The distribution of externally added low-molecular-weight solutes in systems comprising alternating membraneous and aqueous compartments with binding capacity is described for special conditions of unidirectional transport and linear (i.e., far from saturation) binding. The kinetics of the process valid for a certain time period is expressed as the sum of exponential functions of time, their number being equal to the number of compartments investigated. The coefficients of the equation involve the rate and equilibrium parameters of the process as well as the volumes and the connecting areas of individual compartments. The degenerated case resulting from the cellular structure of biosystems is also considered. The description is shown to agree well with the results of drug potency testing, bioactivity being used to monitor concentration of an effector in the receptor region.

Activation of spinach chloroplast NADP-linked glyceraldehyde-3-phosphate dehydrogenase by concerted hysteresis

Kinetic analysis of glyceraldehyde-3-phosphate dehydrogenase showed that the enhancement of the NADP-linked activity by specific chloroplast modulators is a concerted process; either a selected second metabolite or the couple dithiothreitol/thioredoxin-f lowers the concentration of primary modulators (ATP, NADPH, inorganic phosphate, 1,3-diphosphoglycerate) required for maximal stimulation (A0.5). Organic solvents also stimulate NADP-glyceraldehyde-3-phosphate dehydrogenase in the absence of any modulator; the concentration for the highest specific activity correlates inversely with the respective octanol-water partition coefficient. On the other hand, alcohols also enhance enzyme activity by lowering the A0.5 for primary modulators. Another compound--spermine--inhibits both the ATP- and the inorganic phosphate-mediated activation, but it does not influence the NADPH-induced process.

Specific and non-specific ligand binding to serum albumin

The applicability of a model of specific and non-specific binding sites for the binding of organic ligands to serum albumin was examined statistically. It is proposed in this model that there are a limited number of a single class of high affinity sites and an unlimited number of low affinity sites. Ligands bind to the former sites independently, as in the Scatchard model for a single class of binding sites, and to the latter sites in a partition-like manner, the amounts bound increasing linearly with the concentration of free ligands. In this study, data on the binding of warfarin to human serum albumin reported by Wilting et al. [J. Wilting et al., J. biol. Chem. 255, 3032 (1980)] and on the bindings of indomethacin and salicylic acid to human serum albumin reported by Hultmark et al. [D. Hultmark et al., Acta pharm. suecica 12, 259 (1975)] were used. These binding data were analyzed according to the model of specific and non-specific sites as well as the Scatchard model of two classes of binding sites. Statistical analyses showed that the model of specific and non-specific binding sites fitted the data for binding of all compounds very well, indicating the applicability of this model. An index of the upper limit of ligand concentrations necessary for accurate analysis of the binding data was also discussed.

Accurate, wide-range, automated, high-performance liquid chromatographic method for the estimation of octanol/water partition coefficients II: Equilibrium in partition coefficient measurements, additivity of substituent constants, and correlation of biological data

Occasionally, results from the highly reproducible automated log P(o/w) measurement (ALPM) differ from those determined by shake-flask methods. Several specific examples affording different values are presented. One source of these differences may be curvilinearity in plots of log (t - t0) versus percent methanol, which complicate accurate intercept determinations and, thus, estimates of log P(o/w). Other sources of these differences are presented and discussed, although their cause remains unclear. Equilibrium ALPM log P(o/w) measurements of various phenyl-, methyl-, fluoro-, chloro-, and bromobenzenes, suggest substituent constants are not strictly additive. Moreover, the higher values indicate that calculated values may not be accurate for those compounds having multiple substituents or high log P(o/w) values. ALPM gives better predictability of the in vivo concentration process of 8 or 12 toxicants in fish than the shake-flask method, another HPLC method, or even calculated log P(o/w) values. However, it equally correlates the binding to bovine serum albumin by 34 chemicals as predicted by a combination of shake-flask and calculated log P(o/w) values reported elsewhere.

Papain hydrolysis of X-phenyl-N-methanesulfonyl glycinates: a quantitative structure-activity relationship and molecular graphics analysis

The hydrolysis of 32 X-phenyl-N-methanesulfonyl glycinates by papain was investigated. It was found that the variation in the Michaelis constants could be rationalized by the following correlation equation: log 1/Km = 0.61 pi '3 + 0.46 MR4 + 0.55 sigma + 2.00 with a correlation coefficient of 0.945. In this expression, pi '3 is the hydrophobic constant for the more lipophilic of the two possible meta substituents, MR4 is the molar refractivity of 4-substituents, and sigma is the Hammett constant summed for all substituents. Using this equation, we designed, synthesized, and successfully predicted Km for a new congener intended to maximize binding (1/Km). The interactions involved in enzyme-substrate binding, as characterized by the correlation equation, are interpreted using a computer-constructed color three-dimensional-graphics molecular model of the enzyme active site. The nonenzymatic hydrolysis (both acid and basic) of phenyl hippurates yield rate constants which are well correlated by Hammett equations; however, log k for both acid and alkaline hydrolysis are not linearly related to log 1/Km or log kcat/Km.

