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G Britton

Publications and source records attributed to G Britton.

At least 19 recordsLinked to original sources
PubMed1997

13C Magic angle spinning NMR analysis and quantum chemical modeling of the bathochromic shift of astaxanthin in alpha-crustacyanin, the blue carotenoprotein complex in the carapace of the lobster Homarus gammarus.

Selective isotope enrichment, 13C magic angle spinning (MAS) NMR, and semiempirical quantum chemical modeling, have been used to analyze ligand-protein interactions associated with the bathochromic shift of astaxanthin in alpha-crustacyanin, the blue carotenoprotein complex from the carapace of the lobster Homarus gammarus. Spectra of alpha-crustacyanin were obtained after reconstitution with astaxanthins labeled with 13C at positions 4,4', 12,12', 13,13', or 20,20'. The data reveal substantial downfield shifts of 4.9 and 7.0 ppm at positions 12 and 12' in the complex, respectively. In contrast, at the 13 and 13' positions, small upfield shifts of 1.9 ppm were observed upon binding to the protein. These data are in line with previously obtained results for positions 14,14' (3.9 and 6.8 ppm downfield) and 15,15' (0.6 ppm upfield) and confirm the unequal perturbation of both halves after binding of the chromophore. However, these results also show that the main perturbation is of symmetrical origin, since the chemical shift differences exhibit a similar pattern in both halves of the astaxanthin molecule. A small downfield shift of 2.4 ppm was detected for the 4 and 4' positions. Finally, the 20,20' methyl groups are shifted 0.4 ppm upfield by the protein. The full data set provides convincing evidence that charge polarization is of importance for the bathochromic shift. The NMR shifts are compared with calculated charge densities for astaxanthin subjected to variations in protonation states of the ring-functional groups, as models of ligand-protein interactions. Taking into account the color shift and other available optical data, the current model for the mechanisms of interaction with the protein was refined. The results point toward a mechanism in which the astaxanthin is charged and subject to strong electrostatic polarizations originating from both keto groups, most likely a double protonation.

Animals↗
PubMed1996

Dietary supplementation with carotenoids: effects on alpha-tocopherol levels and susceptibility of tissues to oxidative stress.

The ability of dietary supplementation with carotenoids to protect chick tissues against oxidative stress in vitro was examined. Male Leghorn chicks were fed on diets supplemented (100 mg supplement/kg diet) with either beta-carotene, zeaxanthin (beta,beta-carotene-3,3'-diol), canthaxanthin (beta,beta-carotene-4,4'-dione) or alpha-tocopherol, or on a control diet, from 1 d old until 37 d of age. Tissues (liver, heart, skeletal muscle and plasma) were removed and assayed for total carotenoids and alpha-tocopherol content and portions subjected to oxidative stress by incubation of homogenates with cumene hydroperoxide and FeSo4. Animals receiving zeaxanthin and canthaxanthin had significantly greater carotenoid concentrations in liver, heart, muscle and plasma compared with untreated controls (P < 0.05); animals fed on diets supplemented with beta-carotene, or alpha-tocopherol did not have significantly different tissue carotenoid contents compared with untreated controls. alpha-Tocopherol supplementation elevated alpha-tocopherol levels in all tissues examined (P < 0.05). Supplementation with carotenoids did not affect tissue alpha-tocopherol levels, but beta-carotene lowered plasma alpha-tocopherol levels by 50% (P < 0.05). Incubation of plasma or tissue homogenates with oxidant stressors induced lipid peroxidation (production of thiobarbituric-acid reactive substances) in all tissues. Animals given alpha-tocopherol, beta-carotene or zeaxanthin had a reduced susceptibility to oxidant stress in liver compared with unsupplemented controls (P < 0.05), and alpha-tocopherol-supplemented animals had reduced susceptibility in skeletal muscle compared with tocopherol-supplemented animals had reduced susceptibility in skeletal muscle compared with unsupplemented controls (P < 0.05). Canthaxanthin supplementation did not influence the susceptibility to oxidant stress in any tissue examined. These results suggest that zeaxanthin, a carotenoid present in animal and human diets, may have significant activity as an antioxidant against oxidative stress in tissues.

