A heme binding site on myelin basic protein: characterization, location, and significance

Myelin basic protein (MBP), an extrinsic membrane protein from the myelin sheath, binds dicyanohemin. The binding generates absorption bands in the Soret region and quenches the fluorescence emitted by the sole tryptophan residue. The absorption titration curves in the Soret demonstrate that the binding is stoichiometric, one heme per protein, with a large value of the extinction coefficient (8 X 10(4) M-1 cm-1 at 420 nm). Fluorescence quenching titration curves indicate an identical stoichiometry and a low quenching efficiency of 20%. From the heme titration curve the association constant between dicyanohemin and MBP is estimated to be greater than or equal to 10 nM-1 in 50 mM 4-morpholinepropanesulfonic acid buffer, pH 7.0, at 20 degrees C. Digestion of MBP by Staphylococcus aureus V8 protease yields a peptide (38-118) whose heme binding properties are identical to those of MBP. In contrast, peptides obtained by digestion of MBP with cathepsin D do not exhibit any specific binding of dicyanohemin. The cleavage of the Phe-Phe (42-43) bond appears to be critical in this respect. A comparison of the sequence immediately preceding, including these residues with a probable heme binding site of a mitochondrial cytochrome b, reveals a high degree of homology. The possible significance of heme binding is discussed.     

Fluorescence quenching of the buried tryptophan residue of cod parvalbumin

Cod parvalbumin (isotype III) is a single tryptophan-containing protein. The fluorescence characteristics of this tryptophan residue (lambda em approximately 315 nm) suggest that it is buried from solvent and that it is located in an apolar core of the protein. Solute quenching studies of the tryptophan fluorescence of parvalbumin reveal dynamic quenching rate constants, kq, of 1.1 X 10(8) and 2.3 X 10(9) M-1 s-1 (at 25 degrees C) with acrylamide and oxygen, respectively, as quenchers. From temperature dependence studies, activation energies of 6.5 +/- 1.5 and 6.0 +/- 0.5 kcal/mol are found for acrylamide and oxygen quenching. The kq for acrylamide quenching is found to be relatively unchanged (+/- 10%) by an 8-fold increase in the bulk viscosity (glycerol/water mixture). These temperature and viscosity studies argue that the acrylamide quenching process involves a dynamic penetration of the quencher, facilitated by fluctuations in the protein's structure.     

Human placental alkaline phosphatase. An improved purification procedure and kinetic studies.

An improved method for the purification of human placental alkaline phosphatase is described. The partially purified enzyme from Sigma was further purified by successive Concanavalin A-Sepharose and Q-Sepharose chromatography. The whole procedure may be completed in one working day. Highly purified enzyme was obtained with a 39% yield. The intrinsic fluorescence of the enzyme decreased at elevated temperature. The conformation of the enzyme molecule was studied by the fluorescence quenching technique. Upward Stern-Volmer plots were obtained for the quenching data which suggested that, in addition to collisional quenching, static quenching was involved in the quenching mechanism. The dynamic and static quenching constants were found to be 0.7 +/- 0.16 M-1 and 0.44 +/- 0.1 M-1, respectively, using acrylamide as the quenching agent. The corresponding values were 0.43 +/- 0.23 M-1 and 0.84 +/- 0.18 M-1, respectively, with KI as the quenching agent. Mg2+ and PO4(3-) induced protein conformational changes which altered both the dynamic and static quenching constants. Mg2+ was found to be a non-essential activator for the placental alkaline-phosphatase-catalyzed hydrolysis of 4-nitrophenyl phosphate. At pH 9.8, Mg2+ increased Vmax by 1.2-fold without affecting the Kd of the substrate. The tetranitromethane-modified enzyme showed slower migration toward the anode on electrophoresis and increased Kd for Mg2+.     

