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Hydrogenase-Mediated Activities in Isolated Chloroplasts of Chlamydomonas reinhardii¹

Institute for Photobiology of Cells and Organelles, Brandeis University, Waltham, Massachusetts 02254

Isolated intact chloroplasts of Chlamydomonas reinhardii were found to catalyze photoreduction of CO₂ in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea when adapted under an atmosphere of H₂ demonstrating the association of a hydrogenase and anaerobic adaptation system with these plastids. The specific activity of photoreduction was approximately one third that detected in cells and protoplasts. Photoreduction was found to have a lower osmoticum optimum relative to aerobically maintained chloroplasts (50 millimolar versus 120 millimolar mannitol). 3-Phosphoglycerate (3-PGA) stimulated photoreduction up to a peak at 0.25 millimolar beyond which inhibition was observed. In the absence of 3-PGA, inorganic phosphate had no effect on photoreduction but in the presence of 3-PGA, inorganic phosphate also stimulated the reaction. Carbonyl cyanide-p-trifluoromethoxyphenylhydrazone and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone inhibited photoreduction but inhibition by the former could be partially overcome by exogenously added ATP. The intact plastid can also catalyze photoevolution of H₂ while lysed chloroplast extracts catalyzed the reduction of methyl viologen by H₂. Both reactions occurred at rates approximately one-third of those found in cells. The oxyhydrogen reaction in the presence or absence of CO₂ was not detected.

¹ Supported by Department of Energy DE-AC02-76ER03231. The hydrogen electrode used in this study was purchased by BRSG S07 RR07044 awarded by the Biomedical Support Grant Program, Division of Research Resources, National Institute of Health to Brandeis University.

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				A. Melis, L. Zhang, M. Forestier, M. L. Ghirardi, and M. Seibert Sustained Photobiological Hydrogen Gas Production upon Reversible Inactivation of Oxygen Evolution in the Green Alga Chlamydomonas reinhardtii Plant Physiology, January 1, 2000; 122(1): 127 - 136. [Abstract] [Full Text] [PDF]