Membrane effects on drug monooxygenation activity in hepatic microsomes

A Modified Arrhenius Approach to Thermodynamically Study Regioselectivity in Cytochrome P450-Catalyzed Substrate Conversion

The regio- (and stereo-)selectivity and specific activity of cytochrome P450s are determined by the accessibility of potential sites of metabolism (SOMs) of the bound substrate relative to the heme, and the activation barrier of the regioselective oxidation reaction(s). The accessibility of potential SOMs depends on the relative binding free energy (ΔΔGbind ) of the catalytically active substrate-binding poses, and the probability of the substrate to adopt a transition-state geometry. An established experimental method to measure activation energies of enzymatic reactions is the analysis of reaction rate constants at different temperatures and the construction of Arrhenius plots. This is a challenge for multistep P450-catalyzed processes that involve redox partners. We introduce a modified Arrhenius approach to overcome the limitations in studying P450 selectivity, which can be applied in multiproduct enzyme catalysis. Our approach gives combined information on relative activation energies, ΔΔGbind values, and collision entropies, yielding direct insight into the basis of selectivity in substrate conversion.

Phenomenon of Activation of Cytochrome P450 by Nonionic Detergents

Mechanism of substitution of nonionic detergent Emulgen 913 for phospholipid as an activator of N-demethylase activity of cytochrome P450 form 2B4 (LM2) has been studied. It is shown that such an activation takes place at the detergent concentrations below values critical for micelle formation. Under these conditions, Emulgen does not affect the hexameric state of the cytochrome. The stimulating effect proved to be similar in reconstituted monooxygenase systems containing (a) cytochrome P450 2B4 and NADPH-cytochrome P-450 reductase and (b) cytochrome 2B4 and organic hydroperoxides. These results indicate that the activation is due to an effect of the detergent upon P450 2B4 per se rather than upon P450/flavoprotein complex formation. The above conclusion is supported by the sedimentation data and measurement of the CD spectra of cytochrome P450 2B4 at 380–450 nm.

The effect of temperature on monoxygenase reactions in the microsomal membrane

The effect of temperature on the rates of monoxygenase reactions was studied with microsomes prepared from phenobarbital pretreated rats. The rates of the N-demethylation of ethylmorphine, benzphethamine, aminopyrine, and p-nitroanisole were studied. Breaks at temperatures around 24 degrees C were observed in the Arrhenius plots of all these reactions. The energy of activation of these reactions has values of 10-12 and 19-21 kcal per mol at temperature ranges above and below the break temperature, respectively. The break, however, was not observed if 30% glycerol was added to the microsomes. The Arrhenius plot of the microsomal NADPH-cytochrome c reductase activity also did not show any break. The implications of these observations in relationship to the fluidity of the membrane, the translational mobility of membrane enzymes, and the rate of monoxygenase reactions are discussed.

Biphasic kinetic behavior of rat cytochrome P-4501A1-dependent monooxygenation in recombinant yeast microsomes

Rat cytochrome P-4501A1-dependent monooxygenase activities were examined in detail using recombinant yeast microsomes containing rat cytochrome P-4501A1 and yeast NADPH-P-450 reductase. On 7-ethoxycoumarin, which is one of the most popular substrates of P-4501A1, the relationship between the initial velocity (v) and the substrate concentration ([S]) exhibited non-linear Michaelis-Menten kinetics. Hanes-Woolf plots ([S]/v vs. [S]) clearly showed a biphasic kinetic behavior. Aminopyrine N-demethylation also showed a biphasic kinetics. The regression analyses on the basis of the two-substrate binding model proposed by Korzekwa et al. (Biochemistry 37 (1998) 4137-4147) strongly suggest the presence of the two substrate-binding sites in P-4501A1 molecules for those substrates. An Arrhenius plot with high 7-ethoxycoumarin concentration showed a breakpoint at around 28 degrees C probably due to the change of the rate-limiting step of P-4501A1-dependent 7-ethoxycoumarin O-deethylation. However, the addition of 30% glycerol to the reaction mixture prevented observation of the breakpoint. The methanol used as a solvent of 7-ethoxycoumarin was found to be a non-competitive inhibitor. Based on the inhibition kinetics, the real V(max) value in the absence of methanol was calculated. These results strongly suggest that the recombinant yeast microsomal membrane containing a single P-450 isoform and yeast NADPH-P-450 reductase is quite useful for kinetic studies on P-450-dependent monooxygenation including an exact evaluation of inhibitory effects of organic solvents.

Temperature-induced differential kinetic properties between an initial burst and the following steady state in membrane-bound enzymes: studies on lathosterol 5-desaturase

The NADH-dependent lathosterol 5-desaturation reaction that forms 7-dehydrocholesterol is biphasic, an initial burst followed by steady state. The steady-state phase is slower than the burst phase, because the latter diffusion of the lathosterol substrate within the microsomal membrane must occur before the next reaction can take place [Y. Takakuwa, H. Nishino, Y. Ishibe, and T. Ishibashi (1994) J. Biol. Chem. 269, 27889-27893]. In the present study, changes in the structure and function of the membrane were examined by measurement of the Arrhenius activation energy of lathosterol 5-desaturase at various temperatures between 2 and 45 degrees C. At the burst phase, there was a lack of discontinuity in the Arrhenius plots at the presumed phase transition temperature for the microsomal membrane. However, the plots of the activities of the steady state showed breaks at around 17 and 32 degrees C. It was concluded that phospholipid phase transition affects the steady-state phase but not the burst phase. Furthermore, treatment of microsomes with low concentrations of deoxycholate, known to perturb the membrane integrity, resulted in a break of the activation energy of the burst phase. These results have revealed further evidence for our previous model suggesting interaction between the substrate and enzyme within the microsomal membrane via lateral diffusion.

