Modelling of interaction of basic lipophilic ligands with cytochrome P-450 reconstituted in liposomes. Determination of membrane partition coefficients of S-(-)-nicotine and N,N-diethylaniline from spectral binding studies and fluorescence quenching

Compound lipophilicity for substrate binding to human P450s in drug metabolism

Compound lipophilicity is of key importance to P450 binding affinity and enzyme selectivity. Here, lipophilicity is discussed with reference to the human drug-metabolizing P450 enzymes of families CYP1, CYP2 and CYP3. From an extensive compilation of log P values for P450 substrates, and by analysis of relationships between partitioning energy and substrate-binding free energy, the relevance of lipophilicity and other factors pertaining to P450 binding affinity is explained, leading to the formulation of lipophilicity relationships within substrates of each human P450 enzyme involved in drug metabolism. Furthermore, log P values for P450 substrates appear to represent markers for enzyme selectivity. Together with the important roles of hydrogen bonding and pi-pi stacking interaction energies, the desolvation of the P450 active site makes a major contribution to the overall substrate-binding energy and, consequently, a good agreement with experimental information is reported based on this analysis.

Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Optical spectroscopies have been intensively used to determine partition coefficients by a plethora of methodologies. The present review is intended to give detailed and useful information for the determination of partition coefficients and addresses several relevant aspects, namely: (i) definition and calculation of the partition coefficient between aqueous and lipidic phases; (ii) partition coefficients vs. "binding" formalisms; (iii) advantages of spectroscopic methodologies over separation techniques; (iv) formalisms for various experimental approaches based on UV-Vis absorption or fluorescence parameters (fluorescence intensity, lifetime, anisotropy and quenching); (v) experimental hints, artifacts and model limitations; and (vi) a brief survey of nonoptical techniques.

N-oxidative transformation of free and N-substituted amine functions by cytochrome P450 as means of bioactivation and detoxication

Indirect evidence for the participation of cytochrome P450 (P450) in the microsomal N-oxygenation of primary and N-substituted amine functions is presented by studies employing diagnostic modifiers of the hemoprotein system as well as immunochemical approaches. Experiments with recombinant hemoproteins or isozymes purified from the tissues of various animal species support the results obtained by the inhibitor assays. Amine substrates and the redox proteins of the microsomal electron transfer chain reveal to be mutually beneficial in interactions with P450s. Numerous N-substituted amines undergo P450-catalyzed N-oxidative transformation despite the presence of accessible alpha-C hydrogens in these structures rather thought to favor N-dealkylation. In these instances, stabilization of the initially formed aminium radicals by the specific active site orientation of the particular P450s obviously permits oxygen rebound. Apart from common iron-oxenoid chemistry involving a (FeO)3+ species, iron-bound hydroperoxide, (FeO2H)3+, appears to act as an electrophilic oxidant with certain N-substituted amines and P450 subforms. Generally, P450-mediated N-oxygenation of amines can produce cytotoxic and mutagenic metabolites, but equally can well yield hydrophilic products, that are readily excreted and thus promote detoxication.

Influence of membrane partitioning on inhibitors of membrane-bound enzymes

Membrane-water partitioning of inhibitors of acyl-coenzyme A:cholesterol acyltransferase (ACAT) governs the concentration of inhibitor that ACAT is exposed to and determines the corresponding extent of cholesterol esterification inhibition. Partitioning of the ACAT inhibitors CI-976, CL 277,082, and SaH 58-035 into rat liver microsomes containing ACAT was detected by shifts in the level of inhibition that were independent of inhibitor concentration but inversely dependent on microsome membrane concentration. The equilibrium distribution of the ACAT inhibitors between aqueous and membrane phases was derived directly from these data by application of a previously described method of linear analysis. The accuracy of membrane partitioning analysis based on kinetic data was verified for CI-976 by direct measurements of [14C]CI-976 partitioning into phospholipid membranes. The results show that the ACAT inhibitors are highly partitioned into membranes by factors exceeding 1 x 10(6). This result is consistent with the far greater influence of membrane content over aqueous volume on inhibitor activity. The results demonstrate that the size of the membrane phase in aqueous suspension must be taken into account to obtain accurate and reproducible kinetic characterizations of membrane-active molecules. Analyses of the membrane-dependent shifts in activity can be used to calculate the membrane-water partitioning coefficient of membrane-active molecules such as ACAT inhibitors.