Detection of ligand-induced conformation changes in lactate dehydrogenase by using fluorescent probes

The binding of ANS to apolactate dehydrogenase (apo-LDH) is accompanied by a 300-fold increase in dye fluorescence with a shift of the emission maximum from 515 to 479 nm, as well as by quenching of intrinsic protein fluorescence. A tetrameric LDH molecule has 6.4 +/- 1.6 non-interacting dye-binding sites with an association constant equal to (4.3 +/- 1.6) X 10(3) M-1. NAD+ added at saturating concentrations does not alter the number of ANS binding sites or the association constant value. The formation of binary LDH.NAD+, LDH.NADH, LDH.AMP and LDH.pyruvate complexes causes the quenching of fluorescence of the enzyme-bound ANS. The extent of quenching observed at ligand saturating concentrations differs for each ligand. Pyruvate added to the binary LDH.AMP complex exerts no effect on the fluorescence of protein-bound ANS; this indicates that the binding of AMP causes some alterations in the microenvironment of the substrate-binding site. Nicotinamide mononucleotide (NMN) can act as a coenzyme in the LDH-catalyzed reaction. AMP added together with NMN displays an inhibitory effect. The cationic (auramine O) and anionic (ANS) fluorescent probes bound to LDH exhibit different responses to conformational changes accompanying the transition from the apoenzyme to the LDH X NAD-pyruvate complex.     

Detection, characterization, and quenching of the intrinsic fluorescence of bovine heart cytochrome c oxidase

The intrinsic fluorescence of lauryl maltoside solubilized bovine heart cytochrome c oxidase has been determined to arise from tryptophan residues of the oxidase complex. The magnitude of the fluorescence is approximately 34% of that from n-acetyltryptophanamide (NATA). This level of fluorescence is consistent with an average heme to tryptophan distance of 30 A. The majority of the fluorescent tryptophan residues are in a hydrophobic environment as indicated by the fluorescence emission maximum at 328 nm and the differing effectiveness of the quenching agents: Cs+, I-, and acrylamide. Cesium was ineffective up to a concentration of 0.7 M, whereas quenching by the other surface quenching agent, iodide, was complex. Below 0.2 M, KI was ineffective whereas between 0.2 and 0.7 M 15% of the tryptophan fluorescence was found to be accessible to iodide. This pattern indicates that protein structural changes were induced by iodide and may be related to the chaotropic character of KI. Acrylamide was moderately effective as a quenching agent of the oxidase fluorescence with a Stern-Volmer constant of 2 M-1 compared with acrylamide quenching of NATA and the water-soluble enzyme aldolase having Stern-Volmer constants of 12 M-1 and 0.3 M-1, respectively. There was no effect of cytochrome c on the tryptophan emission intensity from cytochrome c oxidase under conditions where the two proteins form a tight, 1:1 complex, implying that the tryptophan residues near the cytochrome c binding site are already quenched by energy transfer to the homes of the oxidase. The lauryl maltoside concentration used to solubilize the enzyme did not affect the fluorescence of NATA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of calf intestinal alkaline phosphatase with 8-anilinonaphthalene-1-sulphonate

Calf intestinal alkaline phosphatase is inhibited by 8-anilinonaphthalene-1-sulphonate (ANS). The inhibition is uncompetitive but non-linear. Hill plots of the inhibition data have slopes of 1.4-1.8 suggestive of positive cooperativity. Fluorescence titration revealed that 2 molecules of ANS bind per molecule of enzyme with no evidence of cooperativity. The Kd for ANS obtained by fluorescence was 1.8 X 10(-6) mol/l but the approximate Ki for inhibition was 1 X 10(-3) mol/l. Thus, the fluorescence and kinetic experiments appear to monitor different events.     

