The presence of reactive SH groups in the enzymatically active dicyano derivative of creatine kinase

Effects of Etanercept on Experimental Osteoarthritis in Rats: Role of Histone Deacetylases

OBJECTIVE

Mounting evidence suggests that histone deacetylases (HDAC) inhibitors reduce cartilage destruction in animal models of osteoarthritis (OA). Tumor necrosis factor (TNF)-α-blocking treatment for OA may provide effective joint protection by slowing joint damage. To investigate the effects of intraperitoneal administration of etanercept (a TNF-α inhibitor) on OA development in rats and changes in the nociceptive behavior of rats and expression of HDACs, RUNX2, and MMP13 in cartilage.

METHODS

Induction of OA in Wistar rats was accomplished through anterior cruciate ligament transection (ACLT). One or five milligrams (mg) of etanercept was administered intraperitoneally for 5 consecutive weeks after ACLT to the ACLT + etanercept (1 and 5 mg/kg) groups. Nociceptive behavior and changes in knee joint width were analyzed. Cartilage was evaluated histologically and immunohistochemically.

RESULTS

ACLT + etanercept significantly improved mechanical allodynia and weight-bearing distribution compared to ACLT alone. In OA rats treated with etanercept, cartilage degeneration and synovitis were significantly less pronounced than those in ACLT rats. OA-affected cartilage also showed reduced expression of HDAC 6, 7, RUNX-2, and MMP-13 in response to etanercept but increased expression of HDAC4.

CONCLUSION

Our study demonstrated that etanercept therapy (1) attenuated the development of OA and synovitis in rats, (2) reduced nociception, and (3) regulated chondrocyte metabolism, possibly by inhibiting cell HDAC6 and HDAC7, RUNX2, and MMP13 and increasing HDAC4 expression. Based on new evidence, etanercept may have therapeutic potential in OA.

Towards creatine kinase aggregation due to the cysteine modification at the flexible active site and refolding pathway

The dimeric native state of creatine kinase (CK) was aggregated at conspicuous levels during cysteine modification at the active site with using 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) under a high enzyme concentration. Measuring the ANS-binding fluorescence revealed that the hydrophobic surface of CK was increased by cysteine modification due to the flexible active site, and this resulted in insoluble aggregation, probably via non-specific hydrophobic interactions. To determine whether the aggregates can be refolded, 3M guanidine hydrochloride (GdnHCl) was used to dissolve the aggregates into the denatured form. Refolding of the solubilized enzyme sample was then conducted, accompanied by deprivation of DTNB from the CK in the presence of DTT. As a result, CK was reactivated by up to 40% with partial recovery of the tertiary (78%) and secondary structures (77%). To further elucidate its kinetic refolding pathway, both time interval measurements and a continuous substrate reaction were performed. The results showed that the refolding behavior was similar to the manner of normal CK folding with respect to the following two-phase kinetic courses. Additionally, the rate constants for the dimerization of the unfolded CK were dependent on the enzyme concentration and this was irrespective to the DTT concentrations, suggesting the rate-limiting steps of CK reassociation. The present study will expand our insight into the flexibility of the enzyme active site, which might act as a risk factor for inducing the unfavorable aggregation and partial refolding pathway of CK, as well as inducing an intermediate-like state recovery from aggregation.

Impact of intra-subunit domain-domain interactions on creatine kinase activity and stability

Creatine kinase (CK) is a key enzyme in vertebrate excitable tissues. In this research, five conserved residues located on the intra-subunit domain-domain interface were mutated to explore their role in the activity and structural stability of CK. The mutations of Val72 and Gly73 decreased both the activity and stability of CK. The mutations of Cys74 and Val75, which had no significant effect on CK activity and structure, gradually decreased the stability and reactivation of CK. Our results suggested that the mutations might modify the correct positioning of the loop contributing to domain-domain interactions, and result in decreased stability against denaturation.

