Loss of conformational stability in calmodulin upon methionine oxidation

We have used electrospray ionization mass spectrometry (ESI-MS), circular dichroism (CD), and fluorescence spectroscopy to investigate the secondary and tertiary structural consequences that result from oxidative modification of methionine residues in wheat germ calmodulin (CaM), and prevent activation of the plasma membrane Ca-ATPase. Using ESI-MS, we have measured rates of modification and molecular mass distributions of oxidatively modified CaM species (CaMox) resulting from exposure to H2O2. From these rates, we find that oxidative modification of methionine to the corresponding methionine sulfoxide does not predispose CaM to further oxidative modification. These results indicate that methionine oxidation results in no large-scale alterations in the tertiary structure of CaMox, because the rates of oxidative modification of individual methionines are directly related to their solvent exposure. Likewise, CD measurements indicate that methionine oxidation results in little change in the apparent alpha-helical content at 28 degrees C, and only a small (0.3 +/- 0.1 kcal mol(-1)) decrease in thermal stability, suggesting the disruption of a limited number of specific noncovalent interactions. Fluorescence lifetime, anisotropy, and quenching measurements of N-(1-pyrenyl)-maleimide (PMal) covalently bound to Cys26 indicate local structural changes around PMal in the amino-terminal domain in response to oxidative modification of methionine residues in the carboxyl-terminal domain. Because the opposing globular domains remain spatially distant in both native and oxidatively modified CaM, the oxidative modification of methionines in the carboxyl-terminal domain are suggested to modify the conformation of the amino-terminal domain through alterations in the structural features involving the interdomain central helix. The structural basis for the linkage between oxidative modification and these global conformational changes is discussed in terms of possible alterations in specific noncovalent interactions that have previously been suggested to stabilize the central helix in CaM.

Albinism: an update and review of the literature

BACKGROUND

Albinism can be a diagnostic challenge to the optometrist, with ocular albinism the entity most likely to be overlooked or misdiagnosed. Albinism should be suspect in a child with nystagmus.

METHODS

Albinism is best diagnosed by electron microscopy of skin or hair bulbs. Measuring the flash visually evoked response (VER) is the best way to confirm the abnormal decussation of the optic nerve fibers. Transillumination of the iris can be performed to see if it lacks pigment on its posterior surface. Optometrists should also be alert for nystagmus, strabismus, lack of stereopsis, and poor fusional capacity. Most people with albinism have photophobia and reduced acuity, and many have defective hearing.

RESULTS

This review describes the various kinds of albinism and summarizes associated ocular manifestations with pertinent forms of the disorder.

CONCLUSIONS

Optometrists are responsible for detection of albinism and provision of optical aids and related advice to afflicted patients. These patients also need appropriate counselling and genetic studies of family members.

Identification of oxidation-sensitive peptides within the cytoplasmic domain of the sarcoplasmic reticulum Ca2+-ATPase

We have examined the oxidative sensitivity of the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum (SR) membranes, exposing isolated SR membranes to the thermolabile water soluble free radical initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH). Incubation with up to 702 microM AAPH-derived radicals results in a concentration- and time-dependent inhibition of calcium-dependent ATPase activity correlating with the loss of monomeric Ca2+-ATPase polypeptides, and the concomitant appearance of higher molecular weight species. However, no oxidant-induced protein fragmentation is detected. The observed formation of oxidant-induced bityrosine accounts for the intermolecular Ca2+-ATPase cross-links, as well as intramolecular cross-links. The oxidation of sulfhydryl groups to disulfides as another possible source of intermolecular cross-links has been ruled out after examination of SDS -PAGE performed under both reducing and non-reducing conditions. Exposure of the SR membranes to AAPH-derived radical species results in a small degree of lipid peroxidation that is not correlated with enzyme inactivation, suggesting that modification of membrane-spanning peptides is not related to enzyme inactivation. Six cytoplasmic peptides have been identified that are modified by exposure to AAPH or, alternatively, to hydrogen peroxide, suggesting that these regions of the Ca2+-ATPase are generally sensitive to oxidants. These oxidized peptides were identified after separation by reversed-phase HPLC followed by N-terminal sequencing and amino acid analysis as corresponding to the following sequences of the Ca2+-ATPase: (i) Glu121 to Lys128, (ii) His190 to Lys218, (iii) Asn330 to Lys352, (iv) Gly432 to Lys436, (v) Glu551 to Arg604, and (vi) Glu657 to Arg671. The Glu551 to Arg604 peptide, located within the nucleotide binding domain, was found to participate in the formation of intermolecular bityrosine cross-links with the identical Glu551 to Arg604 peptide from a neighboring Ca2+-ATPase polypeptide chain.