Physiologically based pharmacokinetic analysis of the concentration-dependent metabolism of halothane

1. Previous studies with the halothane analogue and chlorofluorocarbon replacement 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123) have shown that there are concentration-dependent, sex-specific differences in the rate of uptake during inhalation exposure in rat. Since it is well established that there are sex-specific differences in the control of enzyme activity in drug metabolism, male and female rats were exposed by inhalation to halothane concentrations ranging from 500 to 4000 ppm. 2. A physiologically based pharmacokinetic model describing the concentration-dependent reduction in uptake and metabolism of halothane in male and female rats was developed. The in vivo metabolic rate constants obtained were: for male rats, Km = 0.4 mg litre-1 (2.03 mumol litre-1) and Vmaxc = 9.2 mg kg1 h-1 (46.6 mumol kg1 h-1); for female rats, Km = 0.4 mg litre-1 (2.03 mumol litre-1) and Vmaxc = 10.2 mg kg-1 h-1 (51.7 mumol kg-1 h-1). 3. An equation describing the concentration-dependent decrease of hepatic metabolism of halothane successfully simulated the gas-uptake data. Simulation of cumulative urinary excretion of the major metabolite, trifluoroacetic acid, required introduction of a proportionality constant to limit the extent of reduction of halothane metabolism to 20% of the amount of enzyme activity. Good simulation of urinary excretion data was achieved, which was interpreted to indicate that, when only 20% of the enzyme is inactivated, the rate of enzyme resynthesis was adequate to replenish enzyme activity within 24 h. 4. A rapidly reversible, non-biological inactivation mechanism called "physical toxicity' is discussed as a possible explanation of concentration-dependent gas uptake.

Effects of hypothermia on the elimination of ethanol, diazepam and oxazepam in rat liver slice incubations

The elimination of ethanol, diazepam and oxazepam which are metabolised by different enzymes, has been studied for 30, 60, 90 and 120 min at 37, 27, 17 and 7 degrees C in rat liver slice incubations. Ethanol elimination followed zero-order kinetics at all temperatures, while the benzodiazepines consistently displayed first-order kinetics. No sign of phase transition was observed in the respective Arrhenius-plots. Ethanol elimination was more temperature dependent than the elimination of diazepam, while the elimination of oxazepam was little influenced by temperature. This is shown by the temperature ratios (Q10) and energies of activation (Ea) of 1.76, 1.56, 1.24 and 40.5, 31.9, 15.2 for ethanol, diazepam and oxazepam, respectively. This means that ethanol, diazepam and oxazepam elimination was reduced by 25, 22 and 14%, respectively, for each 10 degrees C of temperature reduction, which is considerably lower than the commonly observed 50% reduction of enzyme activity. We conclude that observations made for one drug on temperature dependent elimination may not apply to other drugs.

Kinetics of cytochromes P-450 IA1 and IIB1 in reconstituted systems with dilauroyl- and distearoyl-glycerophosphocholine

In the present study the effect of changing the fatty acyl moiety of phosphatidylcholine from dilauroyl to distearoyl on the kinetic parameters of O-dealkylation of alkoxyresorufins and ethoxycoumarin dependent on reconstituted cytochromes P-450 IA1 and IIB1 has been investigated. The results demonstrate that (a) the maximum rate of O-dealkylation (V) for both P-450 enzymes was about two times higher in the L-alpha-dilauroyl-sn-glycero-3-phosphocholine (Lau2GroPCho) system and (b) changes in the fatty acyl moiety of phosphatidylcholine (acyl2GroPCho) from dilauroyl to distearoyl affected the apparent Km for the substrate (Kms) of P-450 IA1 and IIB1 in a different way. In addition, (c) the kinetic parameters appeared to be dependent on the acyl2GroPCho/P-450 ratio and a change in this ratio affected the kinetic parameters of P-450 IA1 and IIB1 in a different manner. From these last two observations it was concluded that the mechanism by which phospholipids influence P-450-IIB1-dependent O-dealkylation of ethoxycoumarin is different from that by which they influence P-450-IIB1-dependent O-dealkylation of this substrate. Furthermore, the results of the present study demonstrate that the increase in the rate of O-dealkylation of ethoxycoumarin, reported in the literature for reconstituted systems in the presence of Lau2GroPCho, results from an effect of Lau2GroPCho on both the Kms and the V. In a number of additional experiments possible mechanisms underlying the observed differential effect of Lau2GroPCho and Ste2GroPCho on the Kms and V of P-450 IA1 and IIB1 were investigated. This was done by studying the effect of the two acyl2GroPCho species on the kinetic parameters of some of the different steps of the P-450 cycle, namely substrate binding, oxygen binding and the rate of electron transfer. The results demonstrate an influence of Lau2GroPCho and Ste2GroPCho on (a) substrate binding to cytochrome P-450, (b) the affinity of cytochromes P-450 for NADPH-cytochrome reductase and thus on (c) the electron flow through the reconstituted system. Based on the results from these experiments it was concluded that the increased V of P-450 IA1 and IIB1 in the presence of Lau2GroPCho compared to the systems with Ste2GroPCho was at least in part due to an increased affinity of both P-450 enzymes for NADPH-cytochrome reductase in the presence of Lau2GroPCho compared to Ste2GroPCho.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluidity of the microsomal membrane and cytochrome P450 reduction kinetics of pig liver microsomes as a consequence of organic solvent impact

