Search

Swelling of epidermal cell walls decreases cell-to-cell adhesion and increases cracking susceptibility in sweet cherry. Ca is suggested to decrease cracking susceptibility by crosslinking of cell wall components and, possibly, by decreasing swelling. The objective is to test this hypothesis. The effect of Ca on swelling of anticlinal epidermal cell walls was quantified microscopically in vivo using excised skin sections and in vitro using extracted cell walls. After removal of turgor, cell wall thickness increased. Incubation in CaCl2 decreased cell wall thickness up to 3 mM CaCl2. At higher concentrations thickness remained constant. Decreased cell wall swelling in vivo also occurred with other salts of divalent and trivalent cations, but not with those of monovalent cations. Decreased swelling was due to the Ca cation, the anions had no effect. Ca also decreased swelling of cell walls that were already swollen. CaCl2 also decreased swelling of extracted cell walls in vitro. There was no effect on swelling pressure. The effect on swelling increased as the CaCl2 concentration increased. Chlorides of divalent and trivalent cations, but not those of monovalent cations decreased swelling in vitro. The decrease in swelling among the divalent cations was linearly related to the radius of the cation. The results indicate that Ca decreases cracking susceptibility by decreasing swelling.

Platelet-activating factor (PAF) is a potent phospholipid mediator that exerts various pathophysiological effects by interacting with a G protein-coupled receptor. PAF has been reported to increase the permeability of the blood-brain barrier (BBB) via incompletely characterized mechanisms. We investigated the effect of PAF on rat brain microvascular endothelial cells (RBMVEC), a critical component of the BBB. PAF produced a dose-dependent increase in cytosolic Ca2+ concentration; the effect was prevented by the PAF receptor antagonist, WEB2086. The effect of PAF on cytosolic Ca2+ was abolished in Ca2+-free saline or in the presence of L-type voltage-gated Ca2+ channel inhibitor, nifedipine, indicating that Ca2+ influx is critical for PAF-induced increase in cytosolic Ca2+. PAF produced RBMVEC depolarization; the effect was inhibited by WEB2086. In cells loaded with [(4-amino-5-methylamino-2',7'-difluoro-fluorescein)diacetate] (DAF-FM), a nitric oxide (NO)-sensitive fluorescent dye, PAF increased the NO level; the effect was prevented by WEB2086, nifedipine or by l-NAME, an inhibitor of NO synthase. Immunocytochemistry studies indicate that PAF reduced the immunostaining of ZO-1, a tight junction-associated protein, increased F-actin fibers, and produced intercellular gaps. PAF produced a decrease in RBMVEC monolayer electrical resistance assessed with Electric Cell-Substrate Impedance Sensing (ECIS), indicative of a disruption of endothelial barrier function. In vivo studies indicate that PAF increased the BBB permeability, assessed with sodium fluorescein and Evans Blue methods, via PAF receptor-dependent mechanisms, consequent to Ca2+ influx and increased NO levels. Our studies reveal that PAF alters the BBB permeability by multiple mechanisms, which may be relevant for central nervous system (CNS) inflammatory disorders.

© 2014 American Chemical Society. The dechlorination of trichloroethylene (TCE) by aged Fe0 in the presence of a series of divalent cations was investigated with the result that while no significant degradation of TCE was observed in Milli-Q water or in solutions of Ba2+, Sr2+, or Ca2+, very effective TCE removal was observed in solutions containing Mg2+, Mn2+, Co2+, Fe2+, Ni2+, Zn2+, Cu2+, or Pb2+. The rate constants of TCE removal in the presence of particular cations were positively correlated to the log K representing the affinity of the cations for hydrous ferric oxide (HFO) surface sites though the treatments with Co2+ and Ni2+ were found to provide particularly strong enhancement in TCE degradation rate. The extent of Fe(II) release to solution also increased with increase in log K, while the solution pH from both experimental measurement and thermodynamic calculation decreased with increasing log K. While the peak areas of Fe and O XPS spectra of the passivated ZVI in the presence of Ba2+, Sr2+, and Ca2+ were very close to those in Milli-Q water, very significant increases in surface Fe and O (and OH) were observed in solutions of Mg2+, Mn2+, Co2+, Fe2+, Ni2+, Zn2+, Cu2+ and Pb2+, revealing that the surface oxide layer dissolution is consistent with the recovery of aged Fe0 with respect to TCE degradation. The depassivation process is proposed to involve (i) surface complexation of cations on surface coatings of aged Fe0, (ii) dissolution of the hydrated surface as a consequence of magnetite exposure, and (iii) transport of electrons from underlying Fe0 via magnetite to TCE, resulting in TCE dechlorination and, for some cations (Co2+, Ni2+, Cu2+, and Pb2+), reduction to their zero or +1 valence state (with potential for these reduced metals to enhance TCE degradation).

