The spectrophotometric assay for polyethoxy nonionic detergents in membrane extracts: a critique

Interaction of detergents with biological membranes: Comparison of fluorescence assays with filtration protocols and implications for the rates of detergent association, dissociation and flip-flop

The present study mainly consists of a re-evaluation of the rate at which C₁₂E₈, a typical non-ionic detergent used for membrane studies, is able to dissociate from biological membranes, with sarcoplasmic reticulum membrane vesicles being used as an example. Utilizing a brominated derivative of C₁₂E₈ and now stopped-flow fluorescence instead of rapid filtration, we found that the rate of dissociation of this detergent from these membranes, merely perturbed with non-solubilizing concentrations of detergent, was significantly faster (t_1/2 < 10 ms) than what had previously been determined (t_1/2 ~300–400 ms) from experiments based on a rapid filtration protocol using ¹⁴C-labeled C₁₂E₈ and glass fiber filters (Binding of a non-ionic detergent to membranes: flip-flop rate and location on the bilayer, by Marc le Maire, Jesper Møller and Philippe Champeil, Biochemistry (1987) Vol 26, pages 4803–4810). We here pinpoint a methodological problem of the earlier rapid filtration experiments, and we suggest that the true overall dissociation rate of C₁₂E₈ is indeed much faster than previously thought. We also exemplify the case of brominated dodecyl-maltoside, whose kinetics for overall binding to and dissociation from membranes comprise both a rapid and a sower phase, the latter being presumably due to flip-flop between the two leaflets of the membrane. Consequently, equilibrium is reached only after a few seconds for DDM. This work thereby emphasizes the interest of using the fluorescence quenching associated with brominated detergents for studying the kinetics of detergent/membrane interactions, namely association, dissociation and flip-flop rates.

Immobilized artificial membrane chromatography: rapid purification of functional membrane proteins

A solid-phase membrane mimetic system, denoted as immobilized artificial membranes (IAM), has been developed and utilized as a novel high-performance liquid chromatography (HPLC) matrix for the first step in the rapid purification of functional membrane proteins. IAM phases consist of monolayers of amphiphilic membrane lipid molecules covalently bonded to a rigid silica particle. These monolayers of lipids have proved remarkably effective for the chromatography of biomolecules. Several cytochrome P450 isozymes, an extremely important family of hydrophobic membrane proteins with a labile heme catalytic center, have been partially purified in functional conformations from rat liver, kidney, and adrenal microsomes on IAM supports. Functionality of purified P450 and P450 reductase has been demonstrated by optical difference spectroscopy, by carbon monoxide binding, and by reconstitution of enzymatic activity in vitro. Other membrane proteins, including rat liver plasma membrane NADH oxidase and ferricyanide oxidoreductase have also been partially purified by IAM HPLC. The methods for purification of these proteins are described.

Metabolism of diamantane by rat liver microsomal cytochromes P-450

1. Diamantane binds to liver microsomes from phenobarbital-treated rats with an apparent Ks value of 5.2 x 10(-7) mol/l. This value being lower than that obtained for perhydrophenanthrene indicates that diamantane is very strongly bound to microsomal cytochrome P-450. 2. Metabolic studies show that liver microsomes from phenobarbital-treated rats readily metabolize diamantane to mono-, di- and possibly tri-hydroxy derivatives, whereas liver microsomes from beta-naphthoflavone-induced rats do not bind this hydrocarbon or metabolize it. 3. Reconstituted cytochromes P-450 b and e were more efficient in the hydroxylation of diamantane than liver microsomes; metabolites formed by the reconstituted system do not include all the products formed by microsomes, which indicates the involvement of forms of cytochrome P-450 other than the isozymes b and e.

Chemical modification of the histidine residues of purified hepatic cytochrome P-450: influence on substrate binding and the haemoprotein spin state

A hepatic cytochrome P-450 isolated in an electrophoretically homogeneous form from phenobarbital-treated rats, exists predominantly in the low spin configuration (82% at 20 degrees C). The addition of saturating amounts of the substrate benzphetamine to this haemoprotein shifted the spin equilibrium to the high spin form, resulting in a doubling of the spin equilibrium constant from 0.220 to 0.539 at 20 degrees C. The histidine residues of this low spin, substrate-free cytochrome P-450 were modified in a time- and concentration-dependent manner with diethylpyrocarbonate, and progressive histidine modification resulted in a decrease of both the affinity and extent of substrate interaction with the haemoprotein. Although the histidine-modified haemoprotein maintained the capacity to undergo a temperature-dependent spin transition of the haem iron in the presence of saturating amounts of substrate, this capability was substantially decreased in comparison to the unmodified cytochrome. These results indicate that a histidine residue(s) is involved in the binding of substrate to cytochrome P-450 and hence interferes with the substrate-bound spin equilibrium. Our results further imply that histidine is probably not the sixth ligand of the substrate-free ferric form of the rat liver cytochrome P-450.