The biological effects of high pressures: underlying principles

Physico-chemical principles set constraints on the response of biological systems to high-pressure gases and to pressure per se. They also indicate the mechanisms that may be involved. Classical thermodynamics, intermolecular forces and the theory of solutions and many other areas of physical chemistry have contributed to our understanding of the problems faced by divers breathing gases at high pressure, in particular the high pressure neurological syndrome, inert gas narcosis and decompression sickness. The value and the limitations of physico-chemical arguments when applied to the problems of underwater physiology are analysed.

Molecular mechanisms of general anaesthesia

Important constraints on possible molecular mechanisms of general anaesthesia are derived from a quantitative reappraisal of data on the potency of general anaesthetics on whole animals. Despite their popularity, theories that invoke lipids as the prime target do not look at all promising, and available data point much more plausibly to a direct effect on particularly sensitive proteins. Structural changes of proteins on binding general anaesthetics are probably small but may be sufficient to perturb normal function; alternatively, anaesthetics may compete with an endogenous ligand. The phenomenon of pressure reversal of anaesthesia may simply be due to anaesthetics being squeezed away from their target sites.

Effect of albumin conformation on binding of phenylbutazone and oxyphenbutazone to human serum albumin

The binding of phenylbutazone (I) and oxyphenbutazone (II) to human serum albumin over pH 6.9-9.3 was studied by difference spectrophotometry and equilibrium dialysis. At each pH tested, there was higher binding affinity of I to human serum albumin than II. Equilibrium dialysis showed that over the pH 7-8.2 range both agents had a single high-affinity site and several sites of lower affinity, with the highest binding constant and number of binding sites at pH 7.4 for both I and II. Both techniques showed that the affinity of both drugs to albumin was higher for the neutral form than for the basic form and this transition occurred in both cases around the neutral region (7-7.4). Both the ionized and unionized forms of I and II participated in the binding. In the neutral region, magnesium ion increased the affinity of both drugs to albumin while chloride ion decreased it slightly.

Trehalase activity in extracts of Phycomyces blakesleeanus spores following the induction of germination by heat activation

The heat activation of trehalase in extracts of sporangiospores of Phycomyces blakesleeanus, following the induction of germination by heat activation and the gelatinization of potato starch granules were studied under different conditions in order to discriminate between several phenomena as possible triggers in the activation of trehalase. Short-chain alcohols (from methanol to pentanol) lower the activation temperature of trehalase while long-chain alcohols (from heptanol to nonanol) raise it. Short-chain alcohols also lower the gelatinization temperature of potato starch granules, while long-chain alcohols, hexanol and heptanol have hardly any influence on the gelatinization temperature. Octanol raises the gelatinization temperature. More polar phenols lower the activation temperature of trehalase, while more apolar phenols will raise it. The gelatinization temperature of starch granules is more lowered by the polar polyphenols than by the more apolar phenols. The effect of high pressure on starch gelatinization was investigated in order to compare data from such a model system with the data on trehalase activation. The gelatinization temperature of starch granules is shifted upwards with about 3-5K/1000 atm (1.013 X 10(5) kPa). Pressures higher than 1500 atm do not further increase the gelatinization temperature. However, no reversal of the effect, as occurs with protein conformational changes, is seen with pressure up to 2500 atm. Also for trehalase activation we find a continuous upward shift of the activation temperature with about 5-9K/1000 atm. These data are in agreement with a thermal transition in a polysaccharide matrix, being the trigger in the heat activation of trehalase.

Binding of methyl methacrylate to bovine serum albumin

Binding constants for methacrylates, acrylates, and miscellaneous vinyl monomers with bovine serum albumin were determined using 2-(4'-hydroxyphenylazo) benzoic acid (HABA). The influence of hydrophobic binding appeared to be more dominant in the bindings of methyl methacrylate.

Accumulation of drugs by resting or beating cardiac tissue

The rate and degree of accumulation of 12 neutral, anionic and cationic drugs were studied in resting and 2 Hz-stimulated isolated left auricles of the guinea pig. The uptake process was accelerated in muscles driven electrically. The time needed to reach equilibrium was related to the extent of accumulation which in turn corresponded with the lipophilicity of the drug. The frequency-induced changes in the kinetics of the accumulation disappeared under reduced mechanical activity of the muscle. It is suggested that the increased mechanical activity enhances drug disposition within the extracellular space, so that more drug is available for transmembrane penetration.