Phosphoglycerate mutases (PGAMs) participate in both the glycolytic and the gluconeogenic pathways in reversible isomerization of 3-phosphoglycerate and 2-phosphoglycerate. PGAMs are members of two distinct protein families: enzymes that are dependent on or independent of the 2,3-bisphosphoglycerate cofactor. We determined the X-ray structure of the monomeric Trypanosoma brucei independent PGAM (TbiPGAM) in its apoenzyme form, and confirmed this observation by small angle X-ray scattering data. Comparing the TbiPGAM structure with the Leishmania mexicana independent PGAM structure, previously reported with a phosphoglycerate molecule bound to the active site, revealed the domain movement resulting from active site occupation. The structure reported here shows the interaction between Asp319 and the metal bound to the active site, and its contribution to the domain movement. Substitution of the metal-binding residue Asp319 by Ala resulted in complete loss of independent PGAM activity, and showed for the first time its involvement in the enzyme's function. As TbiPGAM is an attractive molecular target for drug development, the apoenzyme conformation described here provides opportunities for its use in structure-based drug design approaches. Database Structural data for the Trypanosoma brucei 2,3-bisphosphoglycerate-independent phosphoglycerate mutase (iPGAM) has been deposited with the Research Collaboratory for Structural Bioinformatics (RCSB) Protein Data Bank under code 3NVL.

GPRC6A is a novel member of family C of G protein-coupled receptors with so far unknown function. We have recently described both human and mouse GPRC6A as receptors for L-alpha-amino acids. To date, functional characterization of wild-type GPRC6A has been impaired by the lack of activity in quantitative functional assays. The aim of this study was thus to develop such an assay and extend the pharmacological characterization of GPRC6A.

GPRC6A is a novel member of family C of G protein-coupled receptors with so far unknown function. We have recently described both human and mouse GPRC6A as receptors for L-alpha-amino acids. To date, functional characterization of wild-type GPRC6A has been impaired by the lack of activity in quantitative functional assays. The aim of this study was thus to develop such an assay and extend the pharmacological characterization of GPRC6A.

The conformational stability of calreticulin was investigated. Apparent unfolding temperatures (Tm) increased from 31 degrees C at pH 5 to 51 degrees C at pH 9, but electrophoretic analysis revealed that calreticulin oligomerized instead of unfolding. Structural analyses showed that the single C-terminal alpha-helix was of major importance to the conformational stability of calreticulin.

The conformational stability of calreticulin was investigated. Apparent unfolding temperatures (Tm) increased from 31 degrees C at pH 5 to 51 degrees C at pH 9, but electrophoretic analysis revealed that calreticulin oligomerized instead of unfolding. Structural analyses showed that the single C-terminal alpha-helix was of major importance to the conformational stability of calreticulin.

The structure of the Mg(2+)-dependent enzyme human phosphoserine phosphatase (HPSP) was exploited to examine the structural and functional role of the divalent cation in the active site of phosphatases. Most interesting is the biochemical observation that a Ca(2+) ion inhibits the activity of HPSP, even in the presence of added Mg(2+). The sixfold coordinated Mg(2+) ion present in the active site of HPSP under normal physiological conditions, was replaced by a Ca(2+) ion by using a crystallization condition with high concentration of CaCl(2) (0.7 m). The resulting HPSP structure now shows a sevenfold coordinated Ca(2+) ion in the active site that might explain the inhibitory effect of Ca(2+) on the enzyme. Indeed, the Ca(2+) ion in the active site captures both side-chain oxygen atoms of the catalytic Asp20 as a ligand, while a Mg(2+) ion ligates only one oxygen atom of this Asp residue. The bidentate character of Asp20 towards Ca(2+) hampers the nucleophilic attack of one of the Asp20 side chain oxygen atoms on the phosphorus atom of the substrate phosphoserine.

