Aggregation of deoxyhemoglobin subunits.

The formation of deoxyhemoglobin was examined by measuring the heme spectral change that accompanies the aggregation of isolated alpha and beta chains. At low hemeconcentrations (less than 10(-5) M), tetramer formation can be described by two consecutive, second order reactions representing the aggregation of monomers followed by the association of alphabeta dimers. At neutral pH, the rates of monomer and dimer aggregation are roughly the same, approximately 5 X 10(5) M(-1) X(-1) at 20 degrees. Raising or lowering the pH results in a uniform decrease of both aggregation rates due presumably to repulsion of positively charged subunits at acid pH and repulsion of negatively charged subunits at alkaline pH. Addition of p-hydroxymercuribenzoate to alpha chains lowers the rate of monomer aggregation whereas addition of mercurials to the beta subunits appears to lower both the rate of monomer and the rate of dimer aggregation. At high heme concentrations (greater than 10(-5) M) or in the presence of organic phosphates, the rate of chain aggregation becomes limited, in part, by the slow dissociation of beta chain tetramers. In the case of inositol hexaphosphate, the rate of hemoglobin formation exhibits a bell-shaped dependence on phosphate concentration. When intermediate concentrations of inositol hexaphosphate (approximately 10(-4 M) are preincubated with beta subunits, a slow first order time course is observed and exhibits a half-time of about 8 min. As more inositol hexaphosphate is added, the chain aggregation reaction begins to occur more rapidly. Eventually at about 10(-2) M inositol hexaphospate, the time course becomes almost identical to that observed in the absence of phosphates. The increase in the velocity of the chain aggregation reaction at high phosphate concentrations suggests strongly that inositol hexaphosphate binds to beta monomers and, if added in sufficiently large amounts, promotes beta4 dissociation. A quantitative analysis of these results showed that the affinity of beta monomers for inositol hexaphosphate is the same as that of alphabeta dimers. Only when tetramers are formed, either alpha2beta2 or beta4, is a marked increase in affinity for inositol hexaphosphate observed.     

Ligand-induced structural changes in amylose partially complexed with iodine

The influence of complexing agents such as methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, cyclohexanol and 2-octanol on the formation of a blue coloured amylose · iodine complex (pH 4.8), under suboptomum concentrations of iodine and in the absence of potassium iodide, is studied by recording the absorbance at 640 nm. A drop in absorbance at 640 nm accompanied by a blue shift in the spectrum (580–640 nm) was observed at higher concentration of the complexing agents. This behaviour of amylose partially complexed with iodine appears to be due to ligand-induced structural changes in the amylose chain. The fall in absorbance at 640 nm observed when the temeprature of amylose · oidine complex in the presence of complexing agents is raised, and the subsequent regeneration of the absorbance on cooling, indicates the possible helix to random coil transition of the amylose chain in an aqueous system.     

Chemical modification of Escherichia coli RNA polymerase by diethyl pyrocarbonate: evidence of histidine requirement for enzyme activity and intrinsic zinc binding

RNA polymerase (RPase) from Escherichia coli contains five subunits (alpha 2 beta beta' sigma) and two intrinsic Zn ions located in the beta and beta' subunits. This enzyme was rapidly inactivated by diethyl pyrocarbonate (DEP) at pH 6.0 and 25 degrees C. The difference spectrum of the DEP-inactivated and native RPases showed a single peak at 240 nm indicating the formation of N-carbethoxyhistidines. No decrease in absorbance at 278 nm, due to O-carbethoxytyrosine, or modification of amino and sulfhydryl groups was observed. Inactivated RPase with six to nine histidines being modified could be fully reactivated by incubation with 0.5 M hydroxylamine at pH 6.0 and room temperature for 1 h. No structural difference was detected between the native and modified enzymes as evidenced by UV/visible and fluorescence spectra, sodium dodecyl sulfate-polyacrylamide gel electrophoretic pattern, or gel filtration properties. Substrate ATP at 0.11 and 1.14 mM concentrations provided, respectively, 25% and 90% protection against DEP inactivation, while template DNA did not. These results suggest that one or more histidine residues is/are in close proximity to the substrate binding site. The pH dependence of the DEP inactivation of RPase suggested the modification of histidine at the active site with a pK value of 6.9. The inactivation of RPase by DEP and the formation of N-carbethoxyhistidine displayed a similar second-order rate constant of approximately 0.9 mM-1 min-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spectral and oxygen-release kinetic properties of human hemoglobin bound to the cytoplasmic fragment of band 3 protein in solution