Tropomyosin-troponin and tropomyosin-actin interactions: a fluorescence quenching study

Rabbit skeletal alpha alpha-tropomyosin was specifically labeled at Cys-190 with the fluorescent probe N-(iodoacetyl)-N'-(1-naphthyl-5-sulfo)ethylenediamine (1,5-IAE-DANS). The fluorescence decay of the resultant AE-DANS-labeled alpha alpha-tropomyosin (Tm) was monoexponential with a lifetime of 13.55 ns. When acrylamide was used as the quencher, the apparent Stern-Volmer quenching constant Ksv' for Tm was measured to be 5.78 M-1 and the quenching rate constant kq to be 3.20 X 10(8) M-1 s-1. The presence of troponin reduced the magnitude of Ksv' to 4.14 M-1 and induced the appearance of a second decay component. This second component had an amplitude of approximately 20% of the total intensity, a lifetime of approximately 20 ns, and a kq of 4.5 X 10(-7) M-1 s-1. Similarly, the presence of F-actin induced the appearance of a minor longer lived decay component with a decreased kq. On the basis of the increase in the lifetime and the decrease in kq, the appearance of the long-lived decay component was interpreted to be due to troponin or actin interacting with Tm near the Cys-190 site in both cases. Our results further suggest that the label was capable of equilibrating between an exposed hydrophilic environment on the surface of Tm and a buried hydrophobic environment at the troponin-Tm or actin-Tm interaction interfaces.     

Characteristics of murine protoporphyrinogen oxidase.

Protoporphyrinogen oxidase (EC 1.3.3.4) (PPO) is the penultimate enzyme of the heme biosynthetic pathway. Mouse PPO has been purified in low yield and kinetically characterized by this laboratory previously. A new more rapid purification procedure is described herein, and with this protein we detect a noncovalently bound flavin moiety. This flavin is present at approximately stoichiometric amounts in the purified enzyme and has been identified by its fluorescence spectrum and high performance liquid chromatography as flavin mononucleotide (FMN). Fluorescence quenching studies on the flavin yielded a Stern-Volmer quenching constant of 12.08 M-1 for iodide and 1.1 M-1 for acrylamide. Quenching of enzyme tryptophan fluorescence resulted in quenching constants of 6 M-1 and 10 M-1 for iodide and acrylamide, respectively. Plasma scans performed on purified enzyme preparations did not reveal the presence of stoichiometric amounts of protein-bound metal ions, and we were unable to detect any protein-associated pyrroloquinoline quinone (PQQ). Data from circular dichroism studies predict a secondary structure of the native protein consisting of 30.5% alpha helix, 40.5% beta sheet, 13.7% turn, and 15.3% random coil. Denaturation of PPO with urea resulted in a biphasic curve when ellipticity is plotted against urea concentration, typical of amphipathic proteins.     

Carbohydrate-binding proteins of human serum: isolation of two mannose/fucose-specific lectins

Two lectins with specificities for mannose and fucose have been isolated from human serum by affinity chromatography. One mannose-binding protein (MBP 1) has a native Mr of 700,000 with subunits of Mr 32,000 and has specificities for N-acetylglucosamine, N-acetylmannosamine and glucose as well as for mannose and fucose. The other mannose-binding protein (MBP 2) has a native Mr of 200,000 with subunits of Mr 28,000 and is specific only for mannose and fucose. MBP 2 appears to recognize the core sugars of asparagine-linked oligosaccharides as well as the terminal sugars. Both lectins are calcium-dependent, requiring approx. 0.095 mM calcium for half-maximal binding. MBP 1 binds maximally between pH 7-9, whereas MBP 2 has a pH optimum of 6-7. The binding activity of both proteins decreases rapidly below pH 5. The apparent association constants (Ka) for binding to mannon are 2.1 X 10(8) M-1 for MBP 1 and 1.3 X 10(8) M-1 for MBP 2. These data provide further evidence of the complex nature of mammalian carbohydrate recognition systems.     

