Crystal structures of an oxygen-binding cytochrome c from Rhodobacter sphaeroides

The photosynthetic bacterium Rhodobacter sphaeroides produces a heme protein (SHP), which is an unusual c-type cytochrome capable of transiently binding oxygen during autooxidation. Similar proteins have not only been observed in other photosynthetic bacteria but also in the obligate methylotroph Methylophilus methylotrophus and the metal reducing bacterium Shewanella putrefaciens. A three-dimensional structure of SHP was derived using the multiple isomorphous replacement phasing method. Besides a model for the oxidized state (to 1.82 A resolution), models for the reduced state (2.1 A resolution), the oxidized molecule liganded with cyanide (1. 90 A resolution), and the reduced molecule liganded with nitric oxide (2.20 A resolution) could be derived. The SHP structure represents a new variation of the class I cytochrome c fold. The oxidized state reveals a novel sixth heme ligand, Asn(88), which moves away from the iron upon reduction or when small molecules bind. The distal side of the heme has a striking resemblance to other heme proteins that bind gaseous compounds. In SHP the liberated amide group of Asn(88) stabilizes solvent-shielded ligands through a hydrogen bond.

Structure and stability effects of the mutation of glycine 34 to serine in Rhodobacter capsulatus cytochrome c(2)

Gly 34 and the adjacent Pro 35 of Rhodobacter capsulatus cytochrome c(2) (or Gly 29 and Pro 30 in vertebrate cytochrome c) are highly conserved side chains among the class I c-type cytochromes. The mutation of Gly 34 to Ser in Rb. capsulatus cytochrome c(2) has been characterized in terms of physicochemical properties and NMR in both redox states. A comparison of the wild-type cytochrome c(2), the G34S mutation, and the P35A mutation is presented in the context of differences in chemical shifts, the differences in NOE patterns, and structural changes resulting from oxidation of the reduced cytochrome. G34S is substantially destabilized relative to wild-type (2.2 kcal/mol in the oxidized state) but similarly destabilized relative to P35A. Nevertheless, differences in terms of the impact of the mutations on specific structural regions are found when comparing G34S and P35A. Although available data indicates that the overall secondary structure of G34S and wild-type cytochrome c(2) are similar, a number of both perturbations of hydrogen bond networks and interactions with internal waters are found. Thus, the impact of the mutation at position 35 is propagated throughout the cytochrome but with alterations at defined sites within the molecule. Interestingly, we find that the substitution of serine at position 34 results in a perturbation of the heme beta meso and the methyl-5 protons. This suggests that the hydroxyl and beta carbon are positioned away from the solvent and toward the heme. This has the consequence of preferentially stabilizing the oxidized state in G34S, thus, altering hydrogen bond networks which involve the heme propionate, internal waters, and key amino acid side chains. The results presented provide important new insights into the stability and solution structure of the cytochrome c(2).

Worsening of mitral regurgitation secondary to ventricular pacing

A 68-year-old woman was admitted to hospital for possible mitral valve surgery for severe mitral regurgitation (MR). Six months before admission, a DDD pacemaker was implanted for symptomatic bradycardia. She developed atrial fibrillation and the pacer was changed to a VVI mode. At cardiac catheterization, the pulmonary artery and wedge pressures were elevated; coronary angiography showed non-obstructive coronary artery disease. 2-D echocardiography confirmed preserved left ventricular function, severe MR and moderate tricuspid regurgitation. The change in pacing had a dramatic effect on MR severity; ventricular pacing was associated with a MR jet that occupied a larger area of the left atrium than with the unpaced rhythm, the continuous-wave mitral regurgitant Doppler profile was 'dagger'-shaped, and the signal intensity stronger compared with the unpaced rhythm. These findings are consistent with severe MR. The pacer was reprogrammed to reduce the pacing rate from 70 to 45 beats/min, allowing an unpaced rhythm for the most part. The patient improved and was discharged two days later. Six months later she was asymptomatic; repeat echocardiography confirmed mild to moderate MR.