1. The effect of the aromatic solvents toluene, xylene and ethylbenzene on microsomal membrane fluidity and anaerobic NADPH-reduction kinetics were studied. 2. The relation of membrane fluidity to the kinetics of cytochrome P450 reduction by NADPH-cytochrome P450 reductase was examined with regard to a membrane-mediated molecular organization of the multienzyme components of the monooxygenase system. 3. Membrane fluidity changes were detected with the steady-state pyrene excimer formation method and with fluorescence lifetime measurements after incubation of the microsomes with organic solvents. 4. Increase in membrane fluidity in presence of organic solvents leads to a small but significant decrease of the rate constant of the cytochrome P450 reduction kinetics and a change in the relative amplitudes of the components of the biphasic response. 5. The results support the idea of a molecular organization of cytochrome P450 in clusters. Fluidization of the microsomal membrane by organic solvents increase the cytochrome P450 cluster formation.

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its toxic metabolites on the physicochemical property of the liposomal membrane in relation to their cytochrome P-450 inhibition

The effects of the neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and its toxic metabolites MPDP+ (1-methyl-4-phenyl-2,3-dihydropyridinium) and MPP+ (1-methyl-4-phenylpyridinium) on liposomal membrane were assessed using fluorescence-polarization and carboxyfluorescein leakage studies as well as in biological membrane preparations. Of the three compounds, MPTP was found to cause the greatest perturbation of membrane followed by MPDP+ and then MPP+. The ability of the three toxins to inhibit cytochrome P-450 enzyme activity (a microsomal membrane-bound enzyme system) was also studied and their relative potency was again found to be MPTP > MPDP+ > MPP+. The changes in the physicochemical property of the liposomal membrane can be related to the ability of the neurotoxin's ability to inhibit cytochrome P-450 activity.

The interference of polychlorinated biphenyls (Aroclor 1254) with membrane regulation of the activities of cytochromes P-450C21 and P-450(17) alpha,lyase in guinea-pig adrenal microsomes

Regulation of cytochromes P-450 21-hydroxylase (P-450C21) and P-450 17 alpha-hydroxylase/C17,20-lyase (P-450(17) alpha,lyase) activities and impairment of this regulation by Aroclor 1254 was studied in guinea-pig adrenal microsomes. In a membrane depleted system, a decrease in the normally predominant, P-450C21 activity and an increase in P-450(17) alpha,lyase activities was observed. The same deviations were observed in intact microsomes with increase in the reaction temperature (0-40 degrees C). Breaks in Arrhenius plots for activities of P-450C21 and P-450(17) alpha,lyase correlate with transition temperatures reported for the microsomal membrane. These results point to: (1) preference of a gel state membrane for catalytic expression of P-450C21 suggesting a clustered organization of this P-450 species with reductase; (2) preference of a fluid membrane for lyase activity suggesting a random collision mechanism for reduction of P-450(17) alpha,lyase. Aroclor 1254 introduced to reaction mixtures containing intact microsomes elicited basically the same changes as caused by depletion of the microsomal membrane or by increase in the incubation temperature. Lack of effect of Aroclor 1254 on P-450C21 and P-450(17) alpha,lyase activities in the membrane depleted system demonstrates that its interference with monooxygenase activities is mediated by the microsomal membrane. The similarities between altered cytochrome P-450 mediated activities in the presence of Aroclor 1254 and the deviations observed in the membrane depleted system or upon increase in the incubation temperature may suggest that this chemical exerts its impacts by influencing membrane fluidity.

Cytochrome P-455 nm complex formation in the metabolism of phenylalkylamines. XII. Enantioselectivity and temperature dependence in microsomes and reconstituted cytochrome P-450 systems from rat liver

Formation of metabolic intermediate (MI) complexes was studied with the enantiomers of amphetamine, 1-phenyl-2-pentanamine, N-hydroxyamphetamine, and 2-nitroso-1-phenylpropane (the C-nitroso analogue of amphetamine). Three different enzyme systems were used; liver microsomes from phenobarbital pretreated rats and two reconstituted systems containing the P450 2B1 and P450 2C11 forms of cytochrome P-450. Enantioselective complex formation in microsomes was shown for the amines and the nitroso compound, but not for the hydroxylamine. The highly purified P450 2B1 system formed the MI complex with all substrates tested, and the enantioselectivity observed with the microsomal system was reproduced. In the P450 2C11 system the nitroso compounds were completely inactive, whereas the enantiomers of N-hydroxyamphetamine still produced the complex at a high rate. Changes in temperature were shown to affect (R)-2-nitroso-1-phenylpropane more than its enantiomer. Both enantiomers showed biphasic Arrhenius plots for MI complex formation in microsomes (breaks around 22 degrees C), but the activation energies of the (R)-isomer were about five times higher than those of the (S)-isomer. A theory is presented which suggests different modes of interaction with the active site of P-450 to account for the different behaviour of the various substrates.