Gas-phase reactions of glycine, alanine, valine and their N-methyl derivatives with the nitrosonium ion, NO+

The gas-phase reactions of the nitrosonium ion, NO+ with the amino acids glycine, alanine and valine and their N-methyl derivatives were investigated under chemical ionization mass spectrometric (CIMS) conditions. Two products were observed in all cases: the formation of the iminium ion and the formation of an [M-H]+ ion. The latter product is consistent with a reaction channel involving hydride abstraction by NO+, and was confirmed by (i) examining the Ar+CI mass spectra of the same amino acids under similar source conditions and (ii) examining the unimolecular fragmentation reactions of the [M + H]+ ions of the N-nitroso-N-methyl derivatives of each of the amino acids in a tandem mass spectrometer. Further insights into the reaction of glycine with NO+ were obtained by performing ab initio calculations (at the MP2/6-31G* parallel HF/6-31G* level). These results indicate that four reactions are thermodynamically viable for glycine: (i) hydride abstraction; (ii) iminium ion formation (with concomitant loss of HONO and CO); (iii) diazonium ion formation; and (iv) diazonium ion formation followed by loss of N2. Possible reasons why reactions (iii) and (iv) are not observed are discussed, and comparisons with solution reactivity and the gas-phase reactivity of NO+ are also made.

Decreased plasma membrane calcium transport activity in aging brain

We have assessed the functional properties of both calmodulin (CaM) and the plasma membrane Ca(2+)-ATPase in brains of young, middle aged, and old Fisher 344 rats. Under optimal conditions of saturating Ca2+ and ATP, the CaM-activated Ca(2+)-ATPase activity was decreased with increasing age, particularly when CaM isolated from the brains of aged rats was used to stimulate the enzyme. In the case of CaM, structural modifications within the primary sequence of the protein from aged brains were identified. We found that during normal biological aging approximately 6 methionine residues were modified to their corresonding sulfoxide per CaM, and no other amino acids were modified. Some aspects of the age-related decline in the effectiveness of CaM as an activator of Ca(2+)-ATPase could be simulated using a range of reactive oxygen species (including hydrogen peroxide and oxoperoxynitrite) and, in the latter case, the extent of oxidative modification of specific methionine residues was directly related to their surface accessibility. The pattern of oxidative modification of the methionines in the aged CaM was less straightforward, though both in vitro oxidation of CaM and aging within the brain markedly decreased the functional properties of this important Ca(2+)-regulating protein.

Systemic abnormalities associated with iris/nerve head/choroidal/retinal coloboma

A case is reported in which large colobomas of the retina and choroid are found bilaterally in a young female who also has a pituitary tumor and mental retardation. Visual field data are presented and correlated with the retinal appearances. Relevant embryology is reviewed, and a table of systemic abnormalities associated with retinal/choroidal colobomas is presented.

Oxygen-centered spin adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2H-imidazole 1-oxides

The hydroxyl and some alkoxyl spin adducts of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) are difficult to assign due to the remarkable similarity of their EPR spectra. The utility of resolving superhyperfine (SHF) structure followed by computer simulations has been demonstrated to assist in the assignment of EPR spectra with close values of hyperfine splitting constants, e.g., DMPO/ .OH and DMPO/.OR. Here, .OR is the alkoxyl radical derived from thermal decomposition of 2,2' -azobis (2-amidinopropane) hydrochloride (AAPH). In addition, two other spin traps, derivatives of 2H-imidazole 1-oxide, namely, 2,2,4-trimethyl-2H-imidazole 1-oxide (TMIO) and 2,2-dimethyl-4-phenyl-2H-imidazole 1-oxide (DMPIO), have been used in a model study to develop a procedure for distinguishing between oxygen-centered spin adducts. These results are compared with those for DMPO. TMIO and DMPIO spin traps provide more distinguishable individual spectra with .OH and AAPH-derived .OR radicals than the DMPO spin trap. The formation of DMPO/.OR(AAPH) and DMPIO/ .OR(AAPH) spin adducts was confirmed by mass spectrometry. The comparison of spin trapping by DMPO and 2H-imidazole 1-oxides using typical biological sources of other oxygen-centered radicals reveals application limits of these spin traps. For example, 2H-imidazole 1-oxides do not form superoxide spin adducts in the xanthine/xanthine oxidase system. Also, for the first time, experimental evidence is presented for SHF structure in spectra of TMIO and DMPIO spin adducts with .OH/.OD and .CH3/ .CD3 radical species.