The kinetic study of arginine kinase from the sea cucumber Stichopus japonicus with 5,5'-dithiobis-(2-nitrobenzoic acid)

The Stichopus japonicus arginine kinase (AK) is a significant dimeric enzyme. Its modification and inactivation course with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and the reactivation course of DTNB-modified AK by dithiothreitol were investigated on the basis of the kinetic theory of the substrate reaction during the modification of enzyme activity. The results show that the modification is a biphasic course while the inactivation is monophasic, with one essential reactive cysteine per subunit. The Cys274 (numbering from the Stichopus sequence) is exposed to DTNB and is near the ATP binding site. The modified AK can be reactivated by an excess concentration of dithiothreitol in a monophasic kinetic course. The presence of ATP or the transition-state analog markedly slows the apparent reactivation rate constant. The analog components, arginine-ADP-Mg2+ can induce conformational changes of the modified enzyme, but adding NO3- cannot induce further changes that occur with the native enzyme. The reactive cysteines' location and its role in the catalysis of AK are discussed. The results suggest that the cysteine may be located in the hinge area of the two domains of AK. The reactive cysteine of AK, which was proposed to be Cys274, may play an important role not in the binding of the transition-state analog but in the conformational changes caused by the transition-state analog.

Relating structure to mechanism in creatine kinase

Found in all vertebrates, creatine kinase catalyzes the reversible reaction of creatine and ATP forming phosphocreatine and ADP. Phosphocreatine may be viewed as a reservoir of "high-energy phosphate" which is able to supply ATP, the primary energy source in bioenergetics, on demand. Consequently, creatine kinase plays a significant role in energy homeostasis of cells with intermittently high energy requirements. The enzyme is of clinical importance and its levels are routinely used as an indicator of myocardial and skeletal muscle disorders and for the diagnosis of acute myocardial infarction. First identified in 1928, the enzyme has undergone intensive investigation for over 75 years. There are four major isozymes, two cytosolic and two mitochondrial, which form dimers and octamers, respectively. Depending on the pH, the enzyme operates by a random or an ordered bimolecular mechanism, with the equilibrium lying towards phosphocreatine production. Evidence suggests that conversion of creatine to phosphocreatine occurs via the in-line transfer of a phosphoryl group from ATP. A recent X-ray structure of creatine kinase bound to a transition state analog complex confirmed many of the predictions based on kinetic, spectroscopic, and mutagenesis studies. This review summarizes and correlates the more significant mechanistic and structural studies on creatine kinase.

Effect of thiohydroxyl compounds on tyrosinase: inactivation and reactivation study

An unusual thioether bridge (Cys-His) has been detected at the active site of mushroom tyrosinase, and the effects of thiohydroxyl compounds such as dithiothreitol (DTT) and beta-mercaptoethanol (beta-ME) on Cu2+ at the active site have been elucidated. Treatment with DTT and beta-ME on mushroom tyrosinase completely inactivated 3,4-dihydroxyphenylalanine oxidase activity in a dose-dependent manner. Sequential kinetic studies revealed that DTT and beta-ME caused different mixed-type inhibition mechanisms: the slope-parabolic competitive inhibition (Ki = 0.143 mM) by DTT and slope-hyperbolic noncompetitive inhibition (Ki = 0.0128 mM) by beta-ME, respectively. Kinetic Scatchard analysis consistently showed that mushroom tyrosinase had multiple binding sites for DTT and beta-ME with different affinities. Reactivation study of inactivated enzyme by addition of Cu2+ confirmed that DTT and beta-ME directly bound with Cu2+ at the active site. Our results may provide useful information regarding interactions of tyrosinase inhibitor for designing an effective whitening agent targeted to the tyrosinase active site.

Conformational changes and inactivation of rabbit muscle creatine kinase in dimethyl sulfoxide solutions

The effects of dimethyl sulfoxide (DMSO) on creatine kinase (CK) conformation and enzymatic activity were studied by measuring activity changes, aggregation, and fluorescence spectra. The results showed that at low concentrations (< 65% v/v), DMSO had little effect on CK activity and structure. However, higher concentrations of DMSO led to CK inactivation, partial unfolding, and exposure of hydrophobic surfaces and thiol groups. DMSO caused aggregation during CK denaturation. A 75% DMSO concentration induced the most significant aggregation of CK. The CK inactivation and unfolding kinetics were single phase. The unfolding of CK was an irreversible process in the DMSO solutions. The results suggest that to a certain extent, an enzyme can maintain catalytic activity and conformation in water-organic mixture environments. Higher concentrations of DMSO affected the enzyme structure but not its active site. Inactivation occurred along with noticeable conformational change during CK denaturation. The inactivation and unfolding of CK in DMSO solutions differed from other denaturants such as guanidine, urea, and sodium dodecyl sulfate. The exposure of hydrophobic surfaces was a primary reason for the protein aggregation.