Structure and characterization of Ectothiorhodospira vacuolata cytochrome b(558), a prokaryotic homologue of cytochrome b(5)

A soluble cytochrome b(558) from the purple phototropic bacterium Ectothiorhodospira vacuolata was completely sequenced by a combination of automated Edman degradation and mass spectrometry. The protein, with a measured mass of 10,094.7 Da, contains 90 residues and binds a single protoheme. Unexpectedly, the sequence shows homology to eukaryotic cytochromes b(5). As no prokaryotic homologue had been reported so far, we developed a protocol for the expression, purification, and crystallization of recombinant cytochrome b(558). The structure was solved by molecular replacement to a resolution of 1.65 A. It shows that cytochrome b(558) is indeed the first bacterial cytochrome b(5) to be characterized and differs from its eukaryotic counterparts by the presence of a disulfide bridge and a four-residue insertion in front of the sixth ligand (histidine). Eukaryotes contain a variety of b(5) homologues, including soluble and membrane-bound multifunctional proteins as well as multidomain enzymes such as sulfite oxidase, fatty-acid desaturase, nitrate reductase, and lactate dehydrogenase. A search of the Mycobacterium tuberculosis genome showed that a previously unidentified gene encodes a fatty-acid desaturase with an N-terminal b(5) domain. Thus, it may provide another example of a bacterial b(5) homologue.

Structure and mechanism of the flavocytochrome c fumarate reductase of Shewanella putrefaciens MR-1

Fumarate respiration is one of the most widespread types of anaerobic respiration. The soluble fumarate reductase of Shewanella putrefaciens MR-1 is a periplasmic tetraheme flavocytochrome c. The crystal structures of the enzyme were solved to 2.9 A for the uncomplexed form and to 2.8 A and 2.5 A for the fumarate and the succinate-bound protein, respectively. The structures reveal a flexible capping domain linked to the FAD-binding domain. A catalytic mechanism for fumarate reduction based on the structure of the complexed protein is proposed. The mechanism for the reverse reaction is a model for the homologous succinate dehydrogenase (complex II) of the respiratory chain. In flavocytochrome c fumarate reductase, all redox centers are in van der Waals contact with one another, thus providing an efficient conduit of electrons from the hemes via the FAD to fumarate.

Protein dynamics: imidazole binding to class I C-type cytochromes

The oxidized cytochrome c(2) from the purple phototrophic bacteria, Rhodobacter sphaeroides and Rhodobacter capsulatus, bind the neutral species of imidazole (K(a) = 1440 +/- 40 M(-1)) 50 times more strongly than does horse mitochondrial cytochrome c (K(a) = 30 +/- 1 M(-1)). The kinetics of imidazole binding are consistent with a change in rate-limiting step at high ligand concentrations for all three proteins. This is attributed to a conformational change leading to breakage of the iron-methionine bond which precedes imidazole binding. The three-dimensional structure of the Rb. sphaeroides cytochrome c(2) imidazole complex (Axelrod et al., Acta Crystalogr. D50, 596-602) supports the view that the conformational changes are essentially localized to approximately seven residues on either side of the ligated methionine and there is a hydrogen bond between the Phe 102 carbonyl, an internal water, and the bound imidazole. Insertions and deletions in this region of cytochrome c(2), the presence of a proline near the methionine, and the smaller size of the dynamic region of horse cytochrome c suggest that the stabilizing hydrogen bond is not present in horse cytochrome c, hence, the dramatic difference in affinity for imidazole. The kinetics of ligand binding do not correlate with either the strength of the iron-methionine bond as measured by the pK of the 695-nm absorption band or the overall stability of the cytochromes studied. However, the very similar imidazole binding properties of the two cytochromes c(2) indicate that the Rb. sphaeroides cytochrome c(2)-imidazole complex structure is an excellent model for the corresponding Rb. capsulatus cytochrome c(2) complex. It is notable that the movement of the peptide chain in the vicinity of the ligated methionine has been preserved throughout evolution and suggests a role in the function of c-type cytochromes.

Twenty year trends (1975-1995) in the incidence, in-hospital and long-term death rates associated with heart failure complicating acute myocardial infarction: a community-wide perspective

OBJECTIVES

To describe from a population-based perspective, recent and temporal (1975-1995) trends in the incidence, in-hospital and postdischarge case-fatality rates of heart failure (HF) complicating acute myocardial infarction (AMI).

BACKGROUND

Extremely limited data are available describing the incidence and case-fatality rates associated with HF complicating AMI from a community-wide perspective.

METHODS

The medical records of 6,798 residents of the Worcester, Massachusetts metropolitan area with validated MI and without previous HF hospitalized in 10 annual periods between 1975 and 1995 were reviewed.