In 1991 a new type of G-protein-coupled receptor (GPCR) was cloned, the type 1a metabotropic glutamate (mGlu) receptor, which, despite possessing the defining seven-transmembrane topology of the GPCR superfamily, bore little resemblance to the growing number of other cloned GPCRs. Subsequent studies have shown that there are eight mammalian mGlu receptors that, together with the calcium-sensing receptor, the GABA(B) receptor (where GABA is gamma-aminobutyric acid) and a subset of pheromone, olfactory and taste receptors, make up GPCR family C. Currently available data suggest that family C GPCRs share a number of structural, biochemical and regulatory characteristics, which differ markedly from those of the other GPCR families, most notably the rhodopsin/family A GPCRs that have been most widely studied to date. This review will focus on the group I mGlu receptors (mGlu1 and mGlu5). This subgroup of receptors is widely and differentially expressed in neuronal and glial cells within the brain, and receptor activation has been implicated in the control of an array of key signalling events, including roles in the adaptative changes needed for long-term depression or potentiation of neuronal synaptic connectivity. In addition to playing critical physiological roles within the brain, the mGlu receptors are also currently the focus of considerable attention because of their potential as drug targets for the treatment of a variety of neurological and psychiatric disorders.

Recombinant cofactor-independent phosphoglycerate mutase from Trypanosoma brucei was inactivated by EDTA, and reactivated by Co(2+) much more than by Mn(2+) or Fe(2+). It displayed a minor phosphoglycerate phosphatase activity, which was stimulated by Mn(2+) more than by Co(2+). Upon incubation with [(32)P]phosphoglycerate, radioactivity was incorporated into the enzyme, most particularly in the presence of Mn(2+) or Fe(2+). The phosphorylated residue was identified by tandem mass spectrometry as Ser74, a residue homologous to the phosphorylated serine in alkaline phosphatase. However, the rates of formation and of disappearance of this phosphoenzyme were quite low compared to the mutase reaction. This and other properties indicated that the observed phosphoenzyme is an intermediate in the minor phosphatase activity rather than in the phosphomutase reaction.

Isopentenyl diphosphate:dimethylallyl diphosphate (IPP:DMAPP) isomerase catalyses a crucial activation step in the isoprenoid biosynthesis pathway. This enzyme is responsible for the isomerization of the carbon-carbon double bond of IPP to create the potent electrophile DMAPP. DMAPP then alkylates other molecules, including IPP, to initiate the extraordinary variety of isoprenoid compounds found in nature. The crystal structures of free and metal-bound Escherichia coli IPP isomerase reveal critical active site features underlying its catalytic mechanism. The enzyme requires one Mn(2+) or Mg(2+) ion to fold in its active conformation, forming a distorted octahedral metal coordination site composed of three histidines and two glutamates and located in the active site. Two critical residues, C67 and E116, face each other within the active site, close to the metal-binding site. The structures are compatible with a mechanism in which the cysteine initiates the reaction by protonating the carbon-carbon double bond, with the antarafacial rearrangement ultimately achieved by one of the glutamates involved in the metal coordination sphere. W161 may stabilize the highly reactive carbocation generated during the reaction through quadrupole- charge interaction., Journal Article, Research Support, Non-U.S. Gov't, SCOPUS: ar.j, FLWIN, info:eu-repo/semantics/published

Isopentenyl diphosphate:dimethylallyl diphosphate (IPP:DMAPP) isomerase catalyses a crucial activation step in the isoprenoid biosynthesis pathway. This enzyme is responsible for the isomerization of the carbon-carbon double bond of IPP to create the potent electrophile DMAPP. DMAPP then alkylates other molecules, including IPP, to initiate the extraordinary variety of isoprenoid compounds found in nature. The crystal structures of free and metal-bound Escherichia coli IPP isomerase reveal critical active site features underlying its catalytic mechanism. The enzyme requires one Mn(2+) or Mg(2+) ion to fold in its active conformation, forming a distorted octahedral metal coordination site composed of three histidines and two glutamates and located in the active site. Two critical residues, C67 and E116, face each other within the active site, close to the metal-binding site. The structures are compatible with a mechanism in which the cysteine initiates the reaction by protonating the carbon-carbon double bond, with the antarafacial rearrangement ultimately achieved by one of the glutamates involved in the metal coordination sphere. W161 may stabilize the highly reactive carbocation generated during the reaction through quadrupole- charge interaction., Journal Article, Research Support, Non-U.S. Gov't, SCOPUS: ar.j, FLWIN, info:eu-repo/semantics/published