Inhibitive effect of zinc ion on fatty acid synthase from chicken liver

Fatty acid synthase (FAS; acyl-CoA:malonyl-CoA C-acyltransferase [decarboxylating, oxoacyl- and enoyl-reducing and thioester-hydrolyzing], EC 2.3.1.85) is an important enzyme participating in energy metabolism in vivo which is related to adiposis and cancer [Cancer Lett. 167 (1) (2001) 99; Nat. Med. 8 (4) (2002) 335]. Tests of fast- and slow-binding inhibitions showed that fatty acid synthase of chicken liver is rapidly and irreversibly inactivated by low Zn(2+) concentrations. Electrophoresis and FPLC results showed that FAS cross-links occurred in the presence of high Zn(2+) concentrations (>4 microM) which may be another reason that FAS lost its activity. The modification velocity of FAS by DTNB decreased with increasing Zn(2+) concentration, which confirmed that Zn(2+) interacted with SH groups. Substrate protective experiments and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that all three substrates tested had some protective effects on FAS in the presence of Zn(2+), and malonyl-CoA was the most effective of the three substrates. In the presence of malonyl-CoA, the activity loss of FAS decreased sharply and almost no cross-link was observed in SDS-PAGE. This suggests that the phosphopantetheine SH group is the critical group in the cross-link and inhibition of FAS in the presence of Zn(2+).

Dose-dependent biochemical changes in rat central nervous system after 12-week exposure to 1-bromopropane

1-Bromopropane is used as a cleaning agent or adhesive solvent in the workplace. The present study investigated the long-term effects of exposure to 1-bromopropane on biochemical components in the central nervous system (CNS) of rats. Four groups, each of nine male Wistar rats, were exposed to 200, 400, or 800 ppm 1-bromopropane or fresh air only, 8h per day, 7 days a week for 12 weeks. We measured the levels of neuron-specific gamma-enolase, glia-specific beta-S100 protein, creatine kinase (CK) subunits B and M, heat shock protein Hsp27 (by enzyme immunoassay), enzymatic activity of CK and levels of glutathione (GSH), oxidized glutathione (GSSG) and sulfhydrul (SH) base in the cerebrum, cerebellum, brainstem and spinal cord. gamma-Enolase decreased dose-dependently in the cerebrum, which showed a decrease in wet weight, at 400 ppm or over, but no change was noted in beta-S100 protein in any brain region or spinal cord. Hsp27 decreased in the cerebellum, brainstem and spinal cord. Protein-bound SH base, non-protein SH base and total glutathione decreased in every brain region. CK activity decreased dose-dependently at 200 ppm or over, and the ratio of CK activity to CK-B concentration tended to decrease in all regions. The decrease in gamma-enolase in the cerebrum suggests the involvement of biochemical changes in neurons with decrease in the wet weight of the cerebrum. Glutathione depletion and changes in proteins containing SH base as a critical site might be the underlying neurotoxic mechanism of 1-bromopropane. The biochemical changes in the cerebrum indicate that long-term exposure to 1-bromopropane has effects on the CNS.

Biochemical changes in the central nervous system of rats exposed to 1-bromopropane for seven days

1-Bromopropane is used widely as an alternative to ozone-depleting solvents. The neurotoxic effects of this agent have been described in humans and experimental animals. Here we investigated the underlying mechanisms of the neurotoxic effects of 1-bromopropane by examining the initial biochemical changes in the central nervous system. Four groups of 9 Wistar male rats each were exposed to 200, 400, or 800 ppm 1-bromopropane or only fresh air, 8 h per day for 7 days. At the end of the experiment, the cerebrum, cerebellum, brain stem and lumbar enlargement of the spinal cord were dissected out from each rat (n = 8) for biochemical analyses. Morphological examinations of the nervous system were performed in the remaining rat of each group. 1-Bromopropane dose-dependently decreased neurospecific gamma-enolase, total glutathione, and nonprotein sulfhydryl groups in the cerebrum and cerebellum. Creatine kinase activity decreased dose-dependently in the brain and spinal cord. Histopathological examination showed swelling of preterminal axons in gracile nucleus and degeneration of myelin in peripheral nerves. Our results of low levels of gamma-enolase suggested that 1-bromopropane might primarily cause functional or cellular loss of neurons in the cerebrum and cerebellum. Glutathione depletion or modification to functional proteins containing a sulfhydryl base as a critical site might be the underlying mechanism of 1-bromopropane neurotoxicity.