RESULTS

The proportion of AMI patients developing HF during hospitalization declined between 1975-1978 (38%) and 1993-1995 (33%) (p < 0.001). After controlling for potentially confounding factors, the risk of developing HF declined progressively, albeit modestly, over time. In-hospital case-fatality rates of patients with AMI complicated by HF declined by approximately 46% between 1975-1978 (33%) and 1993-1995 (18%) (p < 0.001). Improving trends in hospital survival were observed after adjusting for potentially confounding prognostic factors. The one-year post-discharge mortality rate for hospital survivors of HF did not change over the 20-year period under study, even after controlling for additional prognostic characteristics.

CONCLUSIONS

The results of this community-wide study suggest encouraging declines in the incidence and hospital death rates associated with HF complicating AMI. Continued efforts need to be directed towards the prevention of HF given the magnitude of this clinical syndrome. Efforts of secondary prevention are needed to identify and improve the treatment of patients with symptomatic left ventricular dysfunction following AMI given the lack of improvement in the long-term prognosis of these patients.

Amino acid sequences of two high-potential iron-sulfur proteins (HiPIPs) from the moderately halophilic purple phototrophic bacterium, Rhodospirillum salinarum

The amino acid sequences of two very different high-potential iron-sulfur protein (HiPIP) isozymes have been determined from the moderately halophilic purple phototrophic bacterium, Rhodospirillum salinarum. Iso-1 HiPIP, which is monomeric and contains 57 amino acid residues, is most similar to the Thiobacillus ferrooxidans iron-oxidizing enzyme (45% identity and a 6-residue deletion). On the other hand, iso-2 HiPIP, which is isolated as an oligomer, contains a peptide chain with 54 amino acid residues. It is the smallest reported to date and is only 31% identical to iso-1 HiPIP. A massive deletion of 17 residues is found at the N-terminus, such that only 2 residues remain prior to the first cysteine. Iso-2 HiPIP also has a 12-residue insertion and a 5-residue deletion. Prior to this study, there were only 2 absolutely conserved residues (Tyr 19 and Gly 75, Chromatium numbering) in addition to the 4 iron-sulfur cluster binding cysteine residues among the 13 HiPIPs sequenced to date. We found that Tyr 19 is absent in iso-2 HiPIP along with the entire N-terminal loop. Moreover, Gly 75 is substituted in both R. salinarum HiPIPs. These characteristics make the R. salinarum HiPIPs, and especially iso-2, the most divergent yet characterized.

Nonischemic end-systolic performance. Effect of alterations in regional and global left ventricular contractility

Nonischemic end-systolic performance decreases during ischemia. These changes in performance are likely to be dependent on the size and site of the ischemic zone, as well as the prevailing loading conditions. This study was designed to examine the effect of regional and generalized changes in inotropy on nonischemic end-systolic performance, independent of the ischemic zone size. Twenty dogs were instrumented with sonomicrometers and micromanometer pressure gauges. End-systolic pressure-thickness relationship data were obained during vena-caval balloon inflation. Measurements were obtained before and 90 s after left circumflex (LC) artery occlusion. Then, simultaneous with the occlusion of the LC artery, isoproterenol (0.04 microg/ml) was infused into the left anterior descending artery. After recovery, the same protocol was repeated before and after propranolol (0.5 mg/kg). In a separate set of animals, the same measurements were made following 2.5 and 5 microg/kg/min dobutamine. The effect of ischemia on the nonischemic end-systolic pressure-thickness relationship was expressed as the extent to which the relationship is shifted to the left. Infusion of intracoronary isoproterenol into the perfusion bed of the nonischemic zone produced a significant increase in the slope of the end-systolic pressure-thickness relationship. During ischemia, however, the extent of leftward shift of this relationship was less than that following beta-blockade. Intravenous dobutamine resulted in a dose-dependent increase in the slope of the nonischemic end-systolic pressure thickness relationship, but the extent of leftward displacement of the relationship in response to regional ischemia was less than that following the control occlusion. The nonischemic segment is coupled with the nonfunctioning ischemic zone in such a way that it is required of the nonischemic segment to operate at decreased end-systolic thickness for any end-systolic pressure, the extent of which is to be determined, in part, by the size of the ischemic zone and the contractile state of the nonischemic myocardium. The lower the contractile state prior to coronary occlusion the greater extent of leftward shift of the pressure-thickness relationship.

Exercise-induced hyperkalemia and concentration of Na,K-pumps in skeletal muscle in mitral stenosis: effect of balloon mitral valvotomy

BACKGROUND AND AIMS OF THE STUDY

The study aim was to examine the effects of balloon mitral valvotomy (BMV) on exercise-induced hyperkalemia, and on changes in the concentration of Na,K-pumps in skeletal muscle, as an exaggerated exercise-induced rise in potassium concentration ([K+]) may contribute to exertional fatigue and breathlessness.