The role of rapidly exchanging intracellular Ca2+ stores in the control of Ca2+ homeostasis is reviewed. The following issues are discussed: the reasons why such stores exist in eukaryotic cells; the differences between the terminal cisternae of the skeletal muscle sarcoplasmic reticulum, which have direct, physical connection with the T tubules of the plasmalemma, and the Ca2+ stores located in the depth of the cytoplasm, which are stimulated by second messengers; the cytological nature (subcompartments of the ER) of the rapidly exchanging Ca2+ stores and their functional significance. The conclusions introduce recent developments in which intracellular Ca2+ stores have been investigated also by molecular biology techniques

We have studied the kinetics of the allosteric interactions of pyruvate kinase from Trypanosoma brucei. The kinetics for phosphoenolpyruvate depended strongly on the nature of the bivalent metal ions. Pyruvate kinase activated by Mg2+ had the highest catalytic activity, but also the highest S0.5 for phosphoenolpyruvate, while the opposite was true for pyruvate kinase activated by Mn2+. The reaction rates of Mg(2+)-pyruvate kinase and Mn(2+)-pyruvate kinase were clearly allosteric with respect to phosphoenolpyruvate, while the kinetics with Co(2+)-pyruvate kinase were hyperbolic. However, Co(2+)-pyruvate kinase was still sensitive to heterotropic activation. Trypanosomal pyruvate kinase is unique in that the best activator was fructose 2,6-bisphosphate. Ribulose 1,5-bisphosphate and 5-phosphorylribose 1-pyrophosphate were also strong heterotropic activators, which were much more effective than fructose 1,6-bisphosphate and glucose 1,6-bisphosphate. In the presence of the heterotropic activators, the sigmoidal kinetics with respect to phosphoenolpyruvate and the bivalent metal ions were modified as were the concentrations of phosphoenolpyruvate and the bivalent metal ions needed to attain the maximal activity. Maximal activities were not significantly changed with Mg2+ and Mn2+ as the activating metal ions. Moreover, with Co2+ and fructose 2,6-bisphosphate or ribulose 1,5-bisphosphate or 5-phosphorylribose 1-pyrophosphate, the maximal activity was significantly reduced. Ribulose 1,5-bisphosphate and 5-phosphorylribose 1-pyrophosphate resembled fructose 2,6-bisphosphate rather than fructose 1,6-bisphosphate and glucose 1,6-bisphosphate in their action in that the K0.5 values for the former 3 compounds increased when Mg2+ was replaced by Co2+, while the K0.5 for fructose 1,6-bisphosphate and glucose 1,6-bisphosphate increased.(ABSTRACT TRUNCATED AT 250 WORDS)

In the cofermentation of glycerol with a sugar by Lactobacillus brevis and Lactobacillus buchneri, a 1,3-propanediol:NAD+ oxidoreductase provides an additional method of NADH disposal. The enzyme has been purified from both L. brevis B22 and L. buchneri B190 and found to have properties very similar to those reported for the enzyme from Klebsiella pneumoniae. The enzymes required Mn2+ and are probably octamers with a molecular mass of 350 kDa. Although not absolutely specific for 1,3-propanediol when tested as dehydrogenases, the enzymes have less than 10% activity with glycerol, ethanol, and 1,2-propanediol. These properties contrast sharply with those of a protein isolated from another Lactobacillus species (L. reuteri) that ferments glycerol with glucose and previously designated a 1,3-propanediol dehydrogenase.

Na+/Ca2+ exchange in the B cell was recently characterized by measuring intracellular-Na(+)-dependent 45Ca2+ uptake in isolated rat pancreatic islet cells. The aim of the present study was to investigate the regulation of this process. Extracellular pH (pHo) and intracellular pH (pHi) markedly affected Na+/Ca2+ exchange. A fall of 0.04 unit in pHi decreased the exchange by 45%, whereas a rise of 0.13 unit increased the uptake by 70%. Mitochondrial poisons (oligomycin, antimycin A and 2,4-dinitrophenol) inhibited reverse Na+/Ca2+ exchange by about 25-50%. The exchanger displayed a low Q10 (temperature coefficient), indicating that it is only indirectly dependent on metabolic energy. The phorbol ester phorbol 12-myristate 13-acetate did not affect Na+/Ca2+ exchange. Likewise, lowering the extracellular K+ concentration did not inhibit 45Ca2+ uptake. In conclusion, the pHi and the metabolic state of the cell may represent important modulatory signals by which insulin secretagogues such as glucose could regulate reverse Na+/Ca2+ exchange in the B cell. The process does not appear to co-transport K+ nor to be influenced by protein kinase C., Journal Article, info:eu-repo/semantics/published