Oxidative modification of a carboxyl-terminal vicinal methionine in calmodulin by hydrogen peroxide inhibits calmodulin-dependent activation of the plasma membrane Ca-ATPase

In order to investigate the possibility that calmodulin (CaM) may be a principal target of reactive oxygen species (ROS) produced under conditions of oxidative stress, we have examined wheat germ CaM for the presence of highly reactive sites that correlate with the loss of function. Using reversed-phase HPLC and FAB mass spectrometry after proteolytic digestion, we have identified the sites of modification by hydrogen peroxide. We find that one of the vicinal methionines (i.e., Met146 or Met147) near the C-terminus of CaM is selectively oxidized. The ability of CaM to bind and to activate the plasma membrane (PM)-Ca-ATPase from erythrocytes was measured. There is a 30-fold decrease in the calcium affinity of oxidatively modified CaM. While there is a little change in the binding constant between the carboxyl-terminal domain of calcium-saturated CaM and a peptide homologous to the autoinhibitory sequence of the PM-Ca-ATPase, we find that there is a 9-fold reduction in the affinity of the amino-terminal domain of CaM with respect to the ability to bind target peptides. The extent of oxidative modification to one of the vicinal methionines near the carboxyl-terminal domain correlates with the loss of CaM-dependent activation of the PM-Ca-ATPase. The presence of oxidatively modified CaM prevents native CaM from activating the PM-Ca-ATPase, indicating that the oxidatively modified CaM binds to the autoinhibitory sequence on the Ca-ATPase in an altered nonproductive conformation. We suggest that the functional sensitivity of CaM to the oxidation of one of the C-terminal vicinal methionines permits CAM to serve a regulatory role in modulating cellular metabolism under conditions of oxidative stress. The predominant oxidation of a methionine near the carboxyl terminal of CaM is rationalized in terms of the enhanced solvent accessibility of these vicinal methionines.

A potential anti-oxidant protein in a ferrous iron-oxidizing Sulfolobus species

The production of a 25 kDa protein was greatly increased when a sulfur- and ferrous iron-oxidizing species of Sulfolobus was switched from growth on tetrathionate to growth on ferrous iron. The gene encoding the protein was cloned and sequenced. The predicted amino sequence showed significant similarity to those of the alkyl hydroperoxide reductase/thiol specific anti-oxidant family of proteins that appear to be involved in responses to certain types of oxidative stress.

Information technology for home care--a promising future

Home care faces challenges in the automation arena from changing regulations to improved software and hardware. The past has had its ups and downs in relations between agencies and those vendors from whom they must purchase these necessities. What have been the problems? How can agencies avoid them?

Adaptive changes in lipid composition of skeletal sarcoplasmic reticulum membranes associated with aging

We have undertaken a detailed examination of changes associated with aging in lipid composition and corresponding physical properties of hindlimb skeletal sarcoplasmic reticulum (SR) membranes isolated from young (5 months), middle-aged (16 months), and old (28 months) Fischer strain 344 rats. Silica gel HPLC chromatography was used to separate phospholipid headgroup species. Subsequent reversed-phase HPLC was used to resolve fatty acid chain compositions of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol species. For all three phospholipid pools, significant age-related variations are observed in the abundance of multiple molecular species, particularly those having polyunsaturated fatty acid chains. Using mass spectrometry (fast atom bombardment and tandem techniques) to distinguish ester- from ether-linked phosphatidylethanolamine species, we demonstrate that overall plasmenylethanolamine content is substantially increased with age, from 48 mol% to 62 mol%. A substantial increase is also observed in the single molecular species 18:0-20:4 phosphatidylinositol suggesting implications for signalling pathways. In addition, associated with senescence we find a significant increase in the rigidifying lipid, cholesterol. Despite these changes in lipid composition of different aged animals, the average bilayer fluidity examined at several bilayer depths with stearic acid spin labels, is not altered. Neither do we find differences in the rotational mobility of maleimide spin-labeled Ca(2+)-ATPase, as determined from saturation-transfer electron paramagnetic resonance, which is sensitive to both the fluidity of lipids directly associated with the Ca(2+)-ATPase and to its association with proteins.