METHODS

Eight subjects were evaluated with mitral stenosis (mean age 34 +/- 5.2 years) before, and at two weeks and four months after BMV. Subjects underwent incremental exercise to exhaustion for exercise-induced rise in [K+] and vastus lateralis biopsy for concentration of Na,K-pumps.

RESULTS

Mean (+/- SE) valve area increased from 0.89 +/- 0.03 cm2 before to 1.75 +/- 0.05 cm2 after BMV. There was a progressive increase in VO2,max (15.3 +/- 1.6, 17.2 +/- 1.4 and 19.9 +/- 1.9 l/kg/min) at baseline, early after and later after BMV, respectively (p < 0.01). The rise in [K+] with absolute workload fell progressively at early and late follow up post-BMV (p < 0.05), but was unchanged when plotted against percentage of VO2,max to match for relative workload. The concentration of Na,K-pumps was similar to baseline at early follow up (233 +/- 10 versus 228 +/- 15 pmol/g wet weight), but was significantly increased at late follow up after four months (265 +/- 17 pmol/g; p < 0.05). When the relationship between the concentration of Na,K-pumps and the exercise-induced rise in [K+] was studied, a negative correlation was found. However, correlation analysis for the effects of changes in Na,K-pumps on changes in exercise hyperkalemia after BMV was not significant.

CONCLUSIONS

The progressive reduction in exercise-induced rise in [K+] after BMV may contribute to the progressive improvement in exercise performance. The increased concentration of Na,K-pumps in skeletal muscle may assist in this improvement, and emphasizes the importance of peripheral adaptations in clinical improvement after BMV.

Coronary arterial dimension-to-left ventricular mass ratio in idiopathic dilated cardiomyopathy

This study demonstrated that, compared with normal controls, coronary artery dimensions are not increased appropriately for the increase in left ventricular mass in patients with nonischemic cardiomyopathy. The extent of coronary artery dimension to left ventricular mass mismatch did not correlate with the severity of heart failure.

Evaluation of extent of shortening versus velocity of shortening at the endocardium and midwall in hypertensive heart disease

To assess the incremental value of velocity of shortening velocity parameters compared with simpler, more widely used, extent of shortening parameters in compensated left ventricular hypertrophy, we studied 52 patients with left ventricular hypertrophy and 63 age-matched controls. Velocity parameters did not provide incremental information beyond that obtained by extent of shortening parameters.

Adenosine A1 receptor-mediated antiadrenergic effects are modulated by A2a receptor activation in rat heart

Presently, the physiological significance of myocardial adenosine A2a receptor stimulation is unclear. In this study, the influence of adenosine A2a receptor activation on A1 receptor-mediated antiadrenergic actions was studied using constant-flow perfused rat hearts and isolated rat ventricular myocytes. In isolated perfused hearts, the selective A2a receptor antagonists 8-(3-chlorostyryl)caffeine (CSC) and 4-(2-[7-amino-2-(2-furyl)[1,2, 4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385) potentiated adenosine-mediated decreases in isoproterenol (Iso; 10(-8) M)-elicited contractile responses (+dP/dtmax) in a dose-dependent manner. The effect of ZM-241385 on adenosine-induced antiadrenergic actions was abolished by the selective A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (10(-7) M), but not the selective A3 receptor antagonist 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1, 4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191, 10(-7) M). The A2a receptor agonist carboxyethylphenethyl-aminoethyl-carboxyamido-adenosine (CGS-21680) at 10(-5) M attenuated the antiadrenergic effect of the selective A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA), whereas CSC did not influence the antiadrenergic action of this agonist. In isolated ventricular myocytes, CSC potentiated the inhibitory action of adenosine on Iso (2 x 10(-7) M)-elicited increases in intracellular Ca2+ concentration ([Ca2+]i) transients but did not influence Iso-induced changes in [Ca2+]i transients in the absence of exogenous adenosine. These results indicate that adenosine A2a receptor antagonists enhance A1-receptor-induced antiadrenergic responses and that A2a receptor agonists attenuate (albeit to a modest degree) the antiadrenergic actions of A1 receptor activation. In conclusion, the data in this study support the notion that an important physiological role of A2a receptors in the normal mammalian myocardium is to reduce A1 receptor-mediated antiadrenergic actions.