Citrulline-malate effect on microsome phospholipids and cytochrome P450 in Euglena grown with ethanol

This study indicates for the first time the presence of cytochrome P450 in the microsomes of Euglena grown in lactate medium and substantiates the use of Euglena as a hepatic cell model. Similar effects of ethanol on Euglena and on rat hepatic microsomes were demonstrated: (i) decrements in the quantities of FA per milligram of proteins; (ii) increases in the proportions of PE; (iii) decreases in the proportions of PC; and (iv) production of cytochrome P450, degraded in P420. The citrulline-malate reestablishes in the microsomes the phospholipid environment and the cytochrome P450 concentration. These findings illustrate that the complex acts on the lipid peroxidation via the changes in cytochrome P450 activity.

Lipid regulation of bovine cytochrome P450(11) beta activity

Purified bovine adrenocortical cytochrome P450(11) beta has been reconstituted into phospholipid vesicles using a detergent dialysis procedure. Using this reconstituted system, we have examined the effect of changes in the fatty acyl substituents of the lipids on the catalytic activity of the enzyme. The studies reported here show that cytochrome P450(11) beta exhibits a completely different response to changes in the fatty acyl groups from that shown by cytochrome P450scc. Cytochrome P450(11) beta displays maximal activity in lipid vesicles composed of saturated lipids, such as dipalmitoyl and dimyristoyl phosphatidylcholines, with turnover numbers ranging from 35 to 60 min-1. Incremental increases of phospholipids such as diphytanoyl and dioleoyl phosphatidylcholines result in a progressive inhibition of 11 beta hydroxylase activity; most of this kinetic effect is attributable to a significant decrease in Vmax accompanied by modest changes in Km for the steroid substrate deoxycorticosterone. Diphosphatidyl glycerol (cardiolipin), which has been previously shown to activate cytochrome P450scc, is a potent inhibitor of the 11 beta hydroxylase activity of cytochrome P450(11) beta, with half maximum inhibition observed in vesicles containing 4-5 mol% diphosphatidyl glycerol. Kinetic analysis demonstrates that this inhibition by diphosphatidyl glycerol is reflected in both a decrease in Vmax and relatively large increases (up to sevenfold) in Km for the steroid substrate. These effects on the 11 beta hydroxylase activity may have important implications for the in vivo regulation of not only the 11 beta hydroxylase activity, but also the other catalytic activities of this enzyme, particularly 18- and 19-hydroxylase and oxidase activities.

Role of lipid in the electron transfer between NADPH-cytochrome P-450 reductase and cytochrome P-450 from mammalian liver cells

1. The anaerobic NADPH-reduction of the isozymes cytochrome P-450 LM2 and LM4 was used as a functional tool to study the component interaction in reconstituted monooxygenase systems in dependence on different phospholipids. 2. The isozymes were shown to exhibit similar lipid interaction. The lipids generally favour a catalytically active 1:1 complex formation between reductase and cytochrome P-450 as the rate-determining unit in electron transfer. 3. The cytochrome P-450 reduction proceeds in a biphasic reaction. In dilauroyl phosphatidylcholine (DLPC)-reconstituted systems the amount of the fast reduction psi 1 is stoichiometrically limited by the reductase in deficit: psi 1 corresponds to the 1:1 complex formation capability of the reductase. 4. In vesicle-reconstituted systems an 'overstoichiometric' reductase cycling is observed which gives rise to a significantly increased amount of fast reduction psi 1. Reductase cycling is proposed to occur in protein clusters of cytochrome P-450 and reductase in deficit. 5. The dissociation constant KRP of the functionally active reductase-cytochrome P-450 complex has been determined by means of the amount of psi 1 (DPLC) and the rate constant kapp 1 (vesicles) of the fast reduction as a measure of the complex formation in dependence on the protein molar ratio. Taking into account the actual protein concentration in the vesicular lipid phase, KRP in vesicles has been calculated to be about 3 orders of magnitude increased in comparison to DLPC-reconstituted systems. 6. Vmax data reveal almost the same catalytic activity of both reconstitution modes, which justifies DLPC-reconstitution in model investigations. The vesicle-specific increased accumulation of reduced cytochrome P-450 in the steady state as originated by reductase cycling may offer the physiological advantage of an increased capacity of cytochrome P-450 for synergistic substrate conversion via cytochrome b5.

