Proteolysis of chloroplast thylakoid membranes. II. Evidence for the involvement of the light-harvesting chlorophyll a/b-protein complex in thylakoid stacking and for effects of magnesium ions on photosystem II-light-harvesting complex aggregates in the absence of membrane stacking |
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Authors: | D P Carter L A Staehelin |
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Institution: | Laboratory of Chemoprevention, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20205 USA |
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Abstract: | Experimental evidence indicates that the major pathway of retinoic acid metabolism in hamster liver microsomes follows the sequence: retinoic acid → 4-hydroxy-retinoic acid → 4-keto-retinoic acid → more polar metabolites. Using all-trans-10-3H]retinoic acid, it can be shown by reverse-phase high pressure liquid chromatographic analysis that the first and last steps of this sequence require NADPH, whereas the oxidation of 4-hydroxy to 4-keto-retinoic acid is NAD+ (or NADP+) dependent. Both NADPH-dependent steps, but not the NAD+-dependent dehydrogenase reaction, are strongly inhibited by carbon monoxide. The metabolism of retinoic acid but not of 4-hydroxy-retinoic acid is highly dependent on the vitamin A regimen of the animal. Retinoic acid is rapidly metabolized by liver microsomes either from vitamin A-normal hamsters or from vitamin A-deficient hamsters that have been pretreated with retinoic acid, but not by microsomes from vitamin A-deficient animals; in direct contrast, the rate of metabolism of 4-hydroxy-retinoic acid is equivalent in each of these microsomal preparations. Analysis of the kinetics of these reactions yields the following Michaelis constants with respect to the retinoid substrates: retinoic acid, 1 × 10?6m; 4-hydroxy-retinoic acid, 2 × 10?5m; and 4-keto-retinoic acid, 1 × 10?7m. The 4-hydroxy to 4-keto-retinoic acid oxidation has been shown to be experimentally irreversible, to have a KmNAD+of 2 × 10?5m, to be strongly inhibited by NADH, and to be unaffected by the presence of retinoic acid or its 4-keto-derivative in an equimolar ratio to the 4-hydroxy-substrate. |
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