A hydrophobic quencher of protein fluorescence: 2,2,2-trichloroethanol

Previously the neutral fluorescence quenching probe, acrylamide, was employed to determine the degree of exposure of tryptophan residues in proteins. A less polar neutral quencher 2,2,2-trichloroethanol (trichloroethanol) was used in the present work to investigate whether it would preferentially interact with apolar regions of proteins. For most proteins studied, the degree of quenching by trichloroethanol is found to be about the same as with acrylamide. However, for human and bovine serum albumin hydrophobic interactions between trichloroethanol and these proteins occur, leading to an exalted quenching. The fluorescence quencher thus senses the presence of a hydrophobic domain in the vicinity of the tryptophan residues in these proteins. Trichloroethanol is shown to induce conformational changes in certain proteins and to be a potentially useful quencher for proteins having predominantly tyrosine emission.

Hydrophobic interaction determined by partition in aqueous two-phase systems. Partition of proteins in systems containing fatty-acid esters of poly(ethylene glycol)

In this report we describe a new method which is useful for measuring hydrophobic interactions between aliphatic hydrocarbon chains and proteins in aqueous environment. The method is based on partition of proteins in an aqueous two-phase system containing dextran and poly(ethylene glycol) and different fatty acid esters of poly(ethylene glycol). The partition is measured under conditions where contributions from electrostatic interactions are eliminated. The difference in partition of proteins in phase systems with and without hyrocarbon groups bound to poly(ethylene glycol), deltalog K, where K is the partition coefficient, is taken as a measure of hydrophobic interaction. Deltalog K varies with size of hydrocarbon chain and type of protein. The length of the aliphatic chain should be greater than 8 carbon atoms in order to get a measurable effect in terms of deltalog K. Bovine serum albumin, beta-lactoglobulin, hemoglobin and myoglobin have been shown to have different affinities for palmitic acid ester of poly(ethylene glycol). No hydrophobic effect could be observed for ovalbumin, cytochrome c or alpha-chymotrypsinogen A.

Linear relationships between plasma binding and lipophilicity of disopyramide derivatives

The extent of plasma binding and the partition coefficient of disopyramide and 20 disopyramide derivatives were determined. Structural variations on the four functional groups around the tetrahedral carbon in the disopyramide molecule was found to influence both parameters to varying degrees. Three linear equations were developed to correlate the observed effects, depending on the type of chemical modification. The linear correlation between drug-plasma interaction and lipophilic character was analyzed theoretically. A simple model was derived to relate quantitatively the variation in the extent of plasma binding to the change in lipophilicity of disopyramide derivatives.

Fractionation of protein mixtures through differential adsorption on a gradient of substituted agaroses of increasing hydrophobicity

Data are presented which indicate the feasibility of protein fractionation at high salt concentrations (greater than or equal to 3 M NaLl) through differential hydrophobic (non-ionic) adsorption on a series of columns of agaroses substituted with ligands of increasing hydrophobicity. By means of such a hydrophobicity gradient of connected columns, separation of mixtures of gamma-globulin and serum albumin, as well as group separation of proteins in dialyzed human blood serum, has been achieved.

Cytoplasmic solvent structure of single barnacle muscle cells studied by electron spin resonance

A free radical probe was introduced into single barnacle muscle cells, and its freedom of motion inferred from the spin resonance spectra. The probe reported an average local viscosity of 5-10 cp compared with 1 cp for pure water. From a comparison of the temperature dependence of the probe's tumbling rate in model aqueous systems and in the muscle we concluded that in the muscle the probe was undergoing fast exchange between sites of different mobility. Thus 10 cp must be taken as an upper limit for the viscosity of most cell water.

Determination of thermodynamics of halogen groups in solutions of drug molecules

The contribution of the halogen groups (F, Cl, Br, I) to solute activity and partition coefficients has been investigated using literature data. The activity coefficient for an aromatic solute at infinite dilution is increased as the size of the halogen group increases. Partition coefficients are affected similarly but there is variation among group values for a given halogen. This is related to group position and the partition solvent. Preferred group values for partition have been selected and for different solvent systems mean preferred values can be calculated for two general solvent classifications; polar and non-polar. There is excellent correlation between group values and the size of the halogen function as given by surface area. The group values for non-polar solvents are related to those of the polar solvents through the Hammett electronic substituent constant. The use of the group values in structure-activity relations is discussed with reference to the thermodynamic model of Higuchi and Davis.

Physico-chemical studies of analgesics. The protein-binding of some p-substituted acetanilides

The binding of fifteen p-substituted acetanilides to bovine serum albumin is examined at pH 7·2. An excellent correlation is obtained between the binding enthalpy and Hammett’s substituent constant, s̀. This is interpreted to mean that the binding is non-specific in nature. A very good correlation is also obtained between the entropy of binding and s̀, which suggests that the extent of hydration of unbound drug is a function of the charge separation within the drug molecule. Of the compounds examined, those that have been used clinically as analgesics possess the best thermodynamic properties, being neither so fully bound as to give low free drug concentrations in the bloodstream, nor so little bound that there is no sustained action.