Binding of the cytoplasmic fragment of band 3 protein to oxyhemoglobin in solution caused a spectral change in the absorbance of the hemoglobin beta chain at a ratio of one monomer of band 3 protein per alpha beta dimer of hemoglobin. This spectral change was reversed at higher ratios of cytoplasmic fragment to hemoglobin. The unusual dependence on protein concentration was interpreted as indicating the formation of higher aggregates of the complex between hemoglobin and the cytoplasmic fragment of band 3 protein. Oxygen-release kinetic measurements also showed marked changes as a function of the concentration of the cytoplasmic fragment of band 3 protein. The higher ratio mixture had significantly different kinetic properties as compared with the lower ratio one, which in turn was different from oxyhemoglobin in solution. The significance of the formation of aggregates of band 3 protein containing oxyhemoglobin dimers is discussed in context with evidence suggesting that band 3 protein may exist as an equilibrium mixture of tetramers and dimers in the membrane.     

Intrinsic fluorescence reports a global conformational change in the N-lobe of human serum transferrin following iron release

Transferrins have been extensively studied in order to understand how they reversibly bind and release iron. Human serum transferrin (hTF) is a single polypeptide chain that folds into two lobes (N- and C-lobe); each lobe binds a single ferric ion. Iron release induces a large conformational change in each lobe. At the putative endosomal pH of 5.6, measurement of the increase in intrinsic fluorescence upon iron release from the recombinant N-lobe yields two rate constants: 8.9 min-1 and 1.3 min-1. Direct monitoring of iron release from the N-lobe at pH 5.6 (by the decrease in absorbance at 470 nm) gives a single rate constant of 9.1 min-1, definitively establishing that the faster rate constant in the fluorescent studies is due to iron release. To further elucidate the molecular basis of the intrinsic fluorescence change (and the source of the slower rate constant), we examined the contributions of the three individual tryptophan residues in the N-lobe (Trp8, Trp128, and Trp264). Three double mutants, each containing the single remaining tryptophan residue, were produced. In the iron-bound N-lobe, Trp128 and Trp264 are quenched by iron and account for almost the entire fluorescent signal when iron is released. As for the wild-type N-lobe, the fluorescence increase for each of these mutants is best fit by a double-exponential function indicating two processes. Trp8 is severely quenched under all conditions, making virtually no contribution to the signal. Additionally, a mutant lacking all three Trp residues allows assignment of the fluorescent signal completely to the three tryptophan residues and observation of the presence of one (or more) tyrosinates in the N-lobe that have physiological significance in the uptake of iron.     

Characteristics of the methylammonium/ammonium transport systems of the nitrogen-fixing cyanobacterium Anabaena 7120 (ATCC 27893)

