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Metabolism and Enzymology

Metabolism of 2'-Carboxyarabinitol in Leaves ¹

Department of Biochemistry, University of Nevada, Reno, Nevada 89557-0014

Results presented here indicate that 2'-carboxyarabinitol (CA) is the in vivo precursor and product of 2'-carboxyarabinitol 1-phosphate (CA1P) metabolism in leaves. When [2-¹⁴C]CA was fed in the light to leaves of five species known to be highly active in CA1P metabolism (Phaseolus vulgaris, Lycopersicon esculentum, Helianthus annuus, Petunia hybrida, and Beta vulgaris), [¹⁴C]CA1P was formed in the dark. Reillumination of a Phaseolus leaf caused this [¹⁴C]CA1P to be rapidly metabolized to [¹⁴C]CA (t = 1 min). The epimer 2'-carboxyribitol could not substitute for CA in the dark synthesis of CA1P, and CA in the anionic form was a better substrate than CA in the lactone form. In leaves of Phaseolus vulgaris, the active CA pool size used in the dark synthesis of CA1P is between about 70 and 110 nanomoles per milligram of chlorophyll. The photosynthetic electron transport inhibitor diuron did not affect the dark synthesis of [¹⁴C]CA1P, but did greatly reduce the rate of its subsequent light degradation (t = approximately 10 min). Dark synthesis of [¹⁴C]CA1P was inhibited by dithiothreitol and NaF. From the present data, we suggest that CA1P and CA participate in a metabolic substrate cycle in vivo.

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