Glutathione- and glutathione-S-transferase-dependent oxidative desulfuration of the thione xenobiotic diethyldithiocarbamate methyl ester

Oxidative desulfuration of diethyldithiocarbamate methyl ester (DDTC-Me), a thione xenobiotic and a metabolite of disulfiram, was studied. Using a rat liver microsomal incubation system, DDTC-Me was oxidized at the thionosulfur group, forming DDTC-Me sulfine. Only minimal desulfuration of DDTC-Me to S-methyl-N,N-diethylthiolcarbamate (DETC-Me) occurred. Desulfuration of DDTC-Me increased 4-fold when the microsomal incubation was supplemented with reduced glutathione (GSH) and increased 8-fold when both GSH and glutathione-S-transferase (EC 2.5.1.18) were added. Similar results were obtained using a simplified system containing DDTC-Me sulfine, GSH, and glutathione-S-transferase. This suggested that DDTC-Me sulfine is a stable intermediate formed before DDTC-Me is desulfurated to DETC-Me. This unprecedented desulfuration process can be explained as follows. GSH attacks the oxithiirane isomer of DDTC-Me sulfine, resulting in ring opening followed by loss of glutathione hydrodisulfide, which is reduced by GSH to oxidized glutathione and H2S. GSH can also reduce DDTC-Me sulfine to DDTC-Me. This mechanism is supported by in vitro studies. An approximately 1:1 stoichiometry was observed for the formation of H2S and DETC-Me. A 1:1 stoichiometry was also observed for the consumption of DDTC-Me sulfine, formation of DETC-Me plus DDTC-Me, and formation of oxidized glutathione. Glutathione hydrodisulfide was trapped by derivatization in situ using 4-vinylpyridine. Oxidative desulfuration of a series of dithiocarbamate esters also followed a similar mechanism.

Degradation of O6-benzylguanine in aqueous polyethylene glycol 400 (PEG 400) solutions: concerns with formaldehyde in PEG 400

The degradation of O6-benzylguanine (BG) in aqueous polyethylene glycol (PEG) 400 solution at room temperature had been investigated using chromatographic and spectrometric methods. The degradation of BG in this solvent appeared to arise from a reaction between BG and formaldehyde. The formaldehyde was present as an impurity in PEG 400 and probably formed through air oxidation of PEG 400. The major product of this reaction was believed to be a methylene-bridged compound containing two BG molecules. This was probably produced via an intermediate imine, a schiff base between one BG molecule and formaldehyde. This degradation reaction was the only observable reaction in the 40% PEG/water solvent (pH 8.0) i.e. degradation of the drug via hydrolysis was minimal under these conditions.

Intraspecific variation in egg size and egg composition in birds: effects on offspring fitness

