Evidence for four capital and six auxiliary cation-binding sites on calmodulin: divalent cation interactions monitored by direct binding and microcalorimetry

Intracellular zinc protects Kv7 K+ channels from Ca2+/calmodulin-mediated inhibition

Zinc (Zn) is an essential trace element; it serves as a cofactor for a great number of enzymes, transcription factors, receptors, and other proteins. Zinc is also an important signaling molecule, which can be released from intracellular stores into the cytosol or extracellular space, for example, during synaptic transmission. Amongst cellular effects of zinc is activation of Kv7 (KCNQ, M-type) voltage-gated potassium channels. Here, we investigated relationships between Kv7 channel inhibition by Ca²⁺/calmodulin (CaM) and zinc-mediated potentiation. We show that Zn²⁺ ionophore, zinc pyrithione (ZnPy), can prevent or reverse Ca²⁺/CaM-mediated inhibition of Kv7.2. In the presence of both Ca²⁺ and Zn²⁺, the Kv7.2 channels lose most of their voltage dependence and lock in an open state. In addition, we demonstrate that mutations that interfere with CaM binding to Kv7.2 and Kv7.3 reduced channel membrane abundance and activity, but these mutants retained zinc sensitivity. Moreover, the relative efficacy of ZnPy to activate these mutants was generally greater, compared with the WT channels. Finally, we show that zinc sensitivity was retained in Kv7.2 channels assembled with mutant CaM with all four EF hands disabled, suggesting that it is unlikely to be mediated by CaM. Taken together, our findings indicate that zinc is a potent Kv7 stabilizer, which may protect these channels from physiological inhibitory effects of neurotransmitters and neuromodulators, protecting neurons from overactivity.

Cadmium association with DREAM promotes DREAM interactions with intracellular partners in a similar manner to its physiological ligand, calcium

Cd²⁺exposure has been associated with neurodegenerative diseases and other pathologies, but the underlying mechanism through which it exerts toxic effects remain unresolved.

Defining potential roles of Pb(2+) in neurotoxicity from a calciomics approach

Metal ions play crucial roles in numerous biological processes, facilitating biochemical reactions by binding to various proteins. An increasing body of evidence suggests that neurotoxicity associated with exposure to nonessential metals (e.g., Pb(2+)) involves disruption of synaptic activity, and these observed effects are associated with the ability of Pb(2+) to interfere with Zn(2+) and Ca(2+)-dependent functions. However, the molecular mechanism behind Pb(2+) toxicity remains a topic of debate. In this review, we first discuss potential neuronal Ca(2+) binding protein (CaBP) targets for Pb(2+) such as calmodulin (CaM), synaptotagmin, neuronal calcium sensor-1 (NCS-1), N-methyl-d-aspartate receptor (NMDAR) and family C of G-protein coupled receptors (cGPCRs), and their involvement in Ca(2+)-signalling pathways. We then compare metal binding properties between Ca(2+) and Pb(2+) to understand the structural implications of Pb(2+) binding to CaBPs. Statistical and biophysical studies (e.g., NMR and fluorescence spectroscopy) of Pb(2+) binding are discussed to investigate the molecular mechanism behind Pb(2+) toxicity. These studies identify an opportunistic, allosteric binding of Pb(2+) to CaM, which is distinct from ionic displacement. Together, these data suggest three potential modes of Pb(2+) activity related to molecular and/or neural toxicity: (i) Pb(2+) can occupy Ca(2+)-binding sites, inhibiting the activity of the protein by structural modulation, (ii) Pb(2+) can mimic Ca(2+) in the binding sites, falsely activating the protein and perturbing downstream activities, or (iii) Pb(2+) can bind outside of the Ca(2+)-binding sites, resulting in the allosteric modulation of the protein activity. Moreover, the data further suggest that even low concentrations of Pb(2+) can interfere at multiple points within the neuronal Ca(2+) signalling pathways to cause neurotoxicity.