NH4(+)-transport in Anabaena 7120 was studied using the NH4+ analogue, 14CH3NH3+. At pH 7, two energy-dependent NH4(+)-transport systems were detected in both N2- and NO3(-)-grown cells, but none in NH4(+)-grown cells. Both transport systems showed a low and a high affinity mode of operation depending on the substrate concentration. One of the transport systems showed Km values of 8 microM (Vmax = 1 nmole min-1mg-1protein) and 80 microM (Vmax = 7 nmole min-1mg-1protein), and was insensitive to L-methionine-DL-sulphoximine, a glutamate analogue and irreversible inhibitor of glutamine synthetase. The other transport system showed Km values of 2.5 microM (Vmax = 0.1 nmole min-1mg-1protein) and 70 microM (Vmax = 0.7 nmole min-1mg-1protein), and was sensitive to L-methionine-DL-sulphoximine. Intracellular accumulation of free 14CH3NH3+ showed a biphasic pattern in response to variation in external 14CH3NH3+ concentrations. A maximum intracellular concentration of 2.5 mM and 7.5 mM was reached in the external 14CH3NH3+ concentration range of 1-50 microM and 1-500 microM, respectively. At pH 9, an energy-independent diffusion of 14CH3NH2 leading to a higher intracellular accumulation and assimilation rate, than that at pH 7, was observed.     

pH-induced hysteretic transitions of ovoperoxidase

Ovoperoxidase, the enzyme that catalyzes the dityrosine cross-linking of fertilization membranes of eggs from the sea urchin Stronglyocentrotus purpuratus, exhibits slow changes in catalytic activity upon alterations of pH, with attendant changes in spectral properties. For ovoperoxidase pre-equilibratated at pH 8, abrupt decreases in pH are accompanied by a slow loss in activity that is temporally associated with a change in absorbance at the Soret band. With enzyme pre-equilibrated at pH 4.5 and then shifted to higher pH, there was a slow increase in catalytic activity following a rapid change in the Soret band absorbance. These changes were reversible and led to the same equilibrium state, regardless of the direction of pH shift. The rate of approach to the equilibrium state of ovoperoxidase was independent of enzyme concentration, the presence of substrates, or temperature (from 6.5 to 39.7 degrees C). The pH-induced interconversions of catalytic and spectral properties indicate that ovoperoxidase undergoes hysteretic transitions, in which alterations in the heme environment accompany, but are not sufficient for, the expression of catalytic activity. We present a kinetic mechanism for the hysteretic relaxations and suggest how these transitions may have relevance to the assembly of the fertilization membrane in vivo.     

Anion transport in red blood cells and arginine-specific reagents. Interaction between the substrate-binding site and the binding site of arginine-specific reagents

Phenylglyoxal is found to be a potent inhibitor of sulfate equilibrium exchange across the red blood cell membrane at both pH 7.4 and 8.0. The inactivation exhibits pseudo-first-order kinetics with a reaction order close to one at both pH 7.4 and 8. The rate constant of inactivation at 37 degrees C was found to be 0.12 min-1 at pH 7.4 and 0.19 min-1 at pH 8.0. Saturation kinetics are observed if the pseudo-first order rate constant of inhibition is measured as a function of phenylglyoxal concentration. Sulfate ions as well as chloride ions markedly decrease the rate of inactivation by phenylglyoxal at pH 7.4, suggesting that the modification occurs at or near to the binding site for chloride and sulfate. The decrease of the rate of inactivation produced at pH 8.0 by chloride ions is much higher than that produced by sulfate ions. Kinetic analysis of the protection experiments showed that the loaded transport site is unable to react with phenylglyoxal. From the data it is concluded that the modified amino acid(s) residues, presumably arginine, is (are) important for the binding of the substrate anion.     