Purification and characterization of the rat liver microsomal cytochrome P-450 involved in the 4-hydroxylation of debrisoquine, a prototype for genetic variation in oxidative drug metabolism

Genetic polymorphism in oxidative drug metabolism is perhaps best exemplified in the case of debrisoquine 4-hydroxylase activity, where the incidence of deficient metabolism ranges from 1% to 30% in various populations and this defect is also linked to an impaired ability to metabolize a number of other drugs effectively. Sprague-Dawley (SD) rats possess this activity, but females of the DA strain do not, although total cytochrome P-450 (P-450) levels are similar. We have purified, by using debrisoquine 4-hydroxylase activity as an assay, a minor P-450 to electrophoretic homogeneity from male SD rats and designate this as P-450UT-H. P-450UT-H differs from eight other purified rat liver P-450s as judged by peptide mapping and immunochemical analysis and thus appears to be isozymic with these other P-450s. P-450UT-H exhibited considerably more debrisoquine 4-hydroxylase activity than any of the other purified P-450s and, on a total P-450 basis, more than total microsomal P-450. Antibodies raised against P-450UT-H specifically recognized P-450UT-H and inhibited more than 90% of the debrisoquine hydroxylase activity present in SD rat liver microsomes. The level of P-450UT-H in SD rat liver microsomes accounted for less than 10% of the total P-450, as judged by immunochemical quantitation. These assays also indicated that the level of P-450UT-H in female DA rat liver microsomes is only about 5% of that in male or female SD rat liver microsomes, consonant with the view that deficiency of this form of P-450 is responsible for the defective debrisoquine 4-hydroxylase activity in the former animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Regio-selectivity of purified forms of rabbit liver microsomal cytochrome P-450 in the metabolism of benzo(a)pyrene, n-hexane and 7-ethoxyresorufin

The specificity of electrophoretically homogeneous preparations of rabbit liver microsomal cytochrome P-450LM2-4 towards oxygenation of n-hexane, 7-ethoxyresorufin and benzo(a)pyrene was examined using a reconstituted system consisting of cytochrome P-450, NADPH-cytochrome P-450 reductase and dilauroylphosphatidylcholine. Epoxide hydrase was included when benzo(a)pyrene was used as substrate. Cytochrome P-450LM2 was most active in n-hexane and benzo(a)pyrene oxygenation especially with regard to the formation of 2-hexanol, B(a)P-4,5-dihydrodiol and B(a)P-phenol metabolites. 7-Ethoxyresorufin was, however, a very poor substrate for cytochrome P-450LM2. Cytochrome P-450LM3 had less activity towards the investigated substrates while cytochrome P-450LM4 preferentially formed 2- and 3-hexanol, resorufin and B(a)P-9,10-dihydrodiol. Cytochrome P-450LM4 isolated after pretreatment with 3-methylcholanthrene or phenobarbital showed roughly the same characteristics except in the formation of 1-hexanol where cytochrome P-450LM4 isolated after phenobarbital treatment was the most effective. The formation of B(a)P-4,5- and -9,10-dihydrodiols was greatly increased by incorporation of epoxide hydrase. Our results indicate a certain specificity of the different forms of cytochrome P-450 in the liver microsomes although some overlap in activities was observed.

Incorporation of purified components of the rabbit liver microsomal hydroxylase system into phospholipid vesicles

Methods are described for incorporation of purified forms of rabbit liver microsomal NADPH-cytochrome P-450 reductase and cytochrome P-450LM2, P-450LM3 and P-450LM4 (LM, liver microsomes) into phospholipid vesicles. It was found that each cytochrome could individually be incorporated into preformed phospholipid vesicles in the absence of cholate. However, NADPH-cytochrome P-450 reductase prevented incorporation of P-450 by this method, a phenomenon possibly inherent in the formation of complexes between P-450 and the reductase in solution. Using the cholate-gel filtration technique it was possible to prepare monolamellar phosphatidylcholine vesicles containing any of the cytochromes and P-450 reductase in good yields. It was found that P-450LM3-containing vesicles had a mean diameter of 47 nm, whereas vesicles formed under the same conditions but containing P-450LM4 were much smaller (mean diameter 33 nm). Vesicles formed with P-450LM2 were homogeneous in density (1.04 g/cm3) according to isopycnic centrifugation in Ficoll but not in size (44-72 nm). These findings, taken together with results obtained from treatment of the cytochromes in soluble form and in reconstituted vesicles with the non-penetrating reagent, p-diazobenzene sulphonate, indicate a unidirectional, relatively peripheral orientation of P-450LM4 with the major part localized on the outside of the vesicles. Experiments with trypsin and cytochrome c-reduction demonstrated a unidirectional orientation of P-450 reductase towards the outside of the vesicles.