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Metabolism and Enzymology

Comparative Studies of the Light Modulation of Nitrate Reductase and Sucrose-Phosphate Synthase Activities in Spinach Leaves ¹

U.S. Department of Agriculture, Agricultural Research Service, North Carolina State University, Raleigh, North Carolina 27695-7631, Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27695-7631, Department of Botany, North Carolina State University, Raleigh, North Carolina 27695-7631, Phytotechnology Research Center and Department of Biological Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931-1295

We recently obtained evidence that the activity of spinach (Spinacia oleracea L.) leaf nitrate reductase (NR) responds rapidly and reversibly to light/dark transitions by a mechanism that is strongly correlated with protein phosphorylation. Phosphorylation of the NR protein appears to increase sensitivity to Mg²⁺ inhibition, without affecting activity in the absence of Mg²⁺. In the present study, we have compared the light/dark modulation of sucrose-phosphate synthase (SPS), also known to be regulated by protein phosphorylation, and NR activities (assayed with and without Mg²⁺) in spinach leaves. There appears to be a physiological role for both enzymes in mature source leaves (production of sucrose and amino acids for export), whereas NR is also present and activated by light in immature sink leaves. In mature leaves, there are significant diurnal changes in SPS and NR activities (assayed under selective conditions where phosphorylation status affects enzyme activity) during a normal day/night cycle. With both enzymes, activities are highest in the morning and decline as the photoperiod progresses. For SPS, diurnal changes are largely the result of phosphorylation/dephosphorylation, whereas with NR, the covalent modification is super-imposed on changes in the level of NR protein. Accumulation of end products of photosynthesis in excised illuminated leaves increased maximum NR activity, reduced its sensitivity of Mg²⁺ inhibition, and prevented the decline in activity with time in the light seen with attached leaves. In contrast, SPS was rapidly inactivated in excised leaves. Overall, NR and SPS share many common features of control but are not identical in terms of regulation in situ.

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