Temperature dependence of the microsomal oxidation of ethanol by cytochrome P450 and hydroxyl radical-dependent reactions

The temperature dependence and activation energies for the oxidation of ethanol by microsomes from controls and from rats treated with pyrazole was evaluated to determine whether the overall mechanism for ethanol oxidation by microsomes was altered by the pyrazole treatment. Arrhenius plots of the temperature dependence of ethanol oxidation by pyrazole microsomes were linear and exhibited no transition breaks, whereas a slight break was observed at about 20 +/- 2.5 degrees C with control microsomes. Energies of activation (about 15-17 kcal/mol) were identical for the two microsomal preparations. Although transition breaks were noted for the oxidation of substrates such as dimethylnitrosamine and benzphetamine, activation energies for these two substrates were similar for control microsomes and microsomes from the pyrazole-treated rats. The addition of ferric-EDTA to the microsomes increased the rate of ethanol oxidation by a hydroxyl radical (.OH)-dependent pathway. Arrhenius plots of the .OH-dependent oxidation of ethanol by both microsomal preparations were linear with energies of activation (about 7 kcal/mol) that were considerably lower than values found for the P450-dependent pathway. These results suggest that, at least in terms of activation energy, the increase in microsomal ethanol oxidation by pyrazole treatment is not associated with any apparent change in the overall mechanism or rate-limiting step for ethanol oxidation but likely reflects induction of a P450 isozyme with increased activity toward ethanol. The lower activation energy for the .OH-dependent oxidation of ethanol suggests that different steps are rate limiting for oxidation of ethanol by .OH and by P450, which may reflect the different enzyme components of the microsomal electron transfer system involved in these reactions.

Comparison of in vivo and in vitro methods for assessing the effects of repeated dosing with carbon tetrachloride on the hepatic drug-metabolizing enzyme system

The effect of 7 daily i.p. injections of 0, 2, 20, or 200 microliters/kg carbon tetrachloride on the activity of the hepatic drug-metabolizing enzyme system was measured in the rat by a model substrate assay, employing lindane (gamma-hexachlorocyclohexane), and by a battery of in vitro enzyme assays. The data in this study indicated that repeated administration of CCl4 for 7 days significantly increased phase I and phase II reactions in vivo and in vitro. Though there were differences between the responses of the in vivo and in vitro assays, this is the first report of increased hepatic drug-metabolizing enzyme activity from repeated treatment with CCl4.

Hydroperoxide-dependent folic acid degradation by cytochrome c

Folic acid is degraded by cytochrome c in the presence of hydrogen peroxide/tert-butyl hydroperoxide at the C9-N10 bond. The degradation is increased with increasing temperature. When guanidine HCl or benzoate are included in the reaction medium, the amount of folic acid degradation is enhanced. Catalase, formate, and thiourea inhibited hydrogen peroxide-dependent folic acid degradation only, and not tert-butyl hydroperoxide dependent degradation. Cyanide and azide markedly inhibited both the hydroperoxide-dependent degradations. Superoxide dismutase, EDTA, ethanol, mannitol, and dimethyl sulfoxide did not inhibit the degradation. The mechanism of cytochrome c-catalyzed folic acid degradation is discussed.

Fetal and adult human liver differ markedly in the fluidity and lipid composition of their microsomal membranes

The fluidity and lipid composition of the human hepatic microsomal membrane were studied in 11 livers from 16- to 21-week-old fetuses and in 5 adult livers, and compared with those of fetal and adult rat liver microsomes. Membrane fluidity was analyzed by measurement of fluorescence polarization using the fluorophore 1,6-diphenyl-1,3,5-hexatriene. The lipid apparent microviscosity (eta) of human fetal liver microsomes was 2.17 +/- 0.13 poise, as compared with 1.08 +/- 0.08 poise in adult liver (p less than 0.001). Similar differences in fluidity were found between fetal and adult rat liver microsomes. The more "fluid" adult microsomes had higher phospholipid/cholesterol and phosphatidylcholine/sphingomyelin molar ratios than those of the more "rigid" fetal microsomes. The degree of unsaturation of the adult microsomal lipids was much higher than that of the fetal lipids. The ratios of unsaturated/saturated fatty acids in microsomal lipids highly correlated with the eta values obtained for the combined group of fetal and adult human livers, suggesting that the developmental increase in degree of unsaturation of the microsomal lipids is a major determinant of the increased fluidity of adult as compared with fetal liver microsomes. These differences in fluidity and lipid composition between fetal and adult human liver microsomes may be a critical factor in the regulation of hepatic microsomal drug and carcinogen metabolizing enzyme activity, and could so determine the extent of toxicity and teratogenicity of drugs and/or their metabolites in the developing human fetus.