Effects of acrylamide on creatine kinase from rabbit muscle

The mechanism of inhibition of creatine kinase (CK) by acrylamide (Acr) has been examined (in vitro). Within the concentration range of 0 to 1 M, Acr markedly inhibited CK and depleted the protein thiols. Both inactivation and thiol depletion were time- and Acr concentration-dependent. Addition of dithiothreitol (DTT) did not reactivate CK inactivated by Acr. However, CK with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) pre-blocked thiols can be reactivated by DTT after incubation with Acr. The transition-state analogue also had a significant protective effect on CK against Acr inhibition. We conclude that thiol alkylation is a critical event in inactivation of CK by Acr. Furthermore, Acr binding to CK changed its surface charge, which may be the same effect for the toxicity of Acr towards other proteins.

Folding pathway for partially folded rabbit muscle creatine kinase

Rabbit muscle creatine kinase (CK) was modified by 5,5'-dithio-bis(2-nitrobenzoic acid) accompanied by 3 M guanidine hydrochloride denaturation to produce a partially folded state with modified thiol groups. The partially folded CK was in a monomeric state detected by size exclusion chromatography, native-polyacrylamide gel electrophoresis, circular dichroism, and intrinsic fluorescence studies. After dithiothreitol (DTT) treatment, about 70% CK activity was regained with a two-phase kinetic course. Rate constants calculated for regaining of activity and refolding were compared with those for CK modified with various treatments to show that refolding and recovery of activity were synchronized. To further characterize the partially folded CK state and its folding pathway, the molecular chaperone GroEL was used to evaluate whether it can bind with partly folded CK during refolding, and 1-anilinonaphthalene-8-sulfonate was used to detect the hydrophobic surface of the monomeric state of CK. The monomeric state of CK did not bind with GroEL, although it had a larger area of hydrophobic surface relative to the native state. These results may provide different evidence for the structural requirement of GroEL recognition to the substrate protein compared with previously reported results that GroEL bound with substrate proteins mainly through hydrophobic surface. The present study provides data for a monomeric intermediate trapped by the modification of the SH groups during the refolding of CK. Schemes are given for explaining both the partial folding CK pathway and the refolding pathway.Key words: creatine kinase; partially folded state; reactivation; refolding; GroEL; intermediate.

Reactivation kinetics of guanidine hydrochloride-denatured creatine kinase measured using the substrate reaction

Guanidine hydrochloride-denatured creatine kinase (CK) can very quickly form a dimer with reactivity when the denaturant is diluted into the reaction system in the presence of DTT or EDTA. Tsou's method and its applied equation [Tsou (1988), Adv. Enzymol. Rel. Areas Mol. Biol. 61, 381-436; Yang and Zhou (1998), Biochim. Biophys. Acta 1388, 190-198] were used to measure the kinetic reactivation rate constants and the reactivation degree for reassociated CK dimers. Partial reactivation (about 50% at best) occurred following a monophasic course during the substrate reaction when compared with previous time interval measurements. The reactivation degree increased with increasing DTT (0.1-5 mM) and EDTA (0.1-1 mM) concentrations. The apparent forward rate constants do not change with concentration, showing that the reactivation is a reversible first-order reaction, but not of complex formation type. However, the apparent forward rate constants do change with EDTA concentration, showing that the reactivation with EDTA is a reversible first-order reaction as well as of complex formation type. Excess DTT concentrations have an inhibitory effect, indicating that the excessive EDTA acts as a metal chealate not only for free Mg2+, but also for MgATP during the enzyme catalysis. This study shows that additional information about the reactivation of CK can be obtained from examining the substrate reaction. The possible refolding pathway of CK is discussed.

Reactivation and refolding of reassociated dimers of rabbit muscle creatine kinase

