Direct evidence for electron transfer from ferrous cytochrome b5 to the oxyferrous intermediate of liver microsomal cytochrome P-450 LM2

Interaction and electron transfer between highly purified microsomal cytochrome P-450 from phenobarbital-induced rabbits and cytochrome b5 from uninduced rabbits was investigated by difference and stopped-flow spectrophotometry. Formation of a 1:1 complex between ferric P-450 and b5, demonstrated by difference spectrophotometry, was observed only when both cytochromes were incorporated into micelles of phosphatidylcholine. The dissociation constant (Kd) of the complex was decreased from 2.3 microM to 0.4 microM in the presence of 1 mM benzphetamine. The apparent Kd of benzphetamine was reduced from 220 microM to 50 microM upon addition of b5. The influence of ferrous b5 on the autooxidation of the oxyferrous intermediate of P-450 in the presence and in the absence of substrate was investigated by stopped-flow spectrophotometry. Both cytochromes were reduced photochemically, so that experiments could be carried out in the absence of the corresponding reductases and reduced pyridine nucleotides. Kinetic analysis of the data showed that formation of a 1:1 complex between ferrous P-450 and b5 was a prerequisite for electron transfer between the cytochromes. Here again, incorporation of both cytochromes into micelles was absolutely required for this process. Kd was decreased from 7.5 microM to 2.2 microM in the presence of 1 mM benzphetamine. The rate of electron transfer from b5 to oxyferrous P-450, derived from the kinetics of reoxidation of either b5 or P-450, was increased from 2.5 s-1 to about 4 to 7 s-1 in the presence of 1 mM benzphetamine. These results provide the first quantitative data on the electron transfer between the b5 and P-450, whose rate constant is compatible with the observations made on the effect of b5 on hydroxylation reactions catalyzed by the P-450 enzyme system.

Fast photochemical reactions of cytochrome P450 at subzero temperatures

Several reactions of the cytochrome P450 multi-step cycle have been studied by fast light activation combined with subzero temperatures. A flash device was adapted to an Aminco-Chance DW 2 spectrophotometer equipped for subzero temperature thermostatisation. The first electron can be introduced into the cycle by non specific reducing agents such as reduced flavin mononucleotide (FMNH2) or methylviologen radical (MV.). This first reduction remains a fast process even at subzero temperatures. The oxy-compound Fe2+-O2 can thus be formed either directly from Fe2+ or via the photodissociation of the carboxy-ferro adduct. Fe2+-O2 is stable at subzero temperatures towards spontaneous autoxidation as well as further reduction by FMNH2 or MW.. In addition, the recombination of CO after flash photodissociation of Fe2+-CO was used to study in more details the specific behaviors of the purified microsomal cytochrome.

Enzyme behaviour and molecular environment. The effects of ionic strength, detergents, linear polyanions and phospholipids on the pH profile of soluble cytochrome oxidase

The activity vs. pH profile for the oxidation of ferrocytochrome c by purified cytochrome oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) was investigated as a function of ionic strength (from 10 to 200 mM) in the absence and in the presence of various perturbants: Tween 20, linear polyanions (RNA, heparin, polyglutamic acid) and phospholipids (asolectin, phosphatidylcholine, phosphatidic acid and cardiolipin). The activation induced by Tween 20 and "zero net charge" phospholipid liposomes was not pH dependent. On the other hand, linear polyanions and polyanionic liposomes strongly perturbed the pH profile, mostly at low ionic strength, by shifting the pH optimum about 1.7 pH units towards alkaline pH values. This effect was reversed by increasing ionic strength. These observations are interpreted in the light of polyelectrolyte theory. Since these results show striking with membrane-bound enzyme, it is concluded that in vivo cytochrome oxidase is located within polyanionic sites of the micochondrial membrane. The activation broght about by phospholipids may result from two posible processes: creation of a hydrophobic environment by the non-polar tails, preventing autoaggregation; and creation of a suitable polyelectrolytic environment by the polar heads (of non zero net charge), increasing the intrinsic reaction rate.

Redox potentials in hydro-organic media at normal and subzero temperatures. Ferro-ferricyanide and cytochrome c as models

Redox potentials of ferro-ferricyanide and cytochrome c were measured in water/ethylene glycol and water/dimethylsulfoxide (volume ratio from 100/0 to 50/50) between 25 and -25 degrees C. For both systems, the midpoint potential decreases in the presence of organic solvents and increases by cooling. The magnitude of these variations is larger in dimethylsulfoxide than in ethylene glycol; moreover in the same solvent mixture it is larger with ferro-ferricyanide than with cytochrome c, so that the difference between the redox potentials of these two systems can be strongly affected and even reversed. While in pure water (cacodylate buffer pH 7.0, NaCl 0.1 M) they are respectively +388 and +265 mV, in 50% dimethylsulfoxide at 25 degrees C they decrease to +112 and +208 mV. Reduction of cytochrome c by ferro-ferricyanide, in this mixture, is then expected and was indeed observed. On the other hand, as (deltaE/deltaT)T, (E being the redox potential) is higher for ferro-ferricyanide than for cytochrome c, the oxidative power of the former for the latter is expected to increase as temperature decreases. This effect was observed in 50% ethylene glycol at -16 degrees C. Organic solvents and large temperature variations appear then as powerful perturbants of redox reactions. Their effects should be taken into account in studies of redox reactions carried out in cooled hydro-organic media.