Pleiotropic roles of Ca+2/calmodulin-dependent pathways in regulating cadmium-induced toxicity in human osteoblast-like cell lines

The heavy metal cadmium is a widespread environmental contaminant that has gained public attention due to the global increase in cadmium-containing electronic waste. Human exposure to cadmium is linked to the pathogenesis of osteoporosis. We previously reported cadmium induces apoptosis and decreases alkaline phosphatase mRNA expression via extracellular signal-regulated protein kinase (ERK) activation in Saos-2 bone-forming osteoblasts. This study examines the mechanisms of cadmium-induced osteotoxicity by investigating roles of Ca⁺²/calmodulin-dependent protein kinase (CAMK) pathways. Saos-2 or MG-63 cells were treated for 24 or 48h with 5μM CdCl₂ alone or in combination with calmodulin-dependent phosphodiesterase (PDE) inhibitor CGS-9343β; calmodulin-dependent kinase kinase (CAMKK) inhibitor STO-609; or calmodulin-dependent kinase II (CAMKII) inhibitor KN-93. CGS-9343β protected against cadmium-induced toxicity and attenuated ERK activation; STO-609 enhanced toxicity and exacerbated ERK activation, whereas KN-93 had no detectable effect on cadmium-induced toxicity. Furthermore, CGS-9343β co-treatment attenuated cadmium-induced apoptosis; but CGS-9343β did not recover cadmium-induced decrease in ALP activity. The major findings suggest the calmodulin-dependent PDE pathway facilitates cadmium-induced ERK activation leading to apoptosis, whereas the CAMKK pathway plays a protective role against cadmium-induced osteotoxicity via ERK signaling. This research distinguishes itself by identifying pleiotropic roles for CAMK pathways in mediating cadmium's toxicity in osteoblasts.

FhCaBP2: a Fasciola hepatica calcium-binding protein with EF-hand and dynein light chain domains

FhCaBP2 is a Fasciola hepatica protein which belongs to a family of helminth calcium-binding proteins which combine an N-terminal domain containing two EF-hand motifs and a C-terminal dynein light chain-like (DLC-like) domain. Its predicted structure showed two globular domains joined by a flexible linker. Recombinant FhCaBP2 interacted reversibly with calcium and manganese ions, but not with magnesium, barium, strontium, copper (II), colbalt (II), iron (II), nickel, lead or potassium ions. Cadmium (II) ions appeared to bind non-site-specifically and destabilize the protein. Interaction with either calcium or magnesium ions results in a conformational change in which the protein's surface becomes more hydrophobic. The EF-hand domain alone was able to interact with calcium and manganese ions; the DLC-like domain was not. Alteration of a residue (Asp-58 to Ala) in the second EF-hand motif in this domain abolished ion-binding activity. This suggests that the second EF-hand is the one responsible for ion-binding. FhCaBP2 homodimerizes and the extent of dimerization was not affected by calcium ions or by the aspartate to alanine substitution in the second EF-hand. The isolated EF-hand and DLC-like domains are both capable of homodimerization. FhCaBP2 interacted with the calmodulin antagonists trifluoperazine, chlorpromazine, thiamylal and W7. Interestingly, while chlorpromazine and thiamylal interacted with the EF-hand domain (as expected), trifluoperazine and W7 bound to the DLC-like domain. Overall, FhCaBP2 has distinct biochemical properties compared with other members of this protein family from Fasciola hepatica, a fact which supports the hypothesis that these proteins have different physiological roles.

Negatively charged amino acids near and in transient receptor potential (TRP) domain of TRPM4 channel are one determinant of its Ca2+ sensitivity

Transient receptor potential (TRP) channel melastatin subfamily member 4 (TRPM4) is a broadly expressed nonselective monovalent cation channel. TRPM4 is activated by membrane depolarization and intracellular Ca(2+), which is essential for the activation. The Ca(2+) sensitivity is known to be regulated by calmodulin and membrane phosphoinositides, such as phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). Although these regulators must play important roles in controlling TRPM4 activity, mutation analyses of the calmodulin-binding sites have suggested that Ca(2+) binds to TRPM4 directly. However, the intrinsic binding sites in TRPM4 remain to be elucidated. Here, by using patch clamp and molecular biological techniques, we show that there are at least two functionally different divalent cation-binding sites, and the negatively charged amino acids near and in the TRP domain in the C-terminal tail of TRPM4 (Asp-1049 and Glu-1062 of rat TRPM4) are required for maintaining the normal Ca(2+) sensitivity of one of the binding sites. Applications of Co(2+), Mn(2+), or Ni(2+) to the cytosolic side potentiated TRPM4 currents, increased the Ca(2+) sensitivity, but were unable to evoke TRPM4 currents without Ca(2+). Mutations of the acidic amino acids near and in the TRP domain, which are conserved in TRPM2, TRPM5, and TRPM8, deteriorated the Ca(2+) sensitivity in the presence of Co(2+) or PI(4,5)P2 but hardly affected the sensitivity to Co(2+) and PI(4,5)P2. These results suggest a novel role of the TRP domain in TRPM4 as a site responsible for maintaining the normal Ca(2+) sensitivity. These findings provide more insights into the molecular mechanisms of the regulation of TRPM4 by Ca(2+).