Creatine kinase (ATP:creatine N-phosphotransferase, EC 2.7.3.2) is a good model for studying dissociation and reassociation during unfolding and refolding. This study compares self-reassociated CK dimers and CK dimers that contain hybrid dimers under proper conditions. Creatine kinase forms a monomer when denatured in 6 M urea for 1 h which will very quickly form a dimer when the denaturant is diluted under suitable conditions. After modification by DTNB, CK was denatured in 6 M urea to form a modified CK monomer. Dimerization of this modified subunit of CK occurred upon dilution into a suitable buffer containing DTT. Therefore, three different types of reassociated CK dimers including a hybrid dimer can be made from two different CK monomers in the proper conditions. The CK monomers are from a urea-denatured monomer of DTNB-modified CK and from an unmodified urea dissociated monomer. Equal enzyme concentration ratios of these two monomers were mixed in the presence of urea, then diluted into the proper buffer to form the three types of reassociated CK dimers including the hybrid dimer. Reassociated CK dimers including all three different types recover about 75% activity following a two-phase course (k1 = 4.88 x 10(-3) s(-1), k2 = 0.68 x 10(-3) s(-1)). Intrinsic fluorescence spectra of the three different CK monomers which were dissociated in 6 M urea, dissociated in 6 M urea after DTNB modification, and a mixture of the first two dissociated enzymes were studied in the presence of the denaturant urea. The three monomers had different fluorescence intensities and emission maxima. The intrinsic fluorescence maximum intensity changes of the reassociated CK dimers were also studied. The refolding processes also follow biphasic kinetics (k1 = 3.28 x 10(-)3 s(-1), k2 = 0.11 x 10(-3) S(-1)) after dilution in the proper solutions. Tsou's method [Tsou (1988), Adv. Enzymol. Rel. Areas Mol. Biol. 61, 381-436] was also used to measure the kinetic reactivation rate constants for the different three types of reassociated CK dimers, with different kinetic reactivation rate constants observed for each type. CK dissociation and reassociation schemes are suggested based on the results.

Effect of Mg2+ during reactivation and refolding of guanidine hydrochloride-denatured creatine kinase

Creatine kinase (ATP: creatine N-phosphotransferase, EC 2.7.3.2) was completely denatured using 3 M guanidine hydrochloride for 2 h as in previous studies [Yao et al. (1982), Sci. Sin. 25B, 1296-1302; Yao et al. (1984), Biochemistry 23, 2740-2744; Yao et al. (1982), Sci. Sin. 25B, 1186-1193]. Under suitable conditions, about 60-70% of the activity can be recovered in the presence of different Mg2+ concentrations. Both the reactivation and the refolding processes follow two-phase courses after dilution in the proper solutions. A comparison of the rate constants for the refolding of unfolded creatine kinase with those for the recovery of its catalytic activity at various Mg2+ concentrations shows that these are not synchronized. The reactivity of guanidine hydrochloride-denatured creatine kinase can be inhibited by Mg2+; however, the rates of reactivation are independent of the Mg2+ concentration. In addition, Mg2+ affects the fluorescence intensity, but the rate constants of refolding are independent of Mg2+ concentration. Although the reactivation of GdHCl-denatured creatine kinase is complete about 3 h after dilution with reactivation solutions, the conformational changes during refolding occur in a much slower reaction. Mg2+ can induce complex changes in the relative fluorescence intensity during refolding over a broad range of concentrations.

Reactivation kinetics of 5,5'-dithiobis-(2-nitrobenzoic acid)-modified creatine kinase reactivated by dithiothreitol

The reduction of 5,5'-dithiobis-(2-nitrobenzoic acid)-modified creatine by dithiothreitol has been studied using the kinetic theory of the substrate reaction during modification of enzyme activity as previously described by C.L. Tsou (Adv. Enzymol. Rel. Areas Mol. Biol. 61 (1988) 381-436). The results show that the modified creatine kinase can be fully reactivated by an excess concentration of dithiothreitol in a monophasic kinetic course. The presence of ATP or the transition-state analogue markedly slows the apparent reactivation rate constant, while creatine shows no effect. The substrates creatine-ADP-Mg2+ can induce conformational changes of the modified enzyme but adding NO-3 cannot induce further changes that occur with the native enzyme. The reactive cysteines' location and role in the catalysis of creatine kinase are discussed. It is suggested that the cysteine may be located in the hinge area of the two domains of creatine kinase. The reactive cysteine of creatine kinase may play an important role not in the binding to the transition-state analogue but in the conformational changes caused by the transition-state analogue.