Animals↗
PubMed1996

Arabidopsis carotenoid mutants demonstrate that lutein is not essential for photosynthesis in higher plants.

Lutein, a dihydroxy beta, epsilon-carotenoid, is the predominant carotenoid in photosynthetic plant tissue and plays a critical role in light-harvesting complex assembly and function. To further understand lutein synthesis and function, we isolated four lutein-deficient mutants of Arabidopsis that define two loci, lut1 and lut2 (for lutein deficient). These loci are required for lutein biosynthesis but not for the biosynthesis of beta, beta-carotenoids. The lut1 mutations are recessive, accumulate high levels of zeinoxanthin, which is the immediate precursor of lutein, and define lut1 as a disruption in epsilon ring hydroxylation. The lut2 mutations are semidominant, and their biochemical phenotype is consistent with a disruption of epsilon ring cyclization. The lut2 locus cosegregates with the recently isolated epsilon cyclase gene, thus, providing additional evidence that the lut2 alleles are mutations in the epsilon cyclase gene. It appears likely that the epsilon cyclase is a key step in regulating lutein levels and the ratio of lutein to beta,beta-carotenoids. Surprisingly, despite the absence of lutein, neither the lut1 nor lut2 mutation causes a visible deleterious phenotype or altered chlorophyll content, but both mutants have significantly higher levels of beta, beta-carotenoids. In particular, there is a stable increase in the xanthophyll cycle pigments (violaxanthin, antheraxanthin, and zeaxanthin) in both lut1 and lut2 mutants as well as an increase in zeinoxanthin in lut1 and beta-carotene in lut2. The accumulation of specific carotenoids is discussed as it pertains to the regulation of carotenoid biosynthesis and incorporation into the photosynthetic apparatus. Presumably, particular beta, beta-carotenoids are able to compensate functionally and structurally for lutein in the photosystems of Arabidopsis.

Arabidopsis↗
PubMed1995

Protein-chromophore interactions in alpha-crustacyanin, the major blue carotenoprotein from the carapace of the lobster, Homarus gammarus. A study by 13C magic angle spinning NMR.

MAS (magic angle spinning) 13C NMR has been used to study protein-chromophore interactions in alpha-crustacyanin, the blue astaxanthin-binding carotenoprotein of the lobster, Homarus gammarus, reconstituted with astaxanthins labelled with 13C at the 14,14' or 15,15' positions. Two signals are seen for alpha-crustacyanin containing [14,14'-13C2]astaxanthin, shifted 6.9 and 4.0 ppm downfield from the 134.1 ppm signal of uncomplexed astaxanthin in the solid state. With alpha-crustacyanin containing [15,15'-13C2]astaxanthin, one essentially unshifted broad signal is seen. Hence binding to the protein causes a decrease in electronic charge density, providing the first experimental evidence that a charge redistribution mechanism contributes to the bathochromic shift of the astaxanthin in alpha-crustacyanin, in agreement with inferences based on resonance Raman data [Salares, et al. (1979) Biochim. Biophys. Acta 576, 176-191]. The splitting of the 14 and 14' signals provides evidence for asymmetric binding of each astaxanthin molecule by the protein.

Animals↗
PubMed1995

Structure and properties of carotenoids in relation to function.

The basic principles of structure, stereochemistry, and nomenclature of carotenoids are described and the relationships between structure and the chemical and physical properties on which all the varied biological functions and actions of carotenoids depend are discussed. The conjugated polyene chromophore determines not only the light absorption properties, and hence color, but also the photochemical properties of the molecule and consequent light-harvesting and photoprotective action. The polyene chain is also the feature mainly responsible for the chemical reactivity of carotenoids toward oxidizing agents and free radicals, and hence for any antioxidant role. In vivo, carotenoids are found in precise locations and orientations in subcellular structures, and their chemical and physical properties are strongly influenced by other molecules in their vicinity, especially proteins and membrane lipids. In turn, the carotenoids influence the properties of these subcellular structures. Structural features such as size, shape, and polarity are essential determinants of the ability of a carotenoid to fit correctly into its molecular environment to allow it to function. A role for carotenoids in modifying structure, properties, and stability of cell membranes, and thus affecting molecular processes associated with these membranes, may be an important aspect of their possible beneficial effects on human health.--Britton, G. Structure and properties of carotenoids in relation to function.