Metal toxicity and opportunistic binding of Pb(2+) in proteins

Lead toxicity is associated with various human diseases. While Ca(2+) binding proteins such as calmodulin (CaM) are often reported to be molecular targets for Pb(2+)-binding and lead toxicity, the effect of Pb(2+) on the Ca(2+)/CaM regulated biological activities cannot be described by the primary mechanism of ionic displacement (e.g., ionic mimicry). The focus of this study was to investigate the mechanism of lead toxicity through binding differences between Ca(2+) and Pb(2+) for CaM, an essential intracellular trigger protein with two EF-Hand Ca(2+)-binding sites in each of its two domains that regulates many molecular targets via Ca(2+)-induced conformational change. Fluorescence changes in phenylalanine indicated that Pb(2+) binds with 8-fold higher affinity than Ca(2+) in the N-terminal domain. Additionally, NMR chemical shift changes and an unusual biphasic response observed in tyrosine fluorescence associated with C-terminal domain sites EF-III and EF-IV suggest a single higher affinity Pb(2+)-binding site with a 3-fold higher affinity than Ca(2+), coupled with a second site exhibiting affinity nearly equivalent to that of the N-terminal domain sites. Our results further indicate that Pb(2+) displaces Ca(2+) only in the N-terminal domain, with minimal perturbation of the C-terminal domain, however significant structural/dynamic changes are observed in the trans-domain linker region which appear to be due to Pb(2+)-binding outside of the known calcium-binding sites. These data suggest that opportunistic Pb(2+)-binding in Ca(2+)/CaM has a profound impact on the conformation and dynamics of the essential molecular recognition sites of the central helix, and provides insight into the molecular toxicity of non-essential metal ions.

Distinct mechanisms of calmodulin binding and regulation of adenylyl cyclases 1 and 8

Calmodulin (CaM), by mediating the stimulation of the activity of two adenylyl cyclases (ACs), plays a key role in integrating the cAMP and Ca(2+) signaling systems. These ACs, AC1 and AC8, by decoding discrete Ca(2+) signals can contribute to fine-tuning intracellular cAMP dynamics, particularly in neurons where they predominate. CaM comprises an α-helical linker separating two globular regions at the N-terminus and the C-terminus that each bind two Ca(2+) ions. These two lobes have differing affinities for Ca(2+), and they can interact with target proteins independently. This study explores previous indications that the two lobes of CaM can regulate AC1 and AC8 differently and thereby yield different responses to cellular transitions in [Ca(2+)](i). We first compared by glutathione S-transferase pull-down assays and offline nanoelectrospray ionization mass spectrometry the interaction of CaM and Ca(2+)-binding deficient mutants of CaM with the internal CaM binding domain (CaMBD) of AC1 and the two terminal CaMBDs of AC8. We then examined the influence of these three CaMBDs on Ca(2+) binding by native and mutated CaM in stopped-flow experiments to quantify their interactions. The three CaMBDs show quite distinct interactions with the two lobes of CaM. These findings establish the critical kinetic differences between the mechanisms of Ca(2+)-CaM activation of AC1 and AC8, which may underpin their different physiological roles.

The use of ESI-MS to probe the binding of divalent cations to calmodulin

Proteins have evolved with distinct sites for binding particular metal ions. This allows metalloproteins to perform a myriad of specialized tasks with conformations tailor-made by the combination of its primary sequence and the effect on this of the ligated metal ion. Here we investigate the selectivity of the calcium trigger protein calmodulin for divalent metal ions. This ubiquitous and highly abundant protein exists in equilibrium between its apo and its holo form wherein four calcium ions are bound. Amongst its many functions, calmodulin modulates the calcium concentration present in cells, but this functional property renders it a target for competition from other metal ions. We study the competition posed by four other divalent cations for the calcium binding sites in calmodulin using electrospray ionization mass spectrometry (ESI-MS). We have chosen two other group II cations Mg(2+), Sr(2+), and two heavy metals Cd(2+), Pb(2+). The ease with which each of these metals binds to apo and to holo CaM[4Ca] is described. We find that each metal ion has different properties with respect to calmodulin binding and competition with calcium. The order of affinity for apo CaM is Ca(2+) >> Sr(2+) approximately Mg(2+) > Pb(2+) approximately Cd(2+). In the presence of calcium the affinity alters to Pb(2+) > Ca(2+) > Cd(2+) > Sr(2+) > Mg(2+). Once complexes have been formed between the metal ions and protein (CaM:[xM]) we investigate whether the structural change which must accompanies calcium ligation to allow target binding takes place for a given CaM:[xM] system. We use a 20 residue target peptide, which forms the CaM binding site within the enzyme neuronal nitric-oxide synthase. Our earlier work (Shirran et al. 2005) [1] has demonstrated the particular selectivity of this system for CaM:4Ca(2+). We find that along with Ca(2+) only Pb(2+) forms complexes of the form CaM:4M(2+):nNOS. This work demonstrates the affinity for calcium above all other metals, but also warns about the ability of lead to replace calcium with apparent ease.