The kinetics of assembly of normal and variant human oxyhemoglobins

The kinetics of assembly have been monitored spectrophotometrically for normal and variant human oxyhemoglobins in 0.1 M Tris, 0.1 M NaCl, 1 mM Na2EDTA, pH 7.4, at 21.5 degrees C. Oxyhemoglobin versus oxy chain static difference spectra were performed and revealed subtle but significant absorption changes in both the visible and Soret regions. Kinetic experiments were performed by rapidly mixing equivalent (in heme) concentrations of alpha and beta A chains and following the change in absorbance at 583 nm with time. Over a protein concentration range of 10-100 microM in heme prior to mixing, these time courses were homogeneous and followed first-order kinetics, yielding a value of 0.069 s-1 for the apparent rate constant of dissociation of oxygenated beta A chain tetramers. Under these conditions, the overall assembly of oxyhemoglobins S (beta 6Glu----Val) and N-Baltimore (beta 95Lys----Glu) were also governed by the rates of dissociation of their respective oxygenated beta S and beta N-Baltimore chain tetramers with the apparent first-order rate constants of 0.044 and 0.15 s-1, respectively. In the Soret region, the alpha, beta monomer combination reaction could be observed if the protein concentration (heme basis) was lowered and if protein nonequivalency (beta chain exceeded alpha chain concentration) mixing experiments were performed. A kinetic oxyhemoglobin A, oxy-alpha, oxy-beta A monomer difference spectrum could be generated, and simple second-order kinetics were observed (415 nm) yielding rate constants of 2.3, 3.3, and 4.8 X 10(5) M-1 s-1 for the assembly of oxyhemoglobins S, A, and N-Baltimore, respectively. To our knowledge, this is the first kinetic study to reveal significant differences between the rate of association of alpha and beta monomers of hemoglobin A and those of two distinctly charged hemoglobin variants.     

Conformational change of the heme moiety of the ferrous cytochrome P-450scc-phenyl isocyanide complex upon binding of reduced adrenodoxin

Reduction of cytochrome P-450scc(SF) (SF, substrate free) purified from bovine adrenocortical mitochondria with sodium dithionite (Na2S2O4) or with beta-NADPH mediated by catalytic amounts of adrenodoxin and adrenodoxin reductase in the presence of phenyl isocyanide produced a ferrous cytochrome P-450scc(SF)-phenyl isocyanide complex with Soret absorbance maximum at 455 nm having a shoulder at 425 nm. On the other hand, when a preformed cytochrome P-450scc(SF)-adrenodoxin complex was reduced chemically or enzymatically under the same conditions, the absorbance spectrum showed drastic changes, i.e., an increase in intensity at 425 nm and a concomitant decrease in intensity at 455 nm. Similar spectral changes could be produced by addition of the same amount of reduced adrenodoxin afterward to the ferrous cytochrome P-450scc(SF)-phenyl isocyanide complex. Titration experiments with adrenodoxin showed that (1) a 1:1 stoichiometric saturation of the spectral change was obtained for both the absorbance increase at 425 nm and the absorbance decrease at 455 nm, (2) there was no spectral change in the presence of 0.35 M NaCl, and (3) there was no spectral change for cytochrome P-450scc(SF) whose Lys residue(s) essential to the interaction with adrenodoxin had been covalently modified with PLP. These results suggest that ternary complex formation of ferrous cytochrome P-450scc(SF)-phenyl isocyanide with reduced adrenodoxin caused a conformational change around the ferrous heme moiety. By analysis of temperature and pH dependencies of the spectral change of the ternary complex, it was suggested that this conformational change may reflect the essential step for electron transfer from reduced adrenodoxin to the ferrous-dioxygen complex of cytochrome P-450scc.     

Distinct effects of the C-terminal octapeptide of cholecystokinin and of a cholera toxin pretreatment of the kinetics of rat pancreatic adenylate cyclase activity