Detection of Tryptophan to Tryptophan Energy Transfer in Proteins

Förster resonance energy transfer (FRET) studies usually involve observation of intensity or life-time changes in the donor or acceptor molecule and usually these donor and acceptor molecules differ (heterotransfer). The use of polarization to monitor FRET is far less common, although it was one of the first methods utilized. In 1960, Weber demonstrated that homotransfer between tryptophan molecules contributes to depolarization. He also discovered that the efficiency of homotransfer becomes much less effective upon excitation near the red-edge of the absorption. This “red-edge effect” was shown to be a general phenomenon of homotransfer. We have utilized Weber's red-edge effect to study tryptophan homotransfer in proteins. Specifically, we determined the polarization of the tryptophan fluorescence upon excitation at 295 nm and 310 nm (near the red-edge). Rotational diffusion leads to depolarization of the emission excited at either 295 nm or 310 nm, but homotransfer only contributes to depolarization upon excitation at 295 nm. Hence, the 310/295 polarization ratio gives an indication of tryptophan to tryptophan energy transfer. In single tryptophan systems, the 310/295 ratios are generally below 2 whereas in multi-tryptophan systems, the 310/295 ratios can be greater than 3.     

Venom exonuclease. IV. Intrinsic fluorescence of the exonuclease from Crotalus adamanteus venom

The intrinsic fluorescence of the exonuclease isolated from Crotalus adamanteus venom, was studied. The position of its maximum at 335 nm and half-width of the emission band 55 nm (lambda exc. 295 nm) suggested the existence of at least two types of tryptophan residues in the enzyme molecule. Differential analysis of the fluorescence spectra obtained by excitation at 280 and 295 nm revealed about 12.5% contribution of the tyrosine fluorescence in the overall emission excited at 280 nm. The environment of the tryptophan residues in the exonuclease was studied by quenching of their fluorescence with various ionic (NO3-, NO2-, I-, Br- and Cs+) and non-ionic agents (acrylamide, chloroform-methanol). On this basis, fractions of inner (non-polar) and surface tryptophan residues located in charged and neutral regions of the enzyme molecule were evaluated. More than half of the residues (60%) was found in the inner part of the exonuclease while most of its surface tryptophans--in a neutral region(s).     

Detection of conformational changes in chloroplast coupling factor 1 by 8-anilino-1-naphthalene-sulphonate fluorescence changes

Chloroplast coupling factor 1 (CF1) contains a high-affinity binding site for 8-anilino-1-napthalene sulphonate (ANS,Kd = 5-6 microM). The binding of ANS to the enzyme is associated with a fluorescence enhancement and a blue-shift in the emission spectrum. ANS only slightly inhibits ATP hydrolysis by CF1. Adenine nucleotides and inorganic phosphate induce a fast ANS fluorescence quenching of about 50% which is due to a decrease in the affinity of the enzyme for ANS (Kd increases from 6 microM to 22 microM) and in the fluorescence quantum yield of the bound probe (by 33%) but not in the number of ANS sites (n = 1). Conversely, Mg and Ca ions induce a fluorescence enhancement of bound ANS. Inactivation of the enzyme enhances ANS fluorescence, eliminates the response to adenine nucleotides and inorganic phosphate but increases the response to divalent metals. The affinity of latent CF1 for ADP (Kd = 12 microM) is considerably higher than for ATP (Kd = 95 microM) in buffer containing EDTA. The Kd for inorganic phosphate is 140 microM. Mg increases the apparent affinity for ATP (Kd = 28 microM) but not for ADP or Pi. Binding of ATP to the tight-sites does not inhibit the ADP or Pi-induced fluorescence quenching but decreases the affinity for ADP (Kd = 34 microM) and for inorganic phosphate (Kd = 320 microM). These results suggest that the ADP and phosphate binding sites are different but not independent from the tight sites. Activation of a Mg-specific ATPase in CF1 by octyl glucoside decreases the affinity for ADP and inorganic phosphate by about threefold but increases the affinity for ATP. ATPase activation of CF1 also increases the Ki for ADP inhibition of ATP hydrolysis. ATPase activation also influences the ANS responses to Ca and Mg. Ca-ATPase activation increases the fluorescence enhancement and the apparent affinity for Ca whereas Mg-ATPase activation specifically increases the Mg-induced fluorescence enhancement. The fluorescence of CF1-bound ANS is enhanced by Dio-9 and quenched by phloridzin, quercetin, Nbf-Cl and FITC. Nbf-Cl and FITC completely inhibit the ADP-induced fluorescence quenching whereas Dio-9 inhibits the Mg-induced fluorescence enhancement. ANS does not relieve the quercetin or phloridzin inhibition of ATP hydrolysis indicating that these inhibitors do not compete with ANS for a common binding site. ANS may be used, therefore, as a sensitive probe to detect conformational changes in CF1 in response to activation or inactivation and to binding of substrates and of inhibitors.     