Significance of the extra C-terminal tail of CaLP, a novel calmodulin-like protein involved in oyster calcium metabolism

Oyster (Pinctada fucata) calmodulin-like protein (CaLP), containing a C-terminally extra hydrophilic tail (150D-161K), is a novel protein involved in the regulation of oyster calcium metabolism. To investigate the importance of the extra fragment to the Ca(2+)/Mg(2+)-dependent conformational changes in the intact CaLP molecule and the interactions between CaLP and its target proteins, a truncated CaLP mutant (M-CaLP) devoid of the extended C-terminus was constructed and overexpressed in Escherichia coli. The conformational characteristics of M-CaLP were studied by CD and fluorescence spectroscopy and compared with those of the oyster CaM and CaLP. The far-UV CD results reveal that the extra tail has a strong effect on the Ca(2+)-induced, but a relatively weak effect on the Mg(2+)-induced conformational changes in CaLP. However, upon Ca2+ or Mg2+ binding, only slight changes for intrinsic phenylalanine and tyrosine fluorescence spectra between M-CaLP and CaLP are observed. Our results also indicate that the extra tail can significantly decrease the exposure of the hydrophobic patches in CaLP. Additionally, affinity chromatography demonstrates that the target binding of CaLP is greatly influenced by its additional tail. All our results implicate that the extra tail may play some important roles in the interactions between CaLP and its targets in vivo.

Conformational chemistry of surface-attached calmodulin detected by acoustic shear wave propagation

A thickness shear-mode acoustic wave device, operated in a flow-through format, was used to detect the binding of ions or peptides to surface-attached calmodulin. On-line surface attachment of the protein was achieved by immobilisation of the biotinylated molecule via a neutravidin-biotin linkage onto the surface of the gold electrode of the detector. The interaction between calmodulin, and calcium and magnesium ions induced an increase in resonant frequency and a decrease in motional resistance, which were reversible on washing with buffer. Interestingly, the changes in resonant frequency and motional resistance induced by the binding were opposite to the normal operation of the detector. The response was interpreted as a decrease in surface coupling (partial slip at the liquid/solid interface) instigated by exposure of hydrophobic domains on the protein, and an increase in the thickness, and hence effective wavelength, of the acoustic device, corresponding to an increase in the length of calmodulin by 1.5 A. This result is consistent with the literature value of 4 A. In addition, the interaction of the protein with peptide together with calcium ions was detected successfully, despite the relatively low molecular mass of the 2-kDa peptide. These results confirm the potential of acoustic wave physics for the detection of changes in the conformational chemistry of monolayer of biochemical macromolecules at the solid/liquid interface.

The formation of a complex between calmodulin and neuronal nitric oxide synthase is determined by ESI-MS

Calmodulin (CaM) is an acidic ubiquitous calcium binding protein, involved in many intracellular processes, which often involve the formation of complexes with a variety of protein and peptide targets. One such system, activated by Ca2+ loaded CaM, is regulation of the nitric oxide synthase (NOS) enzymes, which in turn control the production of the signalling molecule and cytotoxin NO. A recent crystallographic study mapped the interaction of CaM with endothelial NOS (eNOS) using a 20 residue peptide comprising the binding site within eNOS. Here the interaction of CaM to the FMN domain of neuronal nitric oxide synthase (nNOS) has been investigated using electrospray ionization mass spectrometry (ESI-MS). The 46 kDa complex formed by CaM-nNOS has been retained in the gas-phase, and is shown to be exclusively selective for CaM.4Ca2+. Further characterization of this important biological system has been afforded by examining a complex of CaM with a 22 residue synthetic peptide, which represents the linker region between the reductase and oxygenase domains of nNOS. This nNOS linker peptide, which is found to be random coil in aqueous solution by both circular dichroism and molecular modelling, also exhibits great discrimination for the form of CaM loaded with 4[Ca2+]. The peptide binding loop is presumed to be configured to an alpha-helix on binding to CaM as was found for the related eNOS binding peptide. Our postulate is supported by gas-phase molecular dynamics calculations performed on the isolated nNOS peptide. Collision induced dissociation was employed to probe the strength of binding of the nNOS binding peptide to CaM.4Ca2+. The methodology taken here is a new approach in understanding the CaM-nNOS binding site, which could be employed in future to inform the specificity of CaM binding to other NOS enzymes.