Ionic regulation in genetic translation systems

The polyelectrolyte theory can provide an interpretation of the interdependence of pH, ionic strength, and polyamines one observes in the activity of ribonuclease acting on RNA. According to this theory: (i) A nucleic acid-enzyme complex and the suspending medium may be considered as two phases in equilibrium, even though within limits, the complex is soluble in water. (ii) The enzymatic catalysis is under tight control of the electrostatic potential generated by the system. Consequently, modification in electrostatic potential will induce a concomitant change in activity. (iii) The electrostatic potential can be modified through action on the system of "modulators", either "external" (ionic strength, pH, temperature, etc.) or "internal" (specific ligands, substrates, protein factors, etc.). Similarities between the reaction of ribonuclease (ribonuclease 3'-pyrimidino-oligonucleotidohydrolase; EC 3.1.4.22) and RNA and those observed with highly organized systems catalyzing DNA, RNA, and protein synthesis suggest that the electrostatic potential also provides an important regulatory mechanism in genetic translation. In this view, an essential function of nucleic acids is to provide their enzyme partners with polyanionic microenvironments within which their catalytic activities are controlled by variation in physicochemical parameters, including the proton concentration induced through "modulation" of the local electrostatic potential.

[Ionic control of biochemical reactions]

It is shown that pH and ionic strength are tightly interdependent in cytochrome oxidase activity at the level of inner membranes of the mitochondrion, as a direct consequence of the polyanionic environment of this enzyme. Application of polyelectrolyte theory explains a number of biochemical reactions controlled by ionic strength fluctuations.

Solvent-temperature perturbations of ionizable groups as a tool for the investigation of the active site of enzymes

Co-solvent and temperature effects on the pK of histidine (imidazolium) residue 46 of trypsin, as well as of weak electrolytes (buffers), which have been reported in two preceding papers, can be satisfactorily explained in terms of enthalpy-entropy compensation patterns. Such patterns have been generated for various mixed solvents between 20 degrees and minus 20 degrees and minus 50 degrees. Under these conditions compensation temperature, T-c, is strongly dependent on the nature of the ionizable group studied: 240 plus or minus 10 K for neutral acids and 310 plus or minus 5 K for cationic acids. This work focuses on the possibilities offered and on the problems raised by the use of this methodology as a tool in the investigation of the active site of enzymes. Furthermore, it is shown in the case of histidine residue 46 of trypsin that the co-solvent effect vanishes at the compensation temperature, a result of great practical significance if applicable to any ionizable group at the active site of enzymes.

The pH dependence of the hydrolysis of benzoyl-L-arginine ethyl ester in cooled mixed solvents

Tables of protonic activity (paH) of a number of buffers, determined in mixed solvents and at subzero temperatures, are reported for the following media: water-1,2-propanediol, water-glycerol, and water-dimethylsulfoxide (50:50, in volume). These data with those previously reported allowed us to study enzymic reactions under these conditions. The paH dependence of the tryptic hydrolysis of benzoyl-L-arginine ethyl ester has been studied in the presence of organic solvents (methanol, ethylene glycol, 1,2-propanediol, glycerol, and dimethylsulfoxide, all 50% by volume) between 20 and -20 degrees. The results have allowed us to show the validity of our paH scales in mixed solvents. The paH profiles obtained under these conditions are similar to those observed in pure water at 20 degrees. They are shifted nevertheless by both solvent and temperature. Such shifts are interpreted in terms of the effects of solvents and temperature on pKES on the basis of the conclusions drawn from a study of the effect of these variables on small dissociable molecules. The results obtained under these conditions of solvents and temperature are consistent with the presence at the active site of the enzyme of a histidine residue, and thus provide, concerning the solvent effect, a direct verification of the method of Findlay et al. (Findlay, D., Mathias, A. P., and Rabin, B. R. (1962) Biochem. J. 85, 139-144). On the other hand, the large temperature interval provided by the low temperature procedure, allows us to vary significantly the pK of ionizable groups of the enzymes and thus makes possible their identification, on the basis of their enthalpy of ionization.

Dipole moment of acetylcholine and its relevance to the chemical synaptic transmission

The dipole moment of acetylcholine (AcCh) has been measured in chloroform and a value of 8.49 D was obtained. Such a value actually represents the total dipole moment of the ion pair (AcCh)(+)(Cl)(-). The dipole moment of the (AcCh)(+) cation alone turned out to be 2.65 D whereas its theoretical value obtained after a vectorial calculation was 1.65 D. The discrepancy was related to an interaction between AcCh and the solvent. The meaning of the measured value is discussed on the basis of a recent theory of chemical synaptic transmission based on the assumption of a much higher dipole moment value for the AcCh molecule.