ADP-binding and ATP-binding sites in native and proteinase-K-digested creatine kinase, probed by reaction-induced difference infrared spectroscopy

Conformational changes induced by nucleotide binding to native creatine kinase (CK) from rabbit muscle and to proteinase-K-digested (nicked) CK, were investigated by infrared spectroscopy. Photochemical release of ATP from ATP[Et(PhNO2)] in the presence of creatine and native CK produced reaction-induced difference infrared spectra (RIDS) of CK related to structural changes of the enzyme that paralleled the reversible phosphoryl transfer from ATP to creatine. Similarly the photochemical release of ADP from ADP[Et(PhNO2)] in the presence of phosphocreatine and native CK allowed us to follow the backward reaction and its corresponding RIDS. Infrared spectra of native CK indicated that carboxylate groups of Asp or Glu, and some carbonyl groups of the peptide backbone are involved in the enzymatic reaction. Native and proteinase nicked CK have similar Stokes' radii, tryptophan fluorescence, fluorescence fraction accessible to iodide, and far-ultraviolet CD spectra, indicating that native and modified enzymes have the same quaternary structures. However, infrared data showed that the binding site of the gamma-phosphate group of the nucleotide was affected in nicked CK compared with that of the native CK. Furthermore, the infrared absorptions associated with ionized carboxylate groups of Asp or Glu amino acid residues were different in nicked CK and in native CK.

Changes of creatine kinase secondary structure induced by the release of nucleotides from caged compounds. An infrared difference-spectroscopy study

Light-induced release of ADP and ATP from their respective caged nucleotides produced small distinct difference infrared spectra of creatine kinase (CK), indicating that ADP and ATP binding to CK promoted different structural alteration. The positive band at 1638-1640 cm-1 and the negative band at about 1650-1652 cm-1 on the reaction-induced infrared difference spectra in the amide I region were insensitive to the deuteration effects. They were assigned to the peptide backbone of the ADP/ATP-binding site. In addition Pi or ATP binding produced another positive band at 1657-1659 cm-1 corresponding to the C = O (amide I band) associated with the gamma-phosphate of ATP. This site was also affected when ADP was added, indicating coupling interactions between both sites. No additional structural changes were observed when creatine and ADP were added, suggesting that the creatine-binding site was uncoupled from the ADP-binding site. The infrared difference spectra of a transition-state-analog complex formed by the addition of ADP, creatine and NO3- (a planar-phosphate-mimicking group) lacked the 1657-1659-cm-1 band indicating that the binding site of gamma-phosphate within CK, was not affected. Infrared changes in the 1560-1590-cm-1 region suggested that carboxylate groups of Asp or Glu were involved in the binding of Pi, ADP and ATP.

Nitric oxide inhibits creatine kinase and regulates rat heart contractile reserve

Cardiac myocytes express both constitutive and cytokine-inducible nitric oxide syntheses (NOS). NO and its congeners have been implicated in the regulation of cardiac contractile function. To determine whether NO could affect myocardial energetics, 31P NMR spectroscopy was used to evaluate high-energy phosphate metabolism in isolated rat hearts perfused with the NO donor S-nitrosoacetylcysteine (SNAC). All hearts were exposed to an initial high Ca2+ (3.5 mM) challenge followed by a recovery period, and then, either in the presence or absence of SNAC, to a second high Ca2+ challenge. This protocol allowed us to monitor simultaneously the effect of SNAC infusion on both contractile reserve (i.e., baseline versus high workload contractile function) and high-energy phosphate metabolism. The initial high Ca2+ challenge caused the rate-pressure product to increase by 74 +/- 5% in all hearts. As expected, ATP was maintained as phosphocreatine (PCr) content briefly dropped and then returned to baseline during the subsequent recovery period. Control hearts responded similarLy to the second high Ca2+ challenge, but SNAC-treated hearts did not demonstrate the expected increase in rate-pressure product. In these hearts, ATP declined significantly during the second high Ca2+ challenge, whereas phosphocreatine did not differ from controls, suggesting that phosphoryl transfer by creatine kinase (CK) was inhibited. CK activity, measured biochemically, was decreased by 61 +/- 13% in SNAC-treated hearts compared to controls. Purified CK in solution was also inhibited by SNAC, and reversal could be accomplished with DTT, a sulfhydryl reducing agent. Thus, NO can regulate contractile reserve, possibly by reversible nitrosothiol modification of CK.