Ligand-linked stability of mutants of the C-domain of calmodulin

There is a necessary energetic linkage between ligand binding and stability in biological molecules. The critical glutamate in Site 4 was mutated to create two mutants of the C-domain of calmodulin yielding E140D and E140Q. These proteins were stably folded in the absence of calcium, but had dramatically impaired binding of calcium. We determined the stability of the mutant proteins in the absence and presence of calcium using urea-induced unfolding monitored by circular dichroism (CD) spectroscopy. These calcium-dependent unfolding curves were fit to models that allowed for linkage of stability to binding of a single calcium ion to the native and unfolded states. Simultaneous analysis of the unfolding profiles for each mutant yielded estimates for calcium-binding constants that were consistent with results from direct titrations monitored by fluorescence. Binding to the unfolded state was not an important energetic contributor to the ligand-linked stability of these mutants.

Relative affinity constants by electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry: calmodulin binding to peptide analogs of myosin light chain kinase

Synthetic RS20 peptide and a set of its point-mutated peptide analogs have been used to analyze the interactions between calmodulin (CaM) and the CaM-binding sequence of smooth-muscle myosin light chain kinase both in the presence and the absence of Ca(2+). Particular peptides, which were expected to have different binding strengths, were chosen to address the effects of electrostatic and bulky mutations on the binding affinity of the RS20 sequence. Relative affinity constants for protein/ligand interactions have been determined using electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry. The results evidence the importance of electrostatic forces in interactions between CaM and targets, particularly in the presence of Ca(2+), and the role of hydrophobic forces in contributing additional stability to the complexes both in the presence and the absence of Ca(2+).

Effects of combined exposure to lead and cadmium on the hypothalamic-pituitary axis function in proestrous rats

The effects of lead and cadmium on the hypothalamic-pituitary axis were studied in proestrous rats. Adult female rats were treated intraperitonially with either lead acetate and cadmium acetate alone or in combination at a dose of 0.05 mg/kg daily for 15 days. Serotonin (5-HT) and norepinephrine (NE) levels decreased in individually and combined metal treated groups whereas dopamine (DA) levels were decreased only in the cadmium-exposed group. The pituitary levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH) were decreased significantly in cadmium and combined treatment groups. In contrast, lead exposure failed to cause any change in serum LH and FSH levels, whereas cadmium and combined treatments showed significant decrease in serum LH and FSH levels as compared with control. The accumulation of both metals increased in the hypothalamus and pituitary after treatment. These data suggest that the metal accumulation disrupts the regulatory mechanisms of the hypothalamic-pituitary axis where the effects produced by the combined treatment of metals are not additive.

Cation- and peptide-binding properties of human calmodulin-like skin protein

Human CLSP, a new Ca(2+)-binding protein specifically expressed in differentiated keratinocytes, is a 15.9 kDa, four EF-hand containing protein with 52% sequence identity to calmodulin (CaM). The protein binds four Ca(2+) ions at two pairs of sites with [Ca(2+)](0.5) values of 1.2 and 150 microM, respectively. Mg(2+) at millimolar concentrations strongly decreases the affinity for Ca(2+) of the two high-affinity sites, but has no effect on the low-affinity sites. The protein can also bind two Mg(2+) ([Mg(2+)](0.5) = 57 microM) at the sites of high Ca(2+) affinity. Thus, as fast skeletal muscle troponin C (TnC), CLSP possesses two high-affinity Ca(2+)-Mg(2+) mixed sites and two low-affinity Ca(2+)-specific sites. Studies on the isolated recombinant N- (N-CLSP) and C-terminal half domains of CLSP (C-CLSP) revealed that, in contrast to the case of TNC, the high-affinity Ca(2+)-Mg(2+) mixed sites reside in the N-terminal half. The binding of cations modifies the intrinsic fluorescence of the two Tyr residues. Upon Ca(2+) binding, hydrophobicity is exposed at the protein surface that can be monitored with a fluorescent probe. The Ca(2+)-dependency of the two conformational changes is biphasic in the absence of Mg(2+), but monophasic in the presence of 2 mM Mg(2+), both corresponding closely to direct binding of Ca(2+) to CLSP. In the presence of Ca(2+), human CLSP forms a high-affinity 1:1 complex with melittin, a natural peptide considered to be a model for the interaction of CaM with its targets. In the complex, CLSP binds Ca(2+) with high affinity to all four binding sites. Isolated N- and C-CLSP show only a weak interaction with melittin, which is enhanced when both halves are simultaneously presented to the model peptide.