Non-enzymatic glycation induces structural modifications of myoglobin

Increased glucose concentration in diabetes mellitus causes glycation of several proteins, leading to changes in their properties. Although glycation-induced functional modification of myoglobin is known, structural modification of the protein has not yet been reported. Here, we have studied glucose-modified structural changes of the heme protein. After in vitro glycation of metmyoglobin (Mb) by glucose at 25°C for 6 days, glycated myoglobin (GMb) and unchanged Mb have been separated by ion exchange (BioRex 70) chromatography, and their properties have been compared. Compared to Mb, GMb exhibits increased absorbance around 280 nm and enhanced fluorescence emission with excitation at 285 nm. Fluorescence quenching experiments of the proteins by acrylamide and KI indicate that more surface accessible tryptophan residues are exposed in GMb. CD spectroscopic study reveals a change in the secondary structure of GMb with decreased α-helix content. 1-anilino-naphthaline-8-sulfonate (ANS) binding with Mb and GMb indicates that glycation increases hydrophobicity of the heme protein. GMb appears to be less stable with respect to thermal denaturation and differential calorimetry experiments. Heme-globin linkage becomes weaker in GMb, as shown by spectroscopic and gel electrophoresis experiments. A correlation between glycation-induced structural and functional modifications of the heme protein has been suggested.     

Hemes and hemoproteins. 3. The reaction of microperoxidase-8 with cyanide: comparison with aquocobalamin and hemoproteins

The monomeric heme octapeptide from cytochrome c, microperoxidase-8, (MP-8), coordinates CN- with log K = 7.55 +/- 0.04 at 25 degrees C in 20% (v/v) aqueous methanol. Log K values are independent of pH between 6 and 9. A spectrophotometric titration of cyanoMP-8 between pH 5.5 and 13.8 gave a single pKa greater than or equal to 13.5 ascribed to ionization of the proximal His ligand. A study of the kinetics of the reaction of MP-8 with cyanide between pH 5.5 and 12, at 25 degrees C and mu = 0.1, indicates that formation of cyanoMP-8 occurs via three routes: attack of CN- on Fe(III) (k1 = 6.0 +/- 0.3 X 10(5) M-1 sec-1); attack of HCN on Fe(III) (k2 = 4.8 +/- 2.0 X 10(3) M-1 sec-1), followed by deprotonation and isomerization to form the C-bound species; and displacement of OH- by CN- when the proximal His ligand is ionized (k5 = 1.8 +/- 0.1 X 10(5) M-1 sec-1). These results are compared with available data for the reaction of cyanide with aquocobalamin and with various hemoproteins.     

Effect of acrylamide on aldolase structure. I. Induction of intermediate states.

Acrylamide is a fluorescence quencher frequently applied for analysis of protein fluorophores exposure with the silent assumption that it does not affect the native structure of protein. In this report, it is shown that quenching of tryptophan residues in aldolase is a time-dependent process. The Stern-Volmer constant increases from 1.32 to 2.01 M-1 during the first 100 s of incubation of aldolase with acrylamide. Two tryptophan residues/subunit are accessible to quenching after 100 s of aldolase interaction with acrylamide. Up to about 1.2 M acrylamide concentration enzyme inactivation is reversible. Independent analyses of the changes of enzyme activity, 1ANS fluorescence during its displacement from aldolase active-site, UV-difference spectra and near-UV CD spectra were carried out to monitor the transition of aldolase structure. From these measurements a stepwise transformation of aldolase molecules from native state (N) through intermediates: I1, T, I2, to denatured (D) state is concluded. The maxima of I1, T, I2 and D states populations occur at 0.2, 1.0, 2.0 and above 3.0 M of acrylamide concentration, respectively. Above 3.5 M, acrylamide aldolase molecules become irreversibly inactivated.     