(1) The kinetic parameters of rat pancreatic adenylate cyclase were evaluated, using GTP, p[NH]ppG or GTP gamma S as nucleotide activator, cholecystokinin as peptide hormone, and GDP beta S and dibutyryl cyclic GMP as inhibitors of guanosine triphosphate and CCK-8, respectively. The time courses of activation and the degree of activation at steady state (EA/ETOT) were compatible with a simple two-state model of activation-deactivation based on a pseudo-monomolecular activation process (rate constant kappa+1), and a deactivation process (rate constant kappa off) that included, depending on the activating nucleotide, the hydrolysis of GTP (rate constant kappa 2) and/or the dissociation of the intact nucleotide (rate constant kappa-1), so that EA/ETOT = kappa+1/(kappa+1 + kappa 2 + kappa-1). (2) The hormone CCK-8 increased the value of kappa+1 with GTP dose-dependently, from 0.2 to 10.9 min-1. The value of kappa-1 increased 0.01 to 0.3 min-1 but the value of kappa 2 was unaltered at 7 min-1, so that EA/ETOT increased 15-fold, from 4% to 61%. (3) A cholera toxin pretreatment at 30 micrograms/ml allowed also a large increase in EA/ETOT with GTP (up to 51%) but the underlying mechanism was different. It consisted of a 14-fold decrease in the kappa off value of the GTP-activated enzyme (from 7 min-1 to 0.5 min-1) that corresponded to a reduction in GTPase activity. When testing the system with p[NH]ppG, two added effects of the cholera toxin pretreatment were observed: a 4-fold increase in the value of kappa+1 (from 0.2 to 0.8 min-1) and the occurrence of a significant 0.3 min-1 value for kappa-1.     

Determination of uranine tracer dye from underground water of Mecsek Hill, Hungary

The underground water system of Mecsek Hill, Hungary was studied using spectrofluorometry. A quantitative dye-tracing technique was used to map the connections between ground water recharge points and the springs and wells. Excitation-emission matrices were used to establish optimal spectroscopic parameters for spectral measurements. Only an EEM gives full spectrofluorimetric information about the sample and makes possible the determination of the optimal wavelength parameters. The uranine concentration was determined by measuring the synchronous excitation spectra of water samples. It was found that under the 4 microg/L uranine concentration, no special chemical treatment was necessary if the pH values remained between 7 and 8. However, above this concentration, the solution should be diluted to avoid spectral distortion from the increasing absorbance of the sample. Long-term concentration measurements were conducted based on spectrofluorometry.     

Absorbance spectral change of the cytochrome P-450S21-phenylisocyanide complex upon binding of reduced NADPH-cytochrome-P-450 reductase.

Reduction of cytochrome P-450S21 (SF) (SF, substrate-free; purified from bovine adrenocortical microsomes) with sodium dithionite (Na2S2O4) in the presence of phenylisocyanide produced a ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex with Soret absorbance maxima at 429 and 456 nm. On the other hand, when a preformed ferric cytochrome P-450S21 (SF)-NADPH-cytochrome-P-450 reductase (Fp2) complex was reduced chemically or enzymatically under the same conditions, the absorbance spectrum of the ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex changed drastically, as characterized by an increase in absorbance intensity at 429 nm and a decrease at 456 nm. Similar spectral changes were observed by addition of reduced Fp2 to the preformed ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex. Experiments to reduce a ferric cytochrome P-450S21 (SF)-phenylisocyanide complex with sodium dithionite in the presence of various amounts of Fp2 showed that; (1), the spectral change reached maxima for both absorption increase at 429 nm and decrease at 456 nm when cytochrome P-450S21 and Fp2 were previously mixed at the cytochrome P-450S21:Fp2 ratio of 1:5; (2), the spectral change was suppressed in 300 mM potassium phosphate buffer (pH 7.4). These results suggest that the absorbance spectral change is due to a conformational change around the heme moiety induced by association with reduced Fp2.     

High-resolution study of the helix–coil transition of poly(L-glutamic acid): Spectroscopic data correlation and the discovery of a new transition

This article reports on both (1) the precision and capability of a computerized multidimensional spectrophotometric system recently developed in our laboratory and (2) the high-resolution study of the helix–coil transition of poly(L-glutamic acid)[poly(Glu)], especially with regard to the discovery of an overlooked transition which is attributable to order–disorder rearrangement of the poly(Glu) side chain in the α-helical conformation. This study was made possible by the high performance of the system used. The simultaneous and continuous measurement of the circular dichroism, the absorbance and light-scattering intensity, and the pH titration curve of poly(Glu) in aqueous salt solution was carried out under continuous scanning of pH ranging from 8 to 2. Besides the well-known random coil to α-helix transition that occurs at about pH 5.5, a highly cooperative transition, which is indicated as a small but definite step in several spectral dimensions, is observed for the first time at pH 4.3. The transition is ascribed to an order–disorder conversion of the side chain on the α-helix backbone.     