Glucocorticoids increase the fluidity of the fetal-rat liver microsomal membrane in the perinatal period

Dexamethasone, a synthetic glucocorticoid, was administered to pregnant rats during the last week of pregnancy in order to examine its effects on the fluidity of the developing fetal-rat liver microsomal membrane. This early prenatal exposure to dexamethasone, which preceded the natural appearance of fetal corticosteroids, markedly accelerated the normal perinatal course of fluidization of this membrane. The lipid apparent microviscosity, which was determined by measurement of fluorescence polarization, decreased in 21-days-old treated fetuses to values that were indistinguishable from those of untreated newborn rats. This dexamethasone-mediated acceleration of membrane fluidization was associated with an increase in the index of unsaturation of the fatty acyl moiety of microsomal lipids. Dexamethasone caused a significant increase in the microsomal content of polyunsaturated fatty acids (arachidonic and linoleic acid), which was accompanied by a decrease in content of monoenoic fatty acids (oleic and palmitoleic acid). This early exposure in utero to dexamethasone precociously induced the changes in fatty acid composition of fetal-rat liver microsomal lipids that normally occur between the last day of pregnancy and the first day of extra-uterine life. These results suggest that endogenous glucocorticoids play a major role in the perinatal fluidization of the rat liver microsomal membrane.

Arrhenius plots of membrane-bound enzymes of mitochondria and microsomes in the brain cortex of developing and old rats

To study changes in lipid-protein-interaction and fluidity in mitochondrial and microsomal membranes of brain during development and aging, the Arrhenius plots of marker enzymes for mitochondrial inner and outer membranes as well as those of microsomal membranes were compared at different ages. The enzymes were, beta-hydroxybutyrate dehydrogenase (BDH) for the inner mitochondrial membrane and rotenone-insensitive-NADH-cytochrome c reductase (Mit-NADH-CytR) for the outer membrane; also antimycin-insensitive-NADH-cytochrome c reductase (Micr-NADH-CytR) and NADPH-cytochrome c reductase for the microsomal membranes. The ages studied were, 1, 5, 30, 60 days postnatal and 2 years. In the microsomes, the plots of NADH-cytochrome c reductase were found to be biphasic at all ages except in the newborn where no break temperature was observed. The activation energy of this enzyme in the physiological range of temperature was found to be high in the newborn, declining with brain maturation. The plot of this enzyme in the old rat showed no difference when compared with the mature. The specific activity of the enzyme, however, was markedly reduced in the old brain microsomal fraction. In contrast, comparison of the plots of microsomal NADPH-cytochrome c reductase at the various ages revealed no break temperature and very similar energies of activation. For the inner mitochondrial membrane beta-hydroxybutyrate dehydrogenase, the Arrhenius plots were generally biphasic at all ages studied with a break temperature of about 20 degrees C. However, the newborn plot was only barely biphasic showing a high energy of activation in the physiological range of temperature. In contrast, for the outer membrane NADH-cytochrome c reductase, the newborn plots were definitely biphasic, exhibiting low activation energy above the breaks. There was also a significant decline in the break temperature with brain development. No significant differences in the plots of this enzyme were found between the old and mature brain cortex. It is concluded that the enzymes of inner and outer mitochondrial membranes as well as the oxidative enzymes of microsomal membranes may show differential patterns of change in lipid-protein-interaction during development and aging, the changes being more marked in development than in aging.

Study of the optimal temperature for the liver microsomal assay with mice S9 fractions

The effect of temperature on enzymatic activity and stability was studied with respect to the monooxygenase activities of aminopyrine-N-demethylase (APD) and p-nitroanisole O-demethylase (pNAD) under incubation conditions for the liver microsomal assay. The activities of S9 liver fractions of mice induced with sodium phenobarbital and beta-naphthoflavone were determined during a period of preincubation in a range of temperatures from 30 to 44 degrees C. The greatest value of the mean specific activity was found at 40-42 degrees C for both APD and pNAD. The rapid increase of lipid peroxidation after 1 h of incubation at temperatures higher than 42 degrees C can provide an explanation of the enhancement of the rate of inactivation. In order to determine whether biological response is affected by the modifications induced by temperature in the metabolic activating system, tester strain D7 of Saccharomyces cerevisiae was used to assay the genetic activity of the well known premutagenic agent cyclophosphamide by incubating the mixtures both at the traditional temperature of 37 degrees C and at 42 degrees C. We suggest that the use of more favourable conditions for LMA with respect to enzymatic activity, than the traditional ones could improve the reliability and the sensitivity of such tests.

Interaction of hexane phosphonic acid diethyl ester with phospholipids in hepatic microsomes and reconstituted liposomes as studied by 31P-NMR

By use of 31P-NMR, quasi-elastic light scattering and freeze-fracture electron microscopy it is shown that hexane phosphonic acid diethyl ester (PAE) is incorporated in hepatic microsomes without any alteration of the bilayer structure at two different sites. These findings proved that PAE can be used as molecular 31P-NMR probe in microsomes to get information about lipid-protein interactions. Extensive studies on reconstituted liposomal systems which contained cytochrome P-450 and cytochrome P-450 reductase showed that both proteins influence the localization of incorporated PAE. The results indicate a specific interaction of phosphatidylethanolamine (PE) with cytochrome P-450 in microsomes.

Spin state studies on cytochrome P-450 in liver microsomes from obese and diabetic animals

The spin state of liver microsomal cytochrome P-450 from obese mice and streptozotocin-diabetic mice and rats has been studied both by the temperature and the type I substrates-induced spectral changes. The high spin cytochrome P-450 is significantly decreased in these animals. Moreover absolute spectra indicate that low spin cytochrome P-450 is stabilized in streptozotocin induced-diabetic animals. Thus the physiopathological state may modify the in vivo spin state of cytochrome P-450 and modifications of the microsomal fatty acid composition might contribute to these changes.