Animals↗
PubMed1990

Localized transposon Tn5 mutagenesis of the photosynthetic gene cluster of Rhodobacter sphaeroides.

Four genes essential for bacteriochlorophyll biosynthesis were known to be encoded within a 45 kb region of the Rhodobacter sphaeroides genome, the boundaries of which are defined by puh and puf genes for reaction-centre and light-harvesting LH1 complexes. The cluster is represented by eight overlapping inserts cloned in the mobilizable vector pSUP202. We have used localized transposon Tn5 mutagenesis to characterize this cluster further; a total of 87 independent insertions were generated which identify nine genes for bacteriochlorophyll biosynthesis, six for carotenoid biosynthesis, and puhA encoding the reaction-centre H subunit. This work provides an essential framework for a detailed study of the structure and expression of genes for photosynthesis in this bacterium.

Bacteriochlorophylls↗
PubMed1987

Resonance Raman study of reconstituted carotenoproteins incorporating astaxanthin and 15,15'-didehydroastaxanthin.

Two reconstituted carotenoproteins have been studied by resonance Raman spectroscopy. They were prepared from the apoprotein of the Asterias rubens carotenoprotein, asteriarubin and either astaxanthin or 15,15'-didehydroastaxanthin. Spectral properties of dehydrocarotenoids are first discussed. The spectral properties of the complexes are compared to those of the free carotenoids and of other carotenoproteins containing astaxanthin, and possible protein-carotenoid interactions are discussed. Greater delocalisation of the pi-electron system in the central part of the polyene chain, and the role of lateral methyl groups in binding is emphasised.

Animals↗
PubMed1984

Detection of carotenoids in blood donors taking Orobronze: a cautionary note.

During the summer, plasma samples from eight blood donors at this transfusion centre were found to have a bright orange colour. All donors appeared to be healthy and haematological tests showed no abnormality. Extracts of lipid from the plasma revealed the presence of the carotenoid canthaxanthin and other carotenoid metabolites which were identified by thin-layer chromatography and high-pressure liquid chromatography, light absorbtion and mass spectrometry. On questioning the donors it was found that they had been taking a course of 'Orobronze' (an oral 'bronzing' treatment containing canthaxanthin). Canthaxanthin was still detectable in the plasma several months after the donors had ceased taking the capsules. At present it seems that there would be no deleterious effects on healthy individuals taking orobronze. The full implications of the presence of canthaxanthin in plasma transfused to patients do, however, require further investigation of carotenoid metabolism in man.

Blood Donors↗
PubMed1981

Relation of glucose to alpha-fetoprotein in amniotic fluid.

A direct relationship between amniotic fluid glucose and alpha-fetoprotein (AFP) has been found between the 16th and 22nd weeks of pregnancy. This relationship is statistically significant (P less than .001) in each of the 6 gestational weeks tested, the coefficients of correlation vary from .74 to .91. The relationship between amniotic fluid AFP and another metabolite, urea nitrogen, was not significant (r = -.34), suggesting that the AFP-glucose relationship was not spurious. Absence of pregnancy-associated macroglobulins in the samples indicates that they were not contaminated by maternal serum. These data reflect an aspect of fetal metabolism or transport that should be investigated more thoroughly.

Amniotic Fluid↗
PubMed1977

The roles of isomers of phytoene, phytofluene and zeta-carotene in carotenoid biosynthesis by a mutant strain of Scenedesmus obliquus.

Considerable changes in pigment composition occur during a period of 10 h when dark-grown cultures of PG1, a zeta-carotenic strain of Scenedesmus obliquus, are illuminated. These changes are consistent with a biosynthetic pathway in which 15-cis-phytoene is converted via 15-cis-phytofluene and 15-cis-zeta-carotene into all-trans-zeta-carotene and trans-bicyclic carotenoids. The findings also support the view that the xanthophylls lutein and zeaxanthin are formed from the corresponding carotenes and are especially important in the development of a normal chloroplast structure.

Carotenoids↗