Enzymatic attack on side chains of synthetic polymers. Chymotrypsin-catalyzed hydrolysis of specific substrate groups attached to acrylamide or acrylic acid co-polymers.

Three vinyl monomers, M-1, M-3, and M-5, in which L-phenylalanine p-nitroanilide was acylated with CH2==CHCONH(CH2)nCO--(n = 1, 3, 5) were synthesized. They were co-polymerized with a large excess of acrylamide (co-polymers PAm-1, PAm-3, and PAm-5) and with a large excess of acrylic acid (co-polymers PAc=1, PAc-3, and PCc-5). In addition, M-5 was co-polymerized with acrylamide containing 2.8 mol % of the hydrophobic monomer N-acrylyl-1-naphthylamine (co-polymer PAm-5N). The rates of the chymotrypsin-catalyzed hydrolysis of the nitroanilide groups of M-5 and the various co-polymers were determined over a range of pH. For some of the systems data were also obtained over a range of substrate concentrations to derive values for Vmax and Km. Results obtained with PAm-5 were found to be independent of the chain length of the co-polymer. At pH 7, 25 degrees and with 2.7 X 10(-6) M enzyme, Vmax values for M-5, PAm-k, PAm-5N, and PAc-5 were 5.5, 5.5, 10, and 3.6 X 10(-8) M/S, while Km values were 8.5, 16.5, 10, and 2.2 X 10(-5),respectively, With PAc-5, the pH activity profile was shifted to higher acidities as compared to the profiles obtained with M-5 and PAm-5. The susceptibility of the co-polymers to chymotrypsin attack decreases sharply with a decreasing spacing of the L-phenylalanine p-nitroanilide residue from the backbone of the polymer chains.     

A photoreversible conformational change in 124 kDa Avena phytochrome

Tryptophan (Trp) fluorescence quenching of phytochrome has been studied using anionic, cationic and neutral quenchers, I-, Cs+ and acrylamide, respectively, in an effort to understand the molecular differences between the Pr and Pfr forms. The data have been analyzed using both Stern-Volmer and modified Stern-Volmer kinetic treatments. The anionic quencher, I-, was proven to be an ineffective quencher with Stern-Volmer constants, Ksv, of 0.60 and 0.63 M-1, respectively, for the Pr and Pfr forms of phytochrome. The cationic quencher, Cs+, showed about a 2-fold difference in the Ksv of Pr and Pfr, indicating a significant change in the fluorescent Trp environments during the Pr to Pfr phototransformation. However, only 25-37% of the fluorescent Trp residues were accessible to the cationic quencher. Most of the fluorescent Trp residues were accessible to acrylamide, but the quenching by acrylamide was indistinguishable for the Pr and Pfr forms. An additional quenching by acrylamide after a saturated quenching with Cs+ showed more than 40% increase in the Ksv of Pfr over Pr. These observations, along with the finding of two distinct components in the Trp fluorescence lifetime, indicate the existence of Trp residues in at least two different sets of environments in the phytochrome protein. The two components of the fluorescence had lifetimes of 1.1 ns (major) and 4.7 ns (minor) for Pr and 0.9 ns (major) and 4.6 ns (minor) for Pfr. Fluorescence quenching was found to be both static and dynamic as the Stern-Volmer constants for the steady-state fluorescence quenching were higher than for the dynamic fluorescence quenching. Based on the quenching results, in combination with the location of Trp residues in the primary structure, we conclude that the Pr to Pfr phototransformation involves a significant conformation change in the phytochrome molecule, preferentially in the 74 kDa chromophore-bearing domain.     