Creatine kinase: the characteristics of the enzyme regenerated from the thio-methylated creatine kinase reflect a differentiation in function between the two reactive thiols

The activity of S-thiomethyl-modified creatine kinase is due to regeneration of the free thiol (Hou and Vollmer, Biochim Biophys Acta (1994) 1205, 83-88). Characteristics of enzyme regenerated from the S-thiomethyl-modified creatine kinase are reported in the present study. The intrinsic fluorescence of the regenerated enzyme is similar to that of the native enzyme in the presence or absence of the dead-end complex. Regenerated CK (rCK) with full activity has only one reactive thiol. The rate constant of the rCK-reactive thiol reacting with DTNB is close to that of the slow phase of the reactive thiols of the native enzyme. If the IAM-modified rCK is treated with the same method as that for obtaining the rCK, the thiol-methylated reactive thiol of the rCK is reduced to a free SH and a regenerated enzyme, RCK, is produced with about 10% of the rCK activity. Therefore, the different roles of the two reactive thiols of creatine kinase may stem from the characteristics of the rCK, which suggests that only one of the two reactive thiols is related to the activity of the enzyme and the slower phase thiol (the first SH) in the modification reaction with DTNB is directly related to the enzymatic activity, while the faster phase thiol (the second SH) assists the first SH. This compensatory mechanism is proposed in the present study to interpret the dispute on the reactive SH role in the enzymatic catalysis.

Effects of pH and KCl on the conformations of creatine kinase from rabbit muscle. Infrared, circular dichroic and fluorescence studies

The activity loss of creatine kinase (CK), observed at low pH (midpoint was 4.8) corresponded to the monomerization of the dimeric protein and was correlated with structural changes. The acid-induced unfolding was not complete at this pH, as probed by circular dichroic (CD) and fluorescence methods. Further decrease of pH, led to a second transition (midpoint was pH 3.5). The loss of activity was irreversible at pH 4.8 (< 20% native activity was recovered) while it was almost fully reversible (> 90% of native activity was recovered) for the enzyme incubated at pH 0.9-2.5. The amount of intermolecular beta-sheets (monitored with the 1620 cm-1 infrared component band) was maximal when the enzyme was incubated at pH 4.8, as a consequence of protein aggregation, while it was minimal at extremes of pH and at low ionic strength. Acid-induced and alkaline-induced denaturations promoted different structural changes, leading to distinct partially unfolded conformational states. The addition of KCl (from 0.05 M to 0.5 M) to an acidic solution of monomeric creatine kinase (pH 1.6) resulted in a highly cooperative transition from the partially unfolded conformation (UA) to the more compact conformation (A) with the properties of a molten globule, as probed by CD spectra and by fluorescence. The formation of intermolecular beta-sheets in the compact conformation was observed by infrared spectroscopy, indicating formation of unstable aggregates.

Unfolding, conformational change of active sites and inactivation of creatine kinase in SDS solutions

It has been reported that inactivation occurs before noticeable conformational change can be detected during denaturation of creatine kinase (ATP: creatine N-phosphotransferase, EC 2.7.3.2) and other enzymes by guanidinium chloride or urea. Therefore, Tsou suggested that enzyme active sites may display more conformational flexibility than the enzyme molecules as a whole [Tsou (1986) Trends Biochem. Sci. 11, 427-429; Tsou (1993) Science 262, 380-381]. In this study, the conformational change of the active site, the unfolding of the whole molecule and the inactivation of creatine kinase in solutions of different concentrations of SDS are compared. The results show that, at low SDS concentrations, the conformational change of the active site and inactivation of the enzyme occur to nearly the same extent. However, both of these changes occur at much lower concentrations of SDS than required to significantly unfold the enzyme molecule. The rates of conformational changes of enzyme active sites are markedly faster than those of inactivation. However, at the same SDS concentration, both the inactivation rate and the rate of the active site conformational change are much faster than that of the unfolding of the enzyme molecule as a whole. The above results provide direct evidence of the flexibility of the active site of creatine kinase.

Creatine kinase compactness and thiol accessibility during sodium dodecyl sulfate denaturation estimated by resonance energy transfer and 2-nitro-5-thiocyanobenzoic acid cleavage

We have investigated the effect of increasing sodium dodecyl sulfate (SDS) concentrations on rabbit muscle cytosolic creatine kinase structure by two methods. We have first determined the variation of accessibility of the thiol groups of the enzyme during SDS denaturation by a technique which involves an irreversible chemical modification of CK accessible thiol groups, followed by NTCB cleavage before the unmodified cysteines in 8 M urea (pH 9) and analysis of the peptides obtained by resolutive gel electrophoresis, without sequencing. We have determined that the order of accessibility of CK MM cysteine residues during SDS denaturation is Cys-282, Cys-145 and then Cys-253. The fourth cysteine residue, Cys-73, is never titrated even at high SDS/CK molar ratio. In contrast, the three last residues are simultaneously titrated when CK is denatured in guanidinium chloride. Thus, SDS-denatured CK seems to retain some residual organized structure. In order to confirm this hypothesis, compactness of the molecule was estimated by fluorescence energy transfer between CK tryptophans and AEDANS, an extrinsic fluorophore. The location of this fluorophore on the accessible thiol of Cys-282 was verified by the previous technique. The results of these experiments do indicate that SDS-denatured CK is more compact than CK completely unfolded in guanidinium chloride.