Na+/Ca2+ exchange in the B cell was recently characterized by measuring intracellular-Na(+)-dependent 45Ca2+ uptake in isolated rat pancreatic islet cells. The aim of the present study was to investigate the regulation of this process. Extracellular pH (pHo) and intracellular pH (pHi) markedly affected Na+/Ca2+ exchange. A fall of 0.04 unit in pHi decreased the exchange by 45%, whereas a rise of 0.13 unit increased the uptake by 70%. Mitochondrial poisons (oligomycin, antimycin A and 2,4-dinitrophenol) inhibited reverse Na+/Ca2+ exchange by about 25-50%. The exchanger displayed a low Q10 (temperature coefficient), indicating that it is only indirectly dependent on metabolic energy. The phorbol ester phorbol 12-myristate 13-acetate did not affect Na+/Ca2+ exchange. Likewise, lowering the extracellular K+ concentration did not inhibit 45Ca2+ uptake. In conclusion, the pHi and the metabolic state of the cell may represent important modulatory signals by which insulin secretagogues such as glucose could regulate reverse Na+/Ca2+ exchange in the B cell. The process does not appear to co-transport K+ nor to be influenced by protein kinase C., Journal Article, info:eu-repo/semantics/published

A CCAAT-binding activity present in nuclear extracts of rat liver and NIH 3T3 fibroblasts was purified using, as assay, DNA binding to a segment of the mouse alpha 2(I) collagen promoter. The activity consists of two components, designated factors A and B, which are separated by ion exchange chromatography on either Mono Q or Mono S columns. Factor A is heat-sensitive, whereas factor B is heat-resistant. Both factors are required for DNA binding and both are present in the DNA protein complex. The A + B complex was extensively purified by heparin-agarose and sequence-specific affinity chromatography. The Mr of factor A is 39,000, whereas the Mr of factor B is 41,000 as determined by renaturation of a highly purified preparation after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Competition experiments indicate that this CCAAT-binding complex has a DNA sequence specificity that is different from those of other CCAAT-binding proteins.

At low pH influenza virus causes membrane fusion. This phenomenon is thought to reflect a part of the infection mechanism of the virus. To obtain more information on the effect of low pH on the virus, the change in morphology of influenza virus particles was studied by electron microscopy. Further, the degradation of haemagglutinin (HA) after trypsin digestion as a function of pH was studied by gel electrophoresis. The results showed that a threshold value existed below which both a change in morphology and an increase in trypsin sensitivity were observed. This threshold pH was found to be strain specific. A number of strains showed a heterogeneity in the particle population with respect to the threshold pH. The various subpopulations appeared to differ genetically. Virus particles with uncleaved precursor HA, HAo, were not effected by the low pH treatment.

SCOPUS: ar.j, info:eu-repo/semantics/published

Alkaline phosphatase is inactivated by mixed function oxidation systems. OH. radicals, generated via an ascorbate-modified Haber-Weiss cycle or a Fenton-type reaction, seem to be responsible for the protein oxidative damage. Experiments with hydroxyl radical scavengers, enzyme substrates, products, and metal cofactors suggest that a "site-specific" radical attack takes place at or near the active center. Vitamin E fails to protect alkaline phosphatase; uric acid, instead, is particularly effective in shielding the protein against covalent modifications.

SCOPUS: ar.j, info:eu-repo/semantics/published

Cu(II)--plastocyanin from French beans (Phaseolus vulgaris) is reduced quantitatively by Cr(II)aq ions to give a substitution-inert Cr(III) adduct of Cu(I)--plastocyanin. Enzymatic proteolysis of this derivative by thermolysin led to the identification of the Cr(III) binding peptide. This contains four potential ligands for the metal ion: aspartate-42 and -44 and glutamate-43 and -45. In the three-dimensional fold of plastocyanin, this stretch is very close to tyrosine-83. The emission intensity and its pH dependence observed for the tyrosines in this tryptophan-devoid protein differ markedly in the Cr(III) adduct. That difference is interpreted as reflecting proximity and interaction between the latter metal ion and tyrosine-83. The distance between the copper center and the suggested Cr(III) binding site is approximately 12 A. The intervening region contains an array of highly invariant aromatic residues. These are proposed to be involved in the electron transfer process. A mechanism for that process is presented that involves interaction between the d electrons of the metal ions with d pi-pi* delocalization through a weakly coupled pi* system. The rationale of this electron transfer pathway for the reactivity of plastocyanin with inorganic redox agents is discussed.

