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W T Griffiths

Publications and source records attributed to W T Griffiths.

33 records · Page 2Linked to original sources
PubMed1980

Substrate-specificity studies on protochlorophyllide reductase in barley (Hordeum vulgare) etioplast membranes.

1. The substrate specificity of the enzyme protochlorophyllide reductase in barley (Hordeum vulgare) etioplasts was investigated. 2. It was shown that naturally occurring esterified protochlorophyllide and chemically prepared protochlorophyllide methyl ester are not substrates for the enzyme, suggesting an important role for the C-7 carboxylic acid group in binding of the porphyrin to the enzyme. 3. Removal of magnesium from the protochlorophyllide leads to inactivity of the compound as a substrate for the enzyme. However, activity can be restored by replacing the magnesium with zinc, whereas nickel, copper or cobalt failed to restore substrate activity. 4. Binding of the second substrate, NADPH, to the enzyme probably occurs through the 2'-phosphate group in the coenzyme.

Chloroplasts↗
PubMed1978

Reconstitution of chlorophyllide formation by isolated etioplast membranes.

1. The reconstitution of chlorophyllide biosynthesis by barley etioplast membranes is described. 2. The process is dependent on the additon of NADPH and protochlorophyllide and on illumination, which can be either continuous or intermittent. 3. The reconstituted process involves spectroscopically similar intermediates to the native reaction in whole leaves. 4. Steps in the process are an initial enzymic formation in the dark of a photoactive complex, P638/652 (probably a ternary protochlorophyllide-NADPH-enzyme complex), followed by a very rapid light-dependent hydrogen transfer from the NADPH to the protochlorophyllide giving chlorophyllide giving chlorophyllide, finally releasing the enzyme for repeating the process. 5. A continuous assay for the system regenerating complex P638/652 was devised on the basis of monitoring chlorophyllide formation. 6. The pH optimum of the reaction is at 6.9 and Km values for protochlorophyllide and NADPH are 0.46 and 35 micron respectively. 7. The reaction is associated specifically with the etioplast membrane fraction. 8. Activities of the system assayed in vitro are more than adequate to account for rates of chlorophyll formation in vivo.

Chlorophyll↗
PubMed1975

Some observations on chlorophyll(ide) synthesis by isolated etioplasts.

1. A modified procedure for the isolation of etioplasts from dark-grown barley is described and the regeneration of phototransformable protchlorophyll(ide) was demonstrated in the isolated plastids. 2. On exposure of the etioplasts to a long-term flash illumination, chlorophyll(ide) synthesis from a precursor pool, which includes all the protochlorophyllide, was demonstrated. 3. Added delta-aminolaevulinic acid failed to be significantly incorporated into chlorophyll(ide) in the etioplasts despite its extensive incorporation into porphyrin precursors of chlorophyll and haem compounds. The findings are discussed in terms of the inability of etioplasts to carry out the metal-insertion step in chlorophyll synthesis. 4. An elevated chlorophyll(ide) concentration was attained in the etioplasts by increasing the size of the utilizable precursor pool by pre-feeding whole plants with delta-aminolaevulinic acid, isolating the etioplasts and subjecting them to the flash illumination.

Aminolevulinic Acid↗
PubMed1975

Characterization of the terminal stages of chlorophyll (ide) synthesis in etioplast membrane preparations.

1. Chlorophyll (ide) formation from protochlorophyll (ide) that is normally inactive was demonstrated in etioplast membranes isolated from maize and barlley plants, the process being dependent on intermittent illumination and the addition of NADPH. 2. The addition of NADPH to the membranes was shown to result in the conversion of inactive protochlorophyll (ide) absorbing at about 630 nm into a form(s) with light-absorption maxima at about 640 and 652 nm, both of which disappear when chlorophyll (ide) is formed on illumination. 3. The temperature-dependence of the activation process and its response to a variety of reagents were examined. From these, the conclusion is drawn that -SH groups are involved in the activation but in the active complex these are unavailable for reaction with -SH reagents. 4. Evidence is presented for the occurrence of glucose 6-phosphate dehydrogenase activity within etioplasts and the suggestion is made that the oxidative pentose phosphate pathway can provide the NADPH required for chlorophyll biosynthesis during the early stages of greening.

Cell Membrane↗
PubMed1967

Nature, intracellular distribution and formation of terpenoid quinones in maize and barley shoots.

1. Maize and barley shoots have been shown to contain phylloquinone, plastoquinone, alpha-tocopherol (and gamma-tocopherol in maize), alpha-tocopherolquinone and ubiquinone-9. 2. No solanesol was detected in any tissue examined. 3. In maize shoots plastoquinone and alpha-tocopherolquinone were localized in the chloroplast; ubiquinone was in the mitochondria. 4. Etiolated (dark-grown) shoots contained smaller amounts of phylloquinone and plastoquinone; alpha-tocopherolquinone was entirely absent; ubiquinone and alpha-tocopherol concentrations were unaffected. 5. On illumination of etiolated shoots the chloroplastidic quinones phylloquinone, plastoquinone and alpha-tocopherolquinone were synthesized in step with chloroplast development. alpha-Tocopherolquinone was not formed at the immediate expense of alpha-tocopherol.

Carotenoids↗
PubMed1967

Biosynthesis of the prenyl side chains of plastoquinone and related compounds in maize and barley shoots.

The incorporation of (14)C by etiolated maize and barley shoots exposed to light of (14)CO(2) and [2-(14)C]mevalonic acid into phylloquinone, plastoquinone, ubiquinone, alpha-tocopherolquinone and alpha-tocopherol was examined. In maize (the principal tissue studied) it was demonstrated that (14)C from [2-(14)C]mevalonic acid is incorporated into phylloquinone, plastoquinone and ubiquinone. alpha-Tocopherol and alpha-tocopherolquinone, although undoubtedly labelled from this substrate, were not purified completely. As expected, (14)C from (14)CO(2) was incorporated into all components examined. Ozonolytic degradation studies showed that (14)C from [2-(14)C]mevalonic acid was incorporated specifically into the prenyl side chains of plastoquinone and ubiquinone, and from this it was inferred that mevalonic acid can be regarded as the specific distal precursor to the prenyl portions of all terpenoid quinones occurring in plant tissues. From a comparison of the relative incorporation of (14)C from (14)CO(2) and [2-(14)C]mevalonic acid into the intra- and extra-chloroplastidic terpenoids evidence was obtained consistent with the tenet that the prenyl portions of the chloroplastidic quinones phylloquinone and plastoquinone, along with beta-carotene, are biosynthesized within the confines of the chloroplast, the side chain of the extraplastidic ubiquinone and phytosterols being synthesized elsewhere within the cell. The results obtained for the incorporation of (14)C from (14)CO(2) and [2-(14)C]mevalonic acid into alpha-tocopherol and alpha-tocopherolquinone were not readily interpretable with regard to the site of synthesis of these compounds.

Carbon Dioxide↗