The activity of S-thiomethyl modified creatine kinase is due to the regeneration of free thiol at the active site

Creatine kinase modified by S-methyl methanethiosulfonate and devoid of reactive thiol group has been reported to retain about 18-40% of the activity of the native enzyme. It has now been found that during the reaction catalyzed by the modified enzyme the rate increases with time and if the reaction is allowed to continue sufficiently long, the enzyme eventually recovers full activity. The presence of substrates is not required for the reactivation as suitable dilution after removal of MMTS in excess leads to complete reactivation of the MMTS modified enzyme with the simultaneous regeneration of reactive thiol per each dimeric molecule as shown by determinations with DTNB and IAN. The addition of MMTS during the course of reactivation again inactivates the reactivated enzyme. The activity recovery is therefore due to the regeneration of reactive thiol and it appears that the active-site thiols are essential for the activity of rabbit muscle creatine kinase.

Conformational changes at the active site of creatine kinase at low concentrations of guanidinium chloride

It has been previously reported that, during denaturation of creatine kinase by guanidinium chloride (GdmCl) or urea [Tsou (1986), Trends Biochem. Sci. 11, 427-429], inactivation occurs before noticeable conformational change can be detected, and it is suggested that the conformation at the active site is more easily perturbed and hence more flexible than the molecule as a whole. In this study, the thiol and amino groups at or near the active site of creatine kinase are labelled with o-phthalaldehyde to form a fluorescent probe. Both the emission intensity and anisotropy decrease during denaturation indicating exposure of this probe and increased mobility of the active site. The above conformational changes take place together with enzyme inactivation at lower GdmCl concentrations than required to bring about intrinsic fluorescence changes of the enzyme. At the same GdmCl concentration, the rate of exposure of the probe is comparable with that of inactivation and is several orders of magnitude faster than that for the unfolding of the molecule as a whole.

Is the subunit the minimal function unit of creatine kinase?

The dimeric rabbit muscle isozyme of creatine kinase (MM) is modified by iodoacetamide to produce the inactive dimer (M'M') and then hybridized with native dimeric brain isozyme (BB). The hybrid enzyme (M'B), as isolated by PAGE, has the same Km for both ATP and creatine but half the specific activity of the brain isozyme (BB). Likewise, the hybrid of the modified brain with the native muscle isozyme (MB') has half the activity of the native muscle enzyme. The M'B, MB' and MB hybrid dimers all have essentially the same electrophoretic properties, and their intrinsic fluorescence and CD spectra in the far-ultraviolet region are very similar to those of the homodimers MM and BB. Similar results were obtained for the hybrid (M"B) containing the muscle enzyme subunit modified at both the thiol group with iodoacetamide and the Trp residue with dimethyl(2-hydroxy-5-nitrobenzyl)sulfonium bromide and the native brain enzyme submit. The above results suggest strongly the independent catalytic function of the subunit of creatine kinase.

Creatine kinase is modified by 2-chloromercuri-4-nitrophenol at the active site thiols with complete inactivation

Creatine kinase modified by mercurials has been reported to be fully reactive as the native enzyme. This was ascribed to the modification of a second class of thiol groups instead of the reactive thiols at the active site (Laue, M.C. and Quiocho, F.A. (1977) Biochemistry 16, 3838-3845). It has now been shown by spectrophotometric titration and fluorescence studies that 2-chloromercuri-4-nitrophenol (MNP) reacts preferentially with the active-site thiol. Moreover, if the activity of the modified enzyme is determined in the absence of added bovine serum albumin or other enzymes, as usually employed in coupled activity assay systems for creatine kinase, the modified enzyme is completely inactive. Addition of an excess of bovine serum albumin or rabbit muscle glyceraldehyde-3-phosphate dehydrogenase restores the activity of the enzyme to over 80% of its original level. It appears that the active thiol groups at the active site of creatine kinase are after all modified by MNP with complete inactivation.