Interaction of dipyridamole with micelles of lysophosphatidylcholine and with bovine serum albumin: fluorescence studies.

The interaction of the coronary vasodilator dipyridamole with biological systems, protein and membranes has been studied through optical absorption and fluorescence spectroscopies. Using the analysis of the spectra and fluorescence intensity of dipyridamole (DIP) in solution, the interaction of this compound with the transport protein albumin (BSA) and with a model of cell membranes, namely micelles of lysophosphatidylcholine (L-PC), was investigated. Measurements were performed at pH 5.0 and pH 7.0 where the molecule of DIP is fully protonated and partially protonated, respectively. The quenching of fluorescence with nitroxide-stable radicals 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) as well as with acrylamide and iodide allowed the localization of the drug in the polar interface of micelles. Quenching by acrylamide and iodide in L-PC micelles demonstrated the effect of micelle protonation which increased the accessibility of iodide to the chromophore. An effective association constant was obtained both at pH 7.0 (7.5 x 10(3) M-1) and pH 5.0 (2.5 x 10(3) M-1) and a very good agreement with the proposed binding model was observed. The quantum yields of fluorescence data agree very well with the fluorescence lifetimes. The measurement of lifetimes was important to understand the kinetic data obtained from Stern-Volmer plots both of radical, acrylamide and iodide quenching of fluorescence. It was observed that, in the presence of micelles, the kq value increased for TEMPO while decreased for TEMPOL. This result, together with the vanishing solubility of DIP in saturated hydrocarbons and the preferential partition of TEMPO in micelles, suggested the localization of DIP in the polar micellar interface. This is also supported by the enhanced iodide quenching at pH 5.0, constancy of acrylamide quenching in the range of pH 7.0-5.0 and the partition of TEMPO and TEMPOL in SDS micelles. The association constant of DIP to BSA was also estimated both at pH 7.0 (2 x 10(4) M-1) and pH 5.0 (4 x 10(3) M-1). Quenching studies with nitroxide radicals, acrylamide and iodide also suggested the binding of the drug to a hydrophobic region of the protein. At pH 5.0, the protein undergo a conformational change which leads to a loosening of the overall structure so that the accessibility of the nitroxide radicals for DIP is increased at this pH. The differences in kq values at pH 7.0 and pH 5.0 suggested that at pH 7.0 the chromophore is protected in the protein site.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cytochrome c peroxidase activity of a protease-modified form of cytochrome c-552 from the denitrifying bacterium Pseudomonas perfectomarina

Protease activity present in aerobically grown cells of Pseudomonas perfectomarina, protease apparently copurified with cytochrome c-552, and trypsin achieved a limited proteolysis of the diheme cytochrome c-552. That partial lysis conferred cytochrome c peroxidase activity upon cytochrome c-552. The removal of a 4000-Da peptide explains the structural changes in the cytochrome c-552 molecule that resulted in the appearance of both cytochrome c peroxidase activity (with optimum activity at pH 8.6) and a high-spin heme iron. The oxidized form of the modified cytochrome c-552 bound cyanide to the high-spin ferric heme with a rate constant of (2.1 +/- 0.1) X 10(3) M-1 s-1. The dissociation constant was 11.2 microM. Whereas the intact cytochrome c-552 molecule can be half-reduced by ascorbate, the cytochrome c peroxidase was not reducible by ascorbate, NADH, ferrocyanide, or reduced azurin. Dithionite reduced the intact protein completely but only half-reduced the modified form. The apparent second-order rate constant for dithionite reduction was (7.1 +/- 0.1) X 10(2) M-1 s-1 for the intact protein and (2.2 +/- 0.1) X 10(3) M-1 s-1 for the modified form. In contrast with other diheme cytochrome c peroxidases, reduction of the low-spin heme was not necessary to permit ligand binding by the high-spin heme iron.