Ascorbic acid recycling in Nb2 lymphoma cells: implications for tumor progression

Analysis of cultured rat "Nb2 lymphoma" cell lines, showing different degrees of malignant progression, can lead to identification of phenotypic changes associated with this phenomenon in T-cell cancers. In the present study we have compared the metastatic sublines, Nb2-11 and Nb2-SFJCD1, with regard to ascorbate and glutathione recycling, important processes in cellular protection from oxidative stresses. Whereas the Nb2-11 subline is prolactin (PRL)-dependent, the genetically related Nb2-SFJCD1 subline is growth factor-independent and shows more chromosomal alterations, indicative of more advanced progression. The Nb2-SFJCD1 cells, compared to the Nb2-11 cells, were less sensitive to toxic effects of dehydroascorbate, a potentially toxic oxidation product of ascorbate. Results were consistent with a significantly higher production of reducing equivalents (e.g., NADPH, GSH) and an accelerated reduction of dehydroascorbate by homogenates of Nb2-SFJCD1 cells. However, the increased resistance was apparently not directly related to the cellular uptake and reduction of dehydroascorbate by whole cells, which was similar in both cell lines. Observations indicate that Nb2 lymphoma cells, in their progression to malignancy, can acquire an enhanced capability to protect themselves from oxidative damage assisting them in withstanding the oxidative stress that anti-neoplastic drugs can cause. The adaptation may also be a mechanism that is utilized by tumor cells in suppressing apoptosis and other protective cellular functions facilitating, or potentiating, a tumor cell's ability to become more metastatic. However, the mechanism leading to this augmented capacity of Nb2 lymphoma cells to resist oxidative stress in not known and is the subject for further study.

Basis for monomer stabilization in Rhodopseudomonas palustris cytochrome c' derived from the crystal structure

The crystal structure of an unusual monomeric cytochrome c' from Rhodopseudomonas palustris (RPCP) has been determined at 2.3 A resolution. RPCP has the four-helix (helices A, B, C and D) bundle structure similar to dimeric cytochromes c'. However the amino acid composition of the surface of helices A and B in RPCP is remarkably different from that of the dimeric cytochromes c'. This surface forms the dimer interface in the latter proteins. RPCP has seven charged residues on this surface contrary to the dimeric cytochromes c', which have only two or three charged groups on the corresponding surface. Moreover, hydrophobic residues on this surface of RPCP are two to three times fewer than in dimeric cytochromes c'. As a result of the difference in amino acid composition, the A-B surface of RPCP is rather hydrophilic compared with dimeric cytochromes c'. We thus suggest that RPCP is monomeric in solution because of the hydrophilic nature of the A-B surface. The amino acid composition of the A-B surface is similar to that of Rhodobacter capsulatus cytochrome c' (RCCP), which is an equilibrium admixture of monomer and dimer. The charge distribution of the A-B surface in RCCP, however, is considerably different from that of RPCP. Due to the difference, RCCP can form dimers by both ionic and hydrophobic interactions. These dimers are quite different from those in proteins which form strong dimers such as in Chromatium vinosum, Rhodospirillum rubrum, Rhodospirillum molischianum and Alcaligenes. Cytochrome c' can be classified into two types. Type 1 cytochromes c' have hydrophobic A-B surfaces and they are globular. The A-B surface of type 2 cytochromes c' is hydrophilic and they take a monomeric or flattened dimeric form.

Effects of chronic adenosine uptake blockade on adrenergic responsiveness and left ventricular chamber function in pressure overload hypertrophy in the rat

BACKGROUND

Increased sympathetic activity contributes to the progression of heart failure. Adenosine counteracts sympathetic activity by inhibition of presynaptic norepinephrine release and attenuation of the metabolic and contractile responses to beta-adrenergic stimulation. In this study, we tested the hypothesis that the adenosinergic effects (uptake blockade) of dipyridamole may retard the progression of pressure overload hypertrophy in the rat.