Mechanism of rate control of the NADPH-dependent reduction of cytochrome P-450 by lipids in reconstituted phospholipid vesicles

The NADPH-supported reduction of cytochrome P-450 LM2 (liver microsomal isozyme 2) in reconstituted phospholipid vesicles in general exhibits two-exponential kinetics. The physiologically relevant rapid partial reaction is favoured in amount with increasing reductase/P-450 ratio. A lipid specificity was observed in that negatively charged lipids favour that process, too. The rate constant increases concomitantly. The data are consistent with the formation of a reactive 1:1 complex the amount of which determines the rate constant. The dissociation constants amount to 0.048 microM for a microsomal lipid extract, 0.051 microM for a 3:1 (w/w) mixture of dioleoylglycerophosphoethanolamine and phosphatidylserine, and 0.47 microM for dioleoylglycerophosphocholine, respectively, in the respective reconstituted systems. At low reductase/P-450 ratio the amount of the rapidly reduced P-450 exceeds the equilibrium concentration of a 1:1 complex. Preformed 1:1 associates, therefore, cannot fit the derived mechanism. Instead, a cluster model based on P-450 association does correspond to the data.

Low-energy Mg2+-induced temperature transitions in liver microsomes

The differential scanning microcalorimetry and fluorescence methods, using probes ANS and pyrene, have been employed to study thermotropic behaviour of rat liver microsomes in the presence and absence of Mg2+. Addition of Mg2+ yields three partially reversible phase transitions at 18, 27 and 32 degrees C, respectively. A character of Mg2+-induced rearrangements in a membrane and their relation to a catalytic function of a cytochrome P-450-dependent enzymatic system is discussed.

Hepatic microsomal NADPH-cytochrome P-450 reductase from little skate, Raja erinacea. Comparison of thermolability and other molecular properties with a mammalian enzyme

Components of little skate (an elasmobranch) and rabbit hepatic microsomal cytochrome P-450 dependent monooxygenase systems were examined for differences which might explain the decreasing xenobiotic-metabolizing activity of little skate microsomes assayed at temperatures above 30 degrees C. The proportion of saturated fatty acids in microsomal lipids and the habitat temperature are both lower in skate as compared to rabbit, which is consistent with the known adaptive pattern. The more thermolabile enzyme of the skate system in microsomal preparations is NADPH-cytochrome P-450 reductase. The optimal assay temperature for purified skate reductase (30 degrees C) is 10 degrees C lower than that for the purified rabbit reductase. The purified skate reductase differs from rabbit reductase in monomeric molecular weight, in peptides produced by partial proteolysis, in immunochemical properties, but not in flavin content.

The influence of cholesterol incorporation and removal on lipid-bilayer viscosity and electron transfer in rat-liver microsomes

The incorporation and removal of cholesterol from rat liver microsomes was used as a methodical approach to investigate the molecular organization of microsomal redox-chains. It was shown that the incorporation of cholesterol in microsomes increases and removal of cholesterol decreases lipid bilayer viscosity as indicated from the rate of fluorescent probe-pyrene eximerisation in cholesterol-enriched and cholesterol-depleted microsomes. The increase of membrane viscosity slows down the initial rates and decreases the rate constants of cytochrome b5 reduction by NAD(P)H, whereas the decrease of membrane viscosity enhances the initial rates and increases the rate constants of these reactions. The rates of cytochrome P450 reduction by reduced pyridine nucleotides do not depend on the viscosity of lipid bilayer. The incorporation and removal of cholesterol from microsomes was not followed by any essential changes in the rates of dimethylaniline N-demethylation, aniline p-hydroxylation, p-nitroanisole O-demethylation, oxygen consumption, oxidation of NADH and NADPH. Thus the reduction of cytochrome b5 by NADH and NADPH is the diffusion-dependent reaction in the redox-chains of microsomes only.

Thermotropic behaviour of phospholipid vesicles reconstituted with rat liver microsomal cytochrome P-450

The interaction of rat liver microsomal cytochrome P-450 with phospholipids has been investigated employing differential scanning microcalorimetry. It is shown that the thermotropic behaviour of phospholipid vesicles reconstituted with cytochrome P-450 depends on liposome composition, protein concentration and the mode of reconstituted system preparation. From the delta H dependence on protein concentration in proteoliposomes it was calculated that one cytochrome P-450 molecule influence 350 +/- 50 dimyristoylphosphatidylcholine (DMPC) molecules. The electrostatic interaction of cytochrome P-450 and negatively charged phospholipid, phosphatidylinositol (PI), mixed with DMPC involves the temperature stabilization of proteoliposomes at a phase transition of phospholipid bilayers. The thermal denaturation temperature is increased due to negatively charged PI added.