Oral cadmium exposure throughout puberty does not inhibit secretion of prolactin, GH and ACTH through dopamine metabolism changes in male rat

This work analyzed possible dopamine-mediated cadmium effects on plasma prolactin, GH and ACTH levels and if these changes were related to metal accumulation. Male rats were treated from day 30 to 60 of life with 50 mg/L of CdCl2 in the drinking water. Cadmium exposure decreased the dopamine (DA) metabolism (DOPAC/DA ratio) in all brain areas studied, and plasma levels of prolactin, GH and ACTH were diminished. The cadmium concentration did not increase nor in hypothalamus nor in the pituitary after the metal exposure. These results suggest that cadmium inhibits the secretion of these pituitary hormones and this inhibitory effect is not mediated by dopamine or the degree of metal accumulation.

Cu2+ and Zn2+ inhibit nitric-oxide synthase through an interaction with the reductase domain

Cu(2+) and Zn(2+) inhibit all of the NADPH-dependent reactions catalyzed by neuronal nitric-oxide synthase (nNOS) including ferricytochrome c reduction, NADPH oxidation, and citrulline formation. Cu(2+) and Zn(2+) also inhibit ferricytochrome c reduction by the independent reductase domain. Zn(2+) affects all activities of the full-length nNOS and the reductase domain to the same extent (estimated IC(50) values from 9 to 31 microm), suggesting Zn(2+) occupation of a single site in the reductase domain. Citrulline formation and NADPH oxidation by the full-length nNOS and ferricytochrome c reduction by the reductase domain are affected similarly by Cu(2+), with estimated IC(50) values ranging from 6 to 33 microm. However, Cu(2+) inhibits ferricytochrome c reduction by the full-length nNOS 2 orders of magnitude more potently, with an estimated IC(50) value of 0.12 microm. These data suggest the possibility that Cu(2+) may interact with nNOS at two sites, one composed exclusively of the reductase domain (which is perhaps also involved in Zn(2+)-mediated inhibition), and another that includes components of both domains. Occupation of the second (higher affinity) site could then promote the selective inhibition of ferricytochrome c reduction in full-length nNOS. Neither the inhibition by Cu(2+) nor that by Zn(2+) is dependent on calmodulin.

Effects of ethanol volume percent on fluorescein-labeled spinach apo- and holocalmodulin

We report the effects of EtOH volume percent (0-70%) on spinach apo- and holocalmodulin that have been site-selectively labeled with fluorescein (F). In these experiments, calmodulin (CaM) has one F reporter group attached to Cys-26, and this site is located immediately adjacent to one of the four primary Ca(2+)-binding sites (EF hands). The optimum analytical CaM-F sensitivity to Ca2+ occurs between approximately 10 and 30% EtOH. Our results also show that added EtOH causes changes in CaM and these changes are surprisingly different for apo- and holo-CaM. Apo-CaM-F appears to lose one of its two waters of hydration at approximately 20% EtOH and retains one water of hydration between approximately 20 and 70% EtOH. In apo-CaM-F, the semiangle that describes the range over which the fluorescein reporter group can precess remains essentially constant (42 +/- 2 degrees) between 0 and 70% EtOH. This shows that the fluorescein reporter group precessional freedom in apo-CaM-F is not affected significantly by EtOH. Holo-CaM-F also appears to lose one water of hydration at approximately 20-30% EtOH but then appears to denature as the EtOH volume percent increases. The fluorescein reporter group semiangle within holo-CaM-F decreases from 43 +/- 1 degrees in neat aqueous buffer to 36 +/- 1 degrees at 70% EtOH. This shows that holo-CaM-F is less nativelike and the EF hand "closes down" about the fluorescein reporter group in holo-CaM-F as the EtOH volume percent increases.