METHODS AND RESULTS

To verify that the administration of dipyridamole increases myocardial adenosine levels in the rat, epicardial adenosine concentrations were measured from 12 isolated, perfused rat hearts exposed to 10(-7) and 10(-6) mol/l dipyridamole. Adenosine concentrations were increased with both doses of dipyridamole. Also, 9 weeks of dipyridamole treatment resulted in decreased sensitivity to the adenosine A1-receptor agonist, 2-chloro-N6-cyclopentyl adenosine, suggesting that dipyridamole increases adenosine levels in the intact rat. In the second part of the study, rats were divided into either abdominal aortic-banded or sham-operated groups and were treated with either dipyridamole or saline. After 9 weeks of treatment, two-dimensional Doppler echocardiographic studies were performed and the adrenergic responsiveness to 10(-8) mol/l isoproterenol was assessed in vitro. The saline-treated banded group demonstrated concentric left ventricular hypertrophy, abnormal diastolic filling, increased wet lung weights and attenuation of adrenergic responsiveness. In contrast, the dipyridamole-treated banded rats exhibited more concentric geometry (higher relative wall thickness with similar left ventricular mass), normal left ventricular filling characteristics and preserved adrenergic responsiveness. Systolic left ventricular chamber and myocardial function, as assessed by stress-endocardial and midwall shortening relationships, were not significantly altered by banding or dipyridamole treatment.

CONCLUSIONS

Dipyridamole treatment prevented the development of abnormal left ventricular chamber filling, preserved adrenergic responsiveness and appeared to attenuate detrimental chamber remodeling in rats with pressure overload hypertrophy.

Characterization of cytochrome c-556 from the purple phototrophic bacterium Rhodobacter capsulatus as a cytochrome-c peroxidase

A cytochrome c-556 was purified from Rhodobacter capsulatus and the complete amino acid sequence was determined. It contains 328 amino acid residues and two typical heme-binding sites at cysteine residues 54 and 57 and at residues 200 and 203. It is homologous to the family of bacterial cytochrome c peroxidases (BCCP) with 69% identity to Paracoccus denitrificans BCCP and 60% identity to Pseudomonas aeruginosa BCCP for which there is a three-dimensional structure. There is lesser similarity to the mauG gene products from methylotrophic bacteria which are thought to be involved in biosynthesis of the quinone cofactor of methylamine dehydrogenase. Translated genes from Escherichia coli and Helicobacter pylori are also related to the bacterial cytochrome c peroxidases. The divergence of this family of proteins is reflected in the fact that the reported sixth heme ligands are not conserved, except in Pseudomonas, Rhodobacter and Paracoccus. This suggests that homologs of BCCP may fold differently and/or may not have the same enzymatic activity as the prototypic protein from Ps. aeruginosa. We found that the Rb. capsulatus BCCP is active with both Rb. capsulatus cytochrome c2 and with horse cytochrome c as substrates (Km values 60 microm and 6 microm, respectively). The turnover number was 40 s(-1) and the Km for peroxide was 33 microm. We have thus confirmed that the Rb. capsulatus protein is a cytochrome c peroxidase.

Adenosine mediates sustained adrenergic desensitization in the rat heart via activation of protein kinase C

Adenosine attenuates the myocardial metabolic and contractile responses induced by ss-adrenergic stimulation. Our study was conducted to investigate the longevity of this antiadrenergic action after adenosine exposure. Adenosine (33 micromol/L) was infused into isolated perfused rat hearts for 1, 5, 30, or 60 minutes, and the adrenergic responsiveness (AR) to isoproterenol (10(-8) mol/L) was determined at the end of each infusion period and during a 45-minute adenosine washout period. Interstitial levels of adenosine, as determined from epicardial surface transudates, returned to preinfusion levels within 10 minutes of washout. The duration of adenosine infusion had no effect on the extent of attenuation of AR at the end of the infusion. Whereas AR returned to preadenosine levels with washout of shorter adenosine infusions (1 and 5 minutes), there was a slow and incomplete recovery of AR after the longer exposures (30 and 60 minutes) to adenosine. The magnitude of this persistent antiadrenergic effect (PAE) of adenosine at 15 minutes of washout was proportional to the epicardial concentration of adenosine during infusion of the nucleoside. Infusion of adenosine either with the nonselective adenosine receptor antagonist 8-p-sulfophenyl theophylline or with the selective A1-receptor antagonist 1,3-dipropyl, 8-cyclopentylxanthine, abolished the PAE during the washout period. In addition, the PAE could be demonstrated only with the selective A1-receptor agonist 2-chloro-N6-cyclopentyladenosine and not with the selective A3-receptor agonist 4-aminobenzyl-5'-N methylcarboxamido-adenosine. When the protein kinase C (PKC) inhibitor chelerythrine was coadministered with adenosine, the PAE of adenosine was not apparent during adenosine washout. A 30-minute infusion of phenylephrine, an alpha-adrenergic agonist that enhances PKC activity, produced a PAE that lasted for up to 30 minutes of washout. This effect was prevented by the coinfusion of chelerythrine. Thus, it is concluded that the PAE of adenosine is determined by the myocardial concentration of this nucleoside and is manifested when myocardial concentrations of adenosine returned to baseline levels. Moreover, a 5-minute duration of adenosine exposure is required for the expression of the PAE. This latter effect seems to be dependent on adenosine-induced PKC activation via A1-receptors.