Fluidity of human erythrocyte membrane and effect of chlorpromazine on fluidity and phase separation of membrane

The fluidity of human erythrocyte membrane, and the effect of chlorpromazine at prelytic and lytic concentrations on the fluidity have been studied by using three kinds of fatty acid spin labels and measuring the temperature dependence of Mg2+-ATPase activity. The Arrhenius plot of the apparent rotational correlation time, tau c, for probes I(12,3) and I(5,10) showed an abrupt discontinuity at about 30 degrees C, and the plot for I(1,14) at 25 degrees C, indicating that a large difference in the fluidity exists between the interior and the outer surface of the lipid bilayer. The portions of the fatty acid chain near the ten carbon bond lengths removed from the bilayer surface became more fluid by chlorpromazine treatment; there was a decrease in the break point to around 26 degrees C following treatment with 0.6 or 1 mM of the drug. Two breaks at 21 and 30 degrees C in the Arrhenius plot of the Mg2+-ATPase activity were observed in normal erythrocyte membrane. The activation energy of the Mg2+-ATPase reaction has the values of 3.0 and 22.1 kcal/mol above the upper break and below the lower break, respectively. The drug exposure induced only a slight shift in the break temperatures, while the treatment significantly enhanced the associated activation energies of the reaction. These results suggest that the boundary phospholipids of the Mg2+-ATPase in the membrane are probably more rigid than the bulk lipids.

Effects of cholesterol on the properties of the membranes of isolated sheep liver nuclei and nuclear envelopes

The exchangeability of cholesterol between sheep liver nuclear membranes and liposomes, and the effect of cholesterol on the fluidity of the membrane lipid were studied. In intact nuclei, the cholesterol/phospholipid ratio increased from 0.102 to 0.145 mol/mol on incubation with cholesterol-rich liposomes, with a time for half-maximal uptake of 4.2 h. In isolated envelopes under the same conditions, the ratio increased from 0.110 to 0.266 mol/mol with a time for half-maximal uptake of about 1.9 h. Moreover, the approximate order parameter of the spin label 5-(N-oxyl-4',4'-dimethyloxazolidino)-stearic acid was 0.677 in intact nuclei and 0.723 in isolated envelopes prior to exchange; after exchange, these values increased to 0.717 and 0.756, respectively. These differences between the preparations could not be attributed to differences in the capacity for cholesterol uptake between the two nuclear membranes, or to a slow rate of exchange between them; the presence of an intact nuclear matrix appeared both to disorder the lipid partially and to inhibit cholesterol uptake. The differences indicate that conclusions based on physical studies of the membrane lipid in isolated envelopes are not necessarily applicable to the intact nucleus.

Effect of fatty acids on physical properties of microsomes from isolated perfused rat liver

A computer-centered spectrofluorimeter was used to examine the physicochemical properties of hepatic microsomes and microsomal lipids obtained from isolated rat livers perfused with medium containing palmitate or oleate. The fatty acid composition and degree of unsaturation of the liver microsomal lipids reflected that the fatty acid present in the perfusate. The absorption corrected fluorescence, relative fluorescence efficiency, polarization, and fluorescence anisotropy of several fluorescent probe molecules were measured to determine if their different microenvironments may be altered by the type of fatty acid infused. The probe molecules beta--parinaric acid and 1,6-diphenyl-1,3,5-hexatriene had higher values for each of these parameters when incorporated into microsomes obtained from livers perfused with a medium containing palmitate than with oleate. The same parameters measured for cholesta-5,7,9(11)-trien-3 beta-ol and N-phenyl-1-naphthylamine were not altered. These differences appeared to be primarily due to alterations in microviscosity of the probe microenvironments since the rotational correlation time of 1,6-diphenyl-1,3,5-hexatriene was 25% lower in the microsomes from livers perfused with oleate as compared to livers perfused with palmitate. Thermal discontinuities in Arrhenius plots were noted in the intact microsomes but not in the isolated microsomal lipids with the fluorescence probe molecule beta-parinaric acid. Break points occurred at 10 degrees C and 26 degrees C for microsomes from livers perfused with palmitate and at 12 degrees C and 17 degrees C for microsomes from livers perfused with oleate containing medium. These results suggest that the physicochemical properties of liver microsomes were determined in part by the fatty acid in the perfusate.

Comparison of the membrane-bound and detergent-solubilised hydrogenase from paracoccus denitrificans. Isolation of the hydrogenase

The hydrogenase from Paracoccus denitrificans is an integral membrane protein and has been solubilised by Triton X-100. The membrane-bound and detergent-solubilised forms of the enzyme have been compared. Both forms of the enzyme show a pH optimum for reduction of benzyl viologen at pH 8.5--9.0 and are both inhibited by concentrations of NaCl greater than 30 mM. An Arrhenius plot of the activity of hydrogenase in the membrane shows no 'break'. The form of the Arrhenius plot and the activation energy are not significantly changed on solubilisation of the enzyme. The Km and V values for benzyl viologen, methyl viologen and H2 are unaltered when the enzyme is extracted from the membrane. Therefore, solubilisation of hydrogenase from the membrane by Triton X-400 is unlikely to disrupt the native conformation of the enzyme. The detergent-solubilised hydrogenase has subsequently been purified using ammonium sulphate precipitation, sucrose density gradient centrifugation and chromatography on hydroxyapatite. The overall yield of activity is 23%, with a final purification of over 100-fold.