Multiphasic modelling of ligand/acceptor interactions. The hydrophobicity-dependent binding of relatively small amphiphilic substances to acceptor proteins and the nature and facedness of acceptor sites

The modelling of multiphasic ligand/acceptor equilibrium binding systems proceeds at three logically distinct levels: (1) A suitable response quantity, e.g. the amount of acceptor-bound ligand nEL, is expressed as a function of the ligand concentrations [Li] (L = A,B,...) in the compartment i that contains the acceptor sites. One thus obtains a response function nEL = f1([Li]). In general, the equilibrium constants KL contained in such mathematical models are physically ill-defined. (2) Each local concentration [Li] is further expressed as a function of [Laq], the corresponding concentration in the aqueous phase, leading to nEL = f2([Laq]). In this way, the constants KL are transformed into effective constants K'L which (i) can be assessed experimentally and (ii) depend on ligand hydrophobicity in a way that is characteristic of the binding site. Formulation of the functions f1 and F2 only requires knowledge of the reactions in which the acceptor sites participate directly. (3) For each ligand, the experimentally accessible total ligand concentration Lt is expressed as a function of [Laq], leading to concentration balance equations Lt = Lt([Laq]). The latter transformation takes account of any reactions, distinct from ligand/acceptor interaction, in which the ligands are involved, e.g. binding to additional protein sites. As a result of steps 2 and 3, each binding system is described by a set of simultaneous equations dependent on the auxiliary variable [Laq]: (i) the response function f2([Laq]) and (ii) a concentration balance for each ligand Lt = Lt([Laq]). The formulae are rendered more conscise and their discussion and application to data fitting are simplified by introducing, for each ligand L, a function FL characterising the distribution of unbound monomeric ligand over the various partition compartments. When the acceptor acts on unbound ligand, the formulae are further expressed in terms of a new auxiliary variable i.e. the total concentration of unbound monomeric ligand microL. In contrast to data analysis as a function of local concentrations, analysis in terms of total ligand concentrations avoids losing sight of alternate hypotheses about the nature of the binding sites. The present formulation has also permitted clarification of several consequences of the multiphasic nature of the binding systems that, as yet, have been poorly recognised.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluorescence studies of the location and membrane accessibility of the palmitoylation sites of rhodopsin

Fluorescent fatty acid labels have been incorporated into the palmitoylation sites of rhodopsin and used to probe the membrane accessibility and location of these sites. The fluorescence properties of anthroyloxy and pyrenyl fatty acids bound to rhodopsin were investigated in a reconstituted vesicle system. Collisional quenching of fluorescence by stearic acid (DSA) labeled with doxyls in the 16, 12, and 5 positions was used to determine the membrane accessibility and disposition of the modifying fatty acids. To properly determine the membrane concentration of these quenchers, the dependence of the Stern-Volmer parameters on both quencher and vesicle concentration was determined. An analysis of these dependences provided a correction for partitioning of the quencher between the aqueous phase and the membrane. After this correction, the relative effectiveness of doxyl quenchers was 16-DSA > 12-DSA > 5-DSA. Parallel studies on free anthroyloxy and pyrenyl fatty acids incorporated into the reconstituted system showed the same dependence on quencher position. These results indicate that the labels at the palmitoylation sites of rhodopsin are situated in the membrane much as a free fatty acid. This anchoring of the palmitates in the membrane results in the formation of a fourth cytoplasmic loop.

Interaction of phospholipids with proteins and peptides. New advances IV

1. The review deals with the newest achievements in the field of the various interactions between phospholipids and proteins and peptides. 2. Interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecular assemblies is also discussed.

Regulatory mechanisms in the activation of nitrogenous compounds by mammalian cytochrome P-450 isozymes

Metabolic activation of nitrogenous compounds by the cytochrome P-450 system is a highly complex process. Inherent substrate factors, such as basicity, electronic state, lipophilicity, and conformation control binding of the diverse classes of amines to cytochrome P-450. Accommodation of these compounds in the enzyme cavity and proper orientation of the molecules are governed by intrinsic properties of the peptide structure of cytochrome P-450, which may be subject to modification by the action of effectors. On the membrane level, phospholipid might have some impact on substrate binding. On the other hand, bound amine substrate is beneficial to the productive interaction of the electron transport chains with the terminal acceptor, improving economy of the system. Certain amines appear to regulate O2 association with cytochrome P-450 and stabilize the various oxy species formed. Considering the selective prerequisites for oxidative attack by cytochrome P-450 at vulnerable nitrogen centers, many cytotoxic amines belonging to the category of relatively rigid, planar molecules undergo N-oxidative activation by the cytochrome P-450IA subfamily, while more bulky amines with flexible conformation are N-oxygenated preferentially by phenobarbital-inducible cytochromes P-450. Small differences in protein structure between the various cytochrome P-450 subforms might serve to stabilize aminium radicals to permit oxygen rebound. Collectively, the selective regulatory mechanisms operative in the bioactivation of nitrogen-containing compounds appear to be determined largely by the type of substrate used and the isozyme involved in catalysis. With respect to the latter, the interplay of the multiple cytochromes P-450 in the various organs of animal species thus serves to rationalize the differences in the particular selectivities for amine substrates. These are responsible for the mode and/or extent to which activation of nitrogenous compounds, including promutagens and procarcinogens, occurs, and this may explain the tissue-specific response to the tumorigenic action of these agents.