Reactivity and pH dependence of thiol conjugation to N-ethylmaleimide: detection of a conformational change in chalcone isomerase

The reactivity of simple alkyl thiolates with N-ethylmaleimide (NEM) follows the Br?nsted equation, log kS- = log G + beta pK, with G = 790 M-1 min-1 and beta = 0.43. The rate constant for the reaction of the thiolate of 2-mercaptoethanol with NEM is 10(7) M-1 min-1, whereas the rate constant for the reaction of the protonated thiol is less than 0.0002 M-1 min-1. The intrinsic reactivity of the protonated thiol (SH) is over (5 X 10(10]-fold less than the thiolate (S-) and makes a negligible contribution to the reactivity of thiols toward NEM. The rate of NEM modification of chalcone isomerase was conveniently measured by following the concomitant loss in enzymatic activity. The pseudo-first-order rate constants for inactivation show a linear dependence on the concentration of NEM up to 200 mM and yield no evidence for noncovalent binding of NEM to the enzyme. Evidence is presented demonstrating that the modification of chalcone isomerase by NEM is limited to a single cysteine residue over a wide range of pH. Kinetic protection against inactivation and modification by NEM is provided by competitive inhibitors and supports the assignment of this cysteine residue to be at or near the active site of chalcone isomerase. The pH dependence of inactivation of the enzyme by NEM indicates a pK of 9.2 for the cysteine residue in chalcone isomerase. At high pH, the enzymatic thiolate is only (3 X 10(-5))-fold as reactive as a low molecular weight alkyl thiolate of the same pK, suggesting a large steric inhibition of reaction on the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)     

The use of 6-(difluoromethyl)indole to study the activation of indole by tryptophan synthase

6-(Difluoromethyl)indole has been characterized and developed as a probe for the turnover of indole by the bifunctional enzyme, tryptophan synthase (alpha 2 beta 2). The neutral form of the indolyl species undergoes a slow and spontaneous hydrolysis to produce 6-formylindole with a rate constant (k1) of 0.0089 +/- 0.0001 min-1. The overall rate is independent of pH in the range of 3.5-10.5. Above pH 10.5, the observed rate increases are due to the high reactivity of the anionic form of the indole; deprotonation at N-1 accelerates hydrolysis by 10(4)-fold (k2, 97 +/- 2 min-1). The magnitude of this effect provides a technique for detecting the formation or stabilization of the anionic form of indole. 6-(Difluoromethyl)indole is recognized and processed by the beta subunit of tryptophan synthase. Selective inactivation of the beta subunit prevents enzymatic processing of 6-(difluoromethyl)indole. Chromatographic isolation and mass spectral analysis has identified 6-(difluoromethyl)tryptophan as the sole turnover product of the indolyl substrate. The lack of enzyme-promoted dehalogenation does not exclude the formation of an indole anion during turnover but rather the data suggest that rapid carbon-carbon bond formation (greater than 5300 min-1) prevents the accumulation of this anion.     