Cadmium effects on hypothalamic activity and pituitary hormone secretion in the male

Cadmium specifically modify amine metabolism at the central nervous system and pituitary hormone secretions. Thus, the physiological functions controlled by these hormones can be modulated by cadmium. This xenobiotic is associated with deleterious effects on the gonadal function and with changes in the secretory pattern of other pituitary hormones like prolactin, ACTH, GH or TSH. The observed changes in pituitary hormone secretion do not correlate with the modifications of central nervous system metabolism of the neurotransmitters involved in their regulation. The accumulative data indicates the existence of a disruption in the regulatory mechanisms of the hypothalamic-pituitary axis. The physiological significance of these effects remains to be elucidated.

Metal ion selectivity for formation of the calmodulin-metal-target peptide ternary complex studied by surface plasmon resonance spectroscopy

Ion selectivities for Ca(2+) signaling pathways of 33 metal ions were examined based on the Ca(2+)-dependent on/off switching mechanism of calmodulin (CaM): Ca(2+) ion-induced selective binding of CaM-Ca(2+) ion complex to the target peptide was observed as an increase in surface plasmon resonance (SPR) signals. As the target peptide, M13 of 26-amino-acid residues derived from skeletal muscle myosin light-chain kinase was immobilized in the dextran matrix, over which sample solutions containing CaM and each metal ion were injected in a flow system. Large changes in SPR signals were also observed for Sr(2+), Ba(2+), Cd(2+), Pb(2+), Y(3+) and trivalent lanthanide ions, thereby indicating that not only Ca(2+) but also these metal ions induce the formation of CaM-M13-metal ion ternary complex. No SPR signal was, however, induced by Mg(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+) and all monovalent metal ions examined. The latter silent SPR signal indicates that these ions, even if they bind to CaM, are incapable of forming the CaM-M13-metal ion ternary complex. Comparing the obtained SPR results with ionic radii of those metal ions, it was found that all cations examined with ionic radii close to or greater than that of Ca(2+) induced the formation of the CaM-metal-M13 ternary complex, whereas those with smaller ionic radii were not effective, or much less so. Since these results are so consistent with earlier systematic data for the effects of various metal ions on the conformational changes of CaM, it is concluded that the present SPR analysis may be used for a simple screening and evaluating method for physiologically relevant metal ion selectivity for the Ca(2+) signaling via CaM based on CaM/peptide interactions.

Effects of chronic alternating cadmium exposure on the episodic secretion of prolactin in male rats

Cadmium increases or decreases prolactin secretion depending on the dose and duration of the exposure to the metal. However, whether there are cadmium effects on the episodic prolactin secretion is less well known. This study was undertaken to address whether chronic alternating exposure to two different doses of cadmium affects the episodic pattern of prolactin and to what extent the effects of cadmium are age-dependent. Male rats were treated s.c. with cadmium chloride (0.5 or 1.0 mg/kg) from day 30 to 60, or from day 60 to 90 of age, with alteration of the doses every 4 days, starting with the smaller dose. Controls received vehicle every 4 days. The last dose of cadmium was given 48 h prior to the pulsatility study. Prolactin secretion in the 4 experimental groups studied was episodic and changed significantly after cadmium exposure. Cadmium administration from day 30 to 60 of life significantly decreased the mean half-life of prolactin. On the other hand, when administered from day 60 to 90 cadmium significantly decreased the mean as well as serum prolactin levels and the absolute amplitude of the prolactin pulses, their duration, the relative amplitude or the mean half-life of the hormone. The frequency of prolactin peaks was not changed by cadmium administration. The results indicate that low intermittent doses of cadmium chronically administered change the episodic secretion pattern of prolactin in rats. The effects of cadmium on prolactin secretion were age dependent.