1. There is little unequivocal evidence to date in support of a positive relationship between egg size and offspring fitness in birds. Although 40 studies (of 34 species) have considered the effect of variation in egg size on chick growth and/or survival up to fledgling only 12 studies have controlled for other characters potentially correlated both with egg size and offspring fitness. Of these only two have reported a significant residual effect of egg size on chick growth (in the roseate tern and European blackbird) and three a residual effect on chick survival (all in seabirds: common tern, lesser black-backed gull and kittiwake). 2. More consistent evidence exists, though from fewer studies, for a positive relationship between egg size and offspring fitness early in the chick-rearing period; chick growth and chick survival being dependent on egg size in 8 of 10 studies and 4 of 5 studies respectively. It is suggested that the most important effect of variation in egg size might be in determining the probability of offspring survival in the first few days after hatching. 3. Egg size explains on average 66% of the variation in chick mass at hatching (n = 35 studies) but only 30% of the variation in chick body size (n = 18). When effects of hatching body size are controlled for chick mass remains significantly correlated with egg size, though the reverse is not true. This supports the hypothesis that large eggs give rise to heavier chicks at hatching, i.e., chicks with more nutrient (yolk) reserves, rather than structurally larger chicks. 4. Egg composition increased isometrically with increasing egg size in about half the studies so far reported (n equals approximately 20). However, in seabirds, and some passerines, larger eggs contain disproportionately more albumen, whilst in some waterfowl percentage yolk content increases with increasing egg size. Changes in albumen content largely reflect variation in the water content of eggs, but changes in yolk content involve variation in lipid content, and therefore in egg 'quality.' The adaptive significance of variation in egg composition is considered; females may adjust egg composition facultatively to maximise the benefits to their offspring of increased reproductive investment. 5. Considerations for future research are discussed with particular emphasis on experimental studies and the application of new techniques.

Plasma prolactin during the breeding season in adult and immature macaroni (Eudyptes chrysolophus) and gentoo (Pygoscelis papua) penguins

Plasma prolactin levels were measured in free-living breeding adult and immature (pre-breeding) macaroni (Eudyptes chrysolophus) and gentoo (Pygoscelis papua) penguins at Bird Island, South Georgia (54 degrees S, 38 degrees W). Macaroni and gentoo penguins first breed at 5-6 and 2 years of age, respectively. In adult birds, of both species, prolactin was low (< 1.0 microgram. liter-1) during the courtship period and then increased during early (gentoo) to mid (macaroni) incubation (to 3.9-4.7 and 2.4-3.5 micrograms.liter-1, respectively), remaining elevated until the creche period, by which time continuous nest attendance by the adults had ceased. This pattern is similar to that seen in other altricial species and is consistent with delayed onset of brood patch development and full incubation efficiency, which has been previously reported in penguins. Adult female macaroni penguins showed a marked, but transient, increase in prolactin concentrations within 24 hr of the first egg being laid (from 1.7 to 7.0 micrograms.liter-1), plasma levels decreasing following clutch completion (to prelaying levels) before increasing again during incubation. Elevated plasma prolactin levels occurred in all age classes of immature (nonbreeding) birds in both macaroni (1- to 5 year-olds) and gentoo (1-year-olds) penguins. However, compared to that in adult birds, the increase in prolactin was more transient in immatures, a smaller proportion of immatures had detectable prolactin levels at each stage of the breeding cycle, and, at least in 1- and 2-year-olds, absolute levels of prolactin were lower.(ABSTRACT TRUNCATED AT 250 WORDS)

Exchangeable GTP binding site of beta-tubulin. Identification of cysteine 12 as the major site of cross-linking by direct photoaffinity labeling

After direct photoaffinity cross-linking of [3H]GTP to the beta-subunit of tubulin, followed by tryptic digestion and alkaline phosphatase treatment, we employed cis-diol-specific boronate gel chromatography and reversed-phase high-pressure liquid chromatography to purify a peptide containing most of the covalently bound radioactivity. The sequence of this peptide corresponded to that of residues 3-19 of beta-tubulin. Residue 10 of the peptide, which is Cys-12 in beta-tubulin, could not be identified. The fast atom bombardment mass spectrum of this peptide showed the presence of a predominant species with a molecular mass of 2022 kDa (2021 kDa for the 12C variant), which is 255 Da greater than the molecular mass of the peptide. Fast atom bombardment collision-activated decomposition mass spectrometry analysis produced fragments which are consistent with the beta(3-19) peptide but having a unit of mass of 358 at position 12. Thermolysin digestion of the tryptic peptide restricted the cross-linking site to the 9-amino acid sequence, I(L)QAGQXGNQ. The molecular mass of this peptide was 1174 kDa, which is equal to the mass of the beta(7-15) peptide containing an extra group of mass 255. To explain the molecular masses of the two labeled peptides, which are 26 atomic mass units less than expected, a mechanism of photolabeling is proposed that involves opening of the guanine ring and loss of the C-6 carbonyl function as CO2.