Tryptic digestion of dynein 1 in low salt medium. Origin and properties of fragment A

Dynein 1 was extracted from sperm flagella of the sea urchin Tripneustes gratilla with 0.6 M NaCl and dialyzed against 0.5 mM EDTA, 14 mM 2-mercaptoethanol, 5 mM imidazole/HCl buffer, pH 7.0, for 24-48 h. In some cases, fractions containing the alpha heavy chain and the beta/intermediate chain 1 complex (beta/IC1) were separated by density gradient centrifugation in the same solution. Treatment of the samples at a trypsin:protein ratio of 1:10 w/w for 32 min at room temperature yields a crude digest from which Fragment A is purified by density gradient centrifugation. The purified Fragment A consists of two principal peptides (Mr = 195,000 and 130,000) that cosediment with the peak of ATPase activity at 12.5 S, which is slightly faster than the 11 S of the original beta/IC1 complex. When digests of the separated alpha chain and of the beta/IC1 complex are followed as a function of time, the early cleavages of the two heavy chains (Mr = 428,000) resemble each other in that both lead to similarly sized peptides of Mr 316,000 and 296,000, but only in the beta/IC1 fraction does the digestion proceed to form Fragment A. The remainder of the beta chain, termed Fragment B, occurs as an Mr 110,000 peptide sedimenting at 5.7 S with no associated ATPase activity. Fragment A has a specific ATPase activity of 4.3 mumol Pi X min-1 X mg-1, with a Km of 29 microM in 0.1 M NaCl medium, and an apparent Ki for inhibition by vanadate of 1.2 microM in the absence of salt, and 22 microM in 0.6 M NaCl. Photoaffinity labeling with [alpha-32P]8-azidoadenosine 5'-triphosphate indicates that the ATP binding site on the beta chain of dynein 1 is located on the Mr 195,000 peptide of Fragment A. The possibility that Fragments A and B of the beta/IC1 complex may correspond to the head and tail regions of the tadpole-shaped particle seen by electron microscopy is discussed.     

Kinetics of electron entry, exit, and interflavin electron transfer during catalysis by sarcosine oxidase.

Sarcosine oxidase contains 1 mol of covalently bound plus 1 mol of noncovalently bound FAD per active site. The first phase of the anaerobic reduction of the enzyme with sarcosine converts oxidized enzyme to an equilibrium mixture of two-electron-reduced forms (EH2) and occurs at a rate (2700 min-1, pH 8.0) similar to that determined for the maximum rate of aerobic turnover in steady-state kinetic studies (2600 min-1). The second phase of the anaerobic half-reaction converts EH2 to the four-electron-reduced enzyme (EH4) and occurs at a rate (k = 350 min-1) which is 7-fold slower than aerobic turnover. Reaction of EH2 with oxygen is 1.7-fold faster (k = 4480 min-1) than aerobic turnover and 13-fold faster than the anaerobic conversion of EH2 to EH4. The results suggest that the enzyme cycles between fully oxidized and two-electron-reduced forms during turnover with sarcosine. The long wavelength absorbance observed for EH2 is attributable to a flavin biradical (FADH.FAD.-) which is generated in about 50% yield at pH 8.0 and in nearly quantitative yield at pH 7.0. The rate of biradical formation is determined by the rate of electron transfer from sarcosine to the noncovalent flavin since electron equilibration between the two flavins (k = 750 s-1 or 45,000 min-1, pH 8.0) is nearly 20-fold faster, as determined in pH-jump experiments. Only two of the three possible isoelectronic forms of EH2 are likely to transfer electrons to oxygen since the reaction is known to occur at the covalent flavin. However, equilibration among EH2 forms is probably maintained during reoxidation, consistent with the observed monophasic kinetics, since interflavin electron transfer is 10-fold faster than electron transfer to oxygen.     

