Kinetics of mitochondrial phosphate transport and rates of respiration and phosphorylation during greening of etiolated Avena leaves

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Abstract
Using the technique of silicone oil filtration of organelles and the inhibitor stop method, the kinetics of transport of inorganic phosphate across the inner mitochondrial membrane were tested in relation to different stages of greening (0 to 24 h) of etiolated laminae of Avena sativa L., and compared to the rates of oxygen consumption and ATP formation. The results demonstrate that there is a pronounced increase in phosphate transport after 3 h of greening, reaching values for Vmax (about 17 μmol mg protein-1 h-1) that are three times as high as those measured with mitochondria from etiolated tissue. This is also mirrored by the rates of respiration and oxidative phosphorylation. After 24 h of light treatment (4 Klx), respiration and ATP formation, as well as V decreased again to levels below those of the etiolated stage. In contrast to V, there was no change in the affinity between inorganic phosphate and the binding sites of the transporting systems involved, as indicated by a rather constant Km (0.23 mM) for phosphate transport. Of the inhibitors of phosphate transport tested, mersalyl and methyl mercuric iodide were most efficient with identical characteristics of inhibition; but compared to animal mitochondria, the concentrations needed to result in similar amounts of inhibition, were more than ten times higher. The results are discussed with respect to plastid development.