Dantrolene sodium, a hydantoin derivative with skeletal muscle relaxant properties, quickly reduces the cytosolic calcium concentration of the Barnacle (Balanus nubilus) giant muscle fibre, by direct action on the sercoplasmic reticulum. This inhibitory effect subsequently reduces the 45Ca efflux out of the cell although the 45Ca influx remains unchanged., Séance du 19 juillet 1976., info:eu-repo/semantics/published

Alkaline phosphatase is inactivated by mixed function oxidation systems. OH. radicals, generated via an ascorbate-modified Haber-Weiss cycle or a Fenton-type reaction, seem to be responsible for the protein oxidative damage. Experiments with hydroxyl radical scavengers, enzyme substrates, products, and metal cofactors suggest that a "site-specific" radical attack takes place at or near the active center. Vitamin E fails to protect alkaline phosphatase; uric acid, instead, is particularly effective in shielding the protein against covalent modifications.

Ca+plus; and Ca tot were measured and compared at 2 years interval (S1 and S2) in 45 normal volunteers: 26 females and 19 males age 20/65 years. ForCa+plus; the mean difference between S1 and S2 was small (0.009 mmol/1) but significant (p < 0.01). For Ca tot the mean difference between S1 and S2 (0.06 mmol/1) was very significant (p < 0.001) due to the combination of various factors: technical, sex difference and proteins. In both series (S1 + S2) Ca+plus; did not differ significantly between females and males, whereas Ca tot was significantly (p < 0.05) higher in males (mean difference 0.06 mmol/1) for S1 and non significant for S2. In 63 normal volunteers we cross correlated the factors of calcium homeostasis and showed significant relationship between Ca+plus;/Ca tot r = 0.428 (p < 0.001), Ca+plus;/PTH r = -0.297 (p < 0.05), Ca tot/Proteins r = 0.518 (p < 0.001). The relationship between Ca tot and PTH does not reach the level of significance. We compared Ca+plus; and Ca tot in renal insufficiency (RI) (n = 16), hyperparathyroidism surgically confirmed (Hyper P) (n = 10), cancer with hypercalcemia (C) (n = 9) and in normal volunteers (N) (n = 63). The relationship between Ca+plus; and Ca tot in the pathologic groups has significantly steeper slopes than the control group (N), p < 0.05 for RI and C and p < 0.01 for Hyper P. The increase of the Ca+plus;/Ca tot ration in RI and Hyper P is positively correlated to PTH levels, nor albumin variations neither pH and P04 variations could completely account for the increase of the ratio. The increase of Ca+plus;/Ca tot in cancer is independent of PTH and cannot entirely be explained by the albumin drop. © 1983 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted., SCOPUS: ar.j, info:eu-repo/semantics/published

Cu(II)--plastocyanin from French beans (Phaseolus vulgaris) is reduced quantitatively by Cr(II)aq ions to give a substitution-inert Cr(III) adduct of Cu(I)--plastocyanin. Enzymatic proteolysis of this derivative by thermolysin led to the identification of the Cr(III) binding peptide. This contains four potential ligands for the metal ion: aspartate-42 and -44 and glutamate-43 and -45. In the three-dimensional fold of plastocyanin, this stretch is very close to tyrosine-83. The emission intensity and its pH dependence observed for the tyrosines in this tryptophan-devoid protein differ markedly in the Cr(III) adduct. That difference is interpreted as reflecting proximity and interaction between the latter metal ion and tyrosine-83. The distance between the copper center and the suggested Cr(III) binding site is approximately 12 A. The intervening region contains an array of highly invariant aromatic residues. These are proposed to be involved in the electron transfer process. A mechanism for that process is presented that involves interaction between the d electrons of the metal ions with d pi-pi* delocalization through a weakly coupled pi* system. The rationale of this electron transfer pathway for the reactivity of plastocyanin with inorganic redox agents is discussed.