Purification and characterization of cytochrome P450 isozymes from beta-naphthoflavone-induced adult hen liver

Cytochromes P450 beta NF-A, beta NF-B, and beta NF-C were purified from beta-naphthoflavone-treated adult hens. Cytochrome P450 beta NF-A, however, appeared at two places in the purification scheme. They were designated as cytochromes P450 beta NF-A1 and beta NF-A2 for property comparison. The cytochromes beta NF-A1 and beta NF-A2 were induced by both phenobarbital and beta-naphthoflavone treatment and were similar to P450 PB-A (previously purified from phenobarbital-induced hen livers) in molecular weights, isoelectric pH, spectral properties, behavior on chromatography columns, catalysis of substrates, immunological cross-reactivity on Ouchterlony plates and by immunoblotting, and NH2-terminal amino acid sequence. However, P450 PB-A differed from beta NF-A1/beta NF-A2 in peptide pattern after partial proteolysis by alpha-chymotrypsin and Staphylococcus aureus V8 protease, and complete digestion of 125I-labeled cytochromes by trypsin. The cytochrome P450 PB-A also differed from beta NF-A1/beta NF-A2, in that its antibodies cross-reacted with P-450 of normal, PB-, and beta-NF-induced rabbit liver microsomes. The cytochromes beta NF-B and beta NF-C, although immunochemically cross-reactive with each other, were distinct enzymes on the basis of molecular weights, spectral characteristics, isoelectric pH, peptide pattern on partial proteolysis, tryptic peptide pattern, cross-reactivity of their antibodies with other species, and NH2-terminal amino acid sequence. The most notable difference between beta NF-B and beta NF-C was that the anti-beta NF-C IgG completely inhibited O-dealkylation of 7-methoxyresorufin and 7-ethoxyresorufin by beta-NF-induced microsomes. These activities increased 40- to 50-fold in beta-NF-induced microsomes as compared to only 2- to 4-fold in PB-treated hens. The amino-terminal sequences of beta NF-B and beta NF-C were different from those of mammalian and other nonmammalian species.     

Detection and identification of intermediates in the reaction of L-serine with Escherichia coli tryptophan synthase via rapid-scanning ultraviolet-visible spectroscopy

Rapid-scanning stopped-flow (RSSF) UV-visible spectroscopy has been used to investigate the UV-visible absorption changes (300-550 nm) that occur in the spectrum of enzyme-bound pyridoxal 5'-phosphate during the reaction of L-serine with the alpha 2 beta 2 and beta 2 forms of Escherichia coli tryptophan synthase. In agreement with previous kinetic studies [Lane, A., & Kirschner, K. (1983) Eur. J. Biochem. 129, 561-570], the reaction with alpha 2 beta 2 was found to occur in three detectable relaxations (1/tau 1 greater than 1/tau 2 greater than 1/tau 3). The RSSF data reveal that during tau 1, the internal aldimine, E(PLP), with lambda max = 412 nm (pH 7.8), undergoes rapid conversion to two transient species, one with lambda max congruent to 420 nm and one with lambda max congruent to 460 nm. These species decay in a biphasic process (1/tau 2, 1/tau 3) to a complicated final spectrum with lambda max congruent to 350 nm and with a broad envelope of absorbance extending out to approximately 525 nm. Analysis of the time-resolved spectra establishes that the spectral changes in tau 2 are nearly identical with the spectral changes in tau 3. Kinetic isotope effects due to substitution of 2H for the alpha-1H of serine were found to increase the amount of the 420-nm transient and to decrease the amount of the species with lambda max congruent to 460 nm. These findings identify the serine Schiff base (the external aldimine) as the 420 nm absorbing, highly fluorescent transient; the species with lambda max congruent to 460 nm is the delocalized carbanion (quinoidal) species derived from abstraction of the alpha proton from the external aldimine. The reaction of L-serine with beta 2 consists of two relaxations (1/tau 1 beta greater than 1/tau 2 beta) and yields a quasi-stable species with lambda max = 420 nm, in good agreement with a previous report [Miles, E. W., Hatanaka, M., & Crawford, I. P. (1968) Biochemistry 7, 2742-2753]. Analysis of the RSSF spectra indicates that the same spectral change occurs in each phase of the reaction. The similarity of the spectral changes that occur in tau 2 and tau 3 of the alpha 2 beta 2 reaction is postulated to originate from the existence of two (slowly) interconverting forms of the enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)