Imidazole binding to Rhodobacter capsulatus cytochrome c2. Effect of site-directed mutants on ligand binding

Although ligand binding in c-type cytochromes is not directly related to their physiological function, it has the potential to provide valuable information on protein stability and dynamics, particularly in the region of the methionine sixth heme ligand and the nearby peptide chain that has been implicated in electron transfer. Thus, we have measured the equilibrium and kinetics of binding of imidazole to eight mutants of Rhodobacter capsulatus cytochrome c2 that differ in overall protein stability. We found that imidazole binding affinity varies 70-fold, but does not correlate with overall protein stability. Instead, each mutant exerts an effect at the local level, with the largest change due to mutant G95E (glycine substituted by glutamate), which shows 30-fold stronger binding as compared with the wild-type protein. The kinetics of imidazole binding are monophasic and reach saturation at high ligand concentrations for all the mutants and wild-type protein, which is attributed to a rate-limiting conformational change leading to breakage of the iron-methionine bond and providing a binding site for imidazole. The mutants show as much as an 18-fold variation in the first-order rate constant for the conformational change, with the largest effect found with mutant G95E. The kinetics also show a lack of correlation with overall protein stability, but are consistent with localized effects on the dynamics of hinge region 88-102 of the protein, which changes conformation to permit ligand binding. These results are consistent with R. capsulatus cytochrome c2 stabilizing the complex through hydrogen bonding to the imidazole. The larger effects of mutant G95E on equilibrium and kinetics are likely to be due to its location within the hinge region adjacent to heme ligand methionine 96, which is displaced by imidazole.

The primary structures of the low-redox potential diheme cytochromes c from the phototrophic bacteria Rhodobacter sphaeroides and Rhodobacter adriaticus reveal a new structural family of c-type cytochromes

The complete amino acid sequence of the low-redox potential cytochrome c-551.5 from Rhodobacter sphaeroides was determined by automated Edman degradation combined with mass spectroscopy. There are 139 residues and two typical Cys-X-X-Cys-His heme-binding sites. A homologous low-redox potential cytochrome was also sequenced from Rhodobacter adriaticus and was found to contain 126 residues. It is 53% identical to that of Rb. sphaeroides and has two internal deletions of one and five residues. The Rhodobacter diheme cytochromes are 21-24% identical to the translated open reading frame SLL1886 from Synechocystis sp. PCC6801. There are at least two deletions of five and eight residues in the 188-residue cyanobacterial protein. Each of the three cytochromes has more histidines than it needs to bind the two hemes, but conserved histidines located 23 residues after the first heme and 14-19 residues before the second heme are likely to be the sixth heme ligands. There is no evidence for gene doubling and no similarity to any other known cytochromes. The measured helix content of 24% is much less than normal for c-type cytochromes. These proteins thus appear to be representative of an entirely new class of c-type cytochromes.

New insights into the photocycle of Ectothiorhodospira halophila photoactive yellow protein: photorecovery of the long-lived photobleached intermediate in the Met100Ala mutant

There are previously two known intermediates (I1 and I2) in the room-temperature photocycle of the photoactive yellow protein (PYP) from Ectothiorhodospira halophila. The three-dimensional structures of ground-state PYP and of I2 have shown that light-induced conformational changes are localized to the active site. Previous site-specific mutagenesis studies of PYP in our laboratories have characterized two active site mutants (Glu46Gln and Arg52Ala). We now report the construction and characterization of a mutant at a third active site position (Met100Ala) in order to establish the role of this residue in the photocycle. Met100Ala PYP has an absorption spectrum which is very similar to wild-type (WT) PYP, but exhibits very different kinetic properties. At pH 7.0, the light-induced bleaching reaction (I2 formation) has a half-life <1 microseconds and the recovery in the dark has a half-life of 5.5 min, as compared with half-lives of 100 microseconds and 140 ms for the same reactions in WT PYP. The slow rate of recovery from I2 for Met100Ala results in the accumulation of the bleached intermediate even under room light illumination. These results are qualitatively similar to what has been observed with the Arg52Ala mutant of PYP, and with WT PYP in the presence of alcohols or urea, and suggest that Met100 acts to stabilize the ground state of the protein. The midpoint for guanidine denaturation confirms this. The slow recovery of I2 in the Met100Ala mutant has allowed us to obtain direct evidence that this intermediate species is also photoactive and can be returned to the ground state by a 365 nm laser flash, with kinetics (half-life = 160 microseconds; k = 6300 s-1) which are 6 orders of magnitude faster than dark recovery. This implies that chromophore reisomerization limits the rate of conversion of I2 to the ground state in PYP. Met100 is in van der Waals contact with the chromophore in the I2 state, and we suggest that the sulfur atom catalyzes cis-trans isomerization in WT PYP.

