Role of Alkaline Fructose-1,6-Diphosphatase in Plants

Abstract

THE splitting of fructose-1,6-diphosphate to fructose-6-phosphate is an important step in the pentose phosphate reductive pathway of carbohydrate metabolism and in photosynthesis. An absolutely specific, alkaline and magnesium-ion-requiring fructose diphosphatase in spinach leaves has been reported^1,2. The leaves contained another fructose diphosphatase with a neutral pH optimum, with no requirement for magnesium ion and capable of splitting also sedoheptulose-1,7-diphosphate². The presence of a third fructose diphosphatase, with an optimum at acid pH and with activity towards ribulose-1,5-diphosphate also, was reported in spinach leaves³. Racker and Schroeder² believed that the alkaline fructose diphosphatase did not participate in photosynthesis, being absent from plastids isolated from homogenates prepared in aqueous media. Smillie⁴, however, obtained evidence to show that alkaline fructose diphosphatase was actively involved in photosynthesis. The enzyme was present in all photosynthetic tissue examined, and, using non-aqueous media for cell disruption, it was possible to show that the activity was localized in the plastids. An investigation by us of the diurnal activity of fructose diphosphatases in the leaves of the tapioca plant (Manihot utillissima) has provided additional evidence, indirectly, for the involvement of the alkaline enzyme in photosynthesis. Homogenates of the leaves showed three distinct pH optima (-acid, neutral and alkaline-) for phosphatase activity towards fructose-1,6-diphosphate. Of these only the alkaline enzyme showed marked diurnal variation in activity, being about three times as active in the daytime as at night.