Evidence of noncovalent dimerization of calmodulin

Calcium-binding proteins, such as S-100, dimerize readily, and this phenomenon plays an important role in their regulation of target enzymes [Krebs, J., Quadroni, M. & Van Eldik, L.J. (1995) Nat. Struct. Biol. 2, 711-714; Kilby, P.M., Van Eldik, L.J. & Roberts, G. C. (1996) Structure 4, 1041-1052]. We have investigated by Fourier-transform ion cyclotron resonance (FTICR) MS the conformational states of the calcium-binding protein calmodulin, and present clear evidence for a calmodulin dimer formed as a result of noncovalent interactions between folded monomers. Ultra-high-resolution electrospray ionization (ESI) mass spectra for calmodulin, obtained with a 9.4 T FTICR mass spectrometer, are presented. With the use of denaturing solutions (1 : 1 acetonitrile/water + 1% formic acid), relatively high charge states (20 < z < 10) of monomeric calmodulin ions were detected, whereas when calmodulin was electrosprayed from buffer, monomers ions with only 5-10 charges were detected. CD measurements for calmodulin in buffered solution revealed that its alpha-helical content was significantly higher than that for calmodulin in acetonitrile/water solutions, consistent with a proposition that changes in charge state distributions observed in the MS experiments reflect differing states of calmodulin folding. Under buffered conditions, noncovalently bound calmodulin dimers were observed by ESI FTICR MS. Analytical ultracentrifugation experiments carried out in the same solution conditions as those used in the MS experiments were consistent with the proposed calmodulin dimer-monomer equilibrium. The ultra-high mass resolution achieved with the 9.4 T FTICR mass spectrometer allowed unequivocal identification of the noncovalent, as opposed to covalent, character of the calmodulin dimer.

Thermodynamic analysis of calcium and magnesium binding to calmodulin

To elucidate some aspects still debated concerning the interaction of Ca2+ and Mg2+ with CaM, the thermodynamic binding parameters of Ca2+-CaM and Mg2+-CaM complexes were characterized by flow dialysis and isothermal microcalorimetry under different experimental conditions. In particular, the enthalpy and entropy changes associated with Ca2+ and Mg2+ binding to their sites were determined, allowing a better understanding of the mechanism underlying cation-CaM interactions. Ca2+-CaM interaction follows an enthalpy-entropy compensation relationship, suggesting that CaM explores a subspace of isoenergetical conformations which is modified by Ca2+ binding. This Ca2+-induced change in CaM dynamics is proposed to play a key role in CaM function, i.e. in its interaction with and/or activation of target proteins. Furthermore, data show that Mg2+ does not act as a direct competitor for Ca2+ binding on the four main Ca2+ binding sites, but rather as an allosteric effector. This implies that the four main Mg2+ binding sites are distinct from the EF-hand Ca2+ binding sites. Finally, Ca2+ is shown to interact with auxiliary binding sites on CaM. These weak affinity sites were thermodynamically characterized. The results presented here challenge the current accepted view of CaM ion binding.

Conformation and thermal denaturation of apocalmodulin: role of electrostatic mutations

Scanning microcalorimetry and circular dichroism were used to study conformational state and heat denaturation of Ca2+-free synthetic calmodulin (SynCaM) and three charge reversal mutants. We produced evidence for the major role of the electrostatic potential in the stability and flexibility of SynCaM. The substitution of 118DEE120 by 118KKK120 (SynCaM12A) does not influence the flexibility of the protein; the replacement of 82EEE84 by 82KKK84 (SynCaM8) decreases its level, while the combination of these two mutations in SynCaM18A significantly increases the flexibility. The heat denaturation of apoSynCaM and its mutants is well approximated by two two-state transitions with the lower-temperature transition corresponding to C-terminal lobe melting and the higher-temperature one to N-terminal lobe melting. The difference in transition temperatures for the two lobes decreases in SynCaM8 and increases in SynCaM18A, suggesting a modification in the influence of one lobe to the other. The electrostatic mutations change the parameters of thermal denaturation of SynCaM lobes in a similar way as pH conditions affect thermal transition parameters of multidomain proteins, leading to a linear temperature dependence of transition enthalpy. One domain of the N-terminal lobe in apoSynCaM18A is unfolded in the native state. Near-UV CD spectra point out the invariability of the local structure of aromatic residues upon mutations, although the secondary structure undergoes striking transformations. Cacodylate ions strongly and specifically alter the helical content of SynCaM. Our data unambiguously demonstrate that the two lobes are not independent, and interactions between the lobes are mediated by the electrostatic potential of the molecule.

Trace element deficiencies in cattle

Deficiency of cobalt, copper, iron, iodine, manganese, selenium, or zinc can cause a reduction in production. Reduced production occurs most commonly when a deficiency corresponds to the phases of growth, reproduction, or lactation. Because of environmental, nutrient, disease, genetic, and drug interactions, deficiencies of single or multiple elements can occur even when the levels recommended by the National Research Council for these nutrients are being fed. Additionally, random supplementation of trace elements above National Research Council recommendations is not justified because of the negative interaction among nutrients and potential toxicosis. Evaluation of trace element status can be difficult because many disease states will alter blood analytes used to evaluate nutrient adequacy. Proper dietary and animal evaluation, as well as response to supplementation, are necessary before diagnosing a trace element deficiency.