The primary structure of soluble cytochrome c-551 from the phototrophic green sulfur bacterium Chlorobium limicola, strain Tassajara, reveals a novel c-type cytochrome

Chlorobium limicola, strain Tassajara, cytochrome c-551 is a soluble dimeric protein containing identical subunits of about 30 kDa. The amino acid sequence was determined by a combination of automated Edman degradation and mass analysis. There are 258 residues with a single heme binding site located at cysteine positions 172 and 175. In addition, there is a disulfide bridge between Cys78 and Cys109, and a free cysteine at position 219 which was found to occur as cysteic acid. The only homologue of soluble cytochrome c-551 is the soxA protein which is part of the thiosulfate utilization operon of Paracoccus denitrificans. They are 32% identical with three small gaps. This is consistent with the observation that cytochrome c-551 is the electron acceptor for a thiosulfate-oxidizing enzyme. On the basis of the redox potential of 135 mV, the sixth heme ligand should be a methionine. Among the seven methionine residues that are present in c-551, only one is conserved, two residues ahead of the heme-binding site. The far-UV circular dichroism spectrum indicates 40% alpha helix and 25% beta secondary structure. No other known cytochrome c has such a mixed structure; they are either all helical or all beta. Thus, Chlorobium soluble cytochrome c-551 and soxA are likely to be representative of a new class of c-type cytochromes.

Changes in glutathione redox cycling and oxidative stress response in the malignant progression of NB2 lymphoma cells

Differential analysis of closely related Nb2-lymphoma cell lines can be used for identification of changes in biochemical properties associated with the malignant progression of certain T-cell cancers. As tumors progress, they tend to show metabolic alterations such as an increased resistance to oxidative stress, a characteristic that may be correlated with changes in intrinsic antioxidant levels (e.g., glutathione) and in activities of associated enzymes such as the glutathione redox pathway. Whether increases in malignancy of Nb2 cells were associated with changes in cellular glutathione levels and activities of glutathione-metabolizing enzymes was addressed. To evaluate this relationship, 3 cell lines, showing increased malignancy, were used: Nb2-U17 (hormone-dependent, non-metastatic), Nb2-11 (hormone-dependent, metastatic), Nb2-SFJCD1 (growth factor-independent, metastatic). Compared to Nb2-U17 and Nb2-11 cells, the highly progressed Nb2-SFJCD1 lymphoma cells maintain low basal glutathione levels. However, the Nb2-SFJCD1 cells display an enhanced capacity to produce glutathione when challenged with an oxidative stress and show a significantly higher resistance to H2O2-induced apoptosis.

New photocycle intermediates in the photoactive yellow protein from Ectothiorhodospira halophila: picosecond transient absorption spectroscopy

Previous studies have shown that the room temperature photocycle of the photoactive yellow protein (PYP) from Ectothiorhodospira halophila involves at least two intermediate species: I1, which forms in <10 ns and decays with a 200-micros lifetime to I2, which itself subsequently returns to the ground state with a 140-ms time constant at pH 7 (Genick et al. 1997. Biochemistry. 36:8-14). Picosecond transient absorption spectroscopy has been used here to reveal a photophysical relaxation process (stimulated emission) and photochemical intermediates in the PYP photocycle that have not been reported previously. The first new intermediate (I0) exhibits maximum absorption at approximately 510 nm and appears in </=3 ps after 452 nm excitation (5 ps pulse width) of PYP. Kinetic analysis shows that I0 decays with a 220 +/- 20 ps lifetime, forming another intermediate (Idouble dagger0) that has a similar difference wavelength maximum, but with lower absorptivity. Idouble dagger0 decays with a 3 +/- 0.15 ns time constant to form I1. Stimulated emission from an excited electronic state of PYP is observed both within the 4-6-ps cross-correlation times used in this work, and with a 16-ps delay for all probe wavelengths throughout the 426-525-nm region studied. These transient absorption and emission data provide a more detailed understanding of the mechanistic dynamics occurring during the PYP photocycle.