Sedoheptulose-1,7-bisphosphate phosphatase activity of chloroplast fructose-1,6-bisphosphatase: identification of enzymes hydrolysing fructose-1,6-bisphosphate and sedoheptulose-1,7-bisphosphate in stromal extracts from chloroplasts of spinach (Spinaci

Abstract

The identity of enzymes present in soluble extracts of spinach(Spinacia oleracea) chloroplasts that are capable ofhydrolysing fructose-1,6-bisphosphate and sedoheptulose-1,7-bisphosphate hasbeen investigated using antibodies against purified spinach chloroplastfructose-1,6-bisphosphatase (EC 3.1.3.11). The activity of purifiedfructose-1,6-bisphosphatase, which can exist in a less active oxidised form ora more active reduced form as well as total fructose-1,6-bisphosphatase instromal extracts is inhibited completely by the antiserum. Apparently, only asingle enzyme, which can exist in an oxidised or reduced form, is responsiblefor hydrolysis of fructose-1,6-bisphosphate in the chloroplast. Purifiedchloroplast fructose-1,6-bisphosphatase can also exhibitsedoheptulose-1,7-bisphosphatase activity, but only when reduced. Oxidisedchloroplast stromal extracts contain little or nosedoheptulose-1,7-bisphosphatase activity whereas reduced extracts containsedoheptulose-1,7-bisphosphatase activity. Antiserum against fructose-1,6-bisphosphatase does not inhibit sedoheptulose-1,7-bisphosphatase activitydetectable at pH 8 or less with 2 mM Mg²⁺ butsubstantially inhibits (up to 60%) the sedoheptulose-1,7-bisphosphataseactivity at higher pH or Mg²⁺ concentration, i.e.conditions under which the chloroplast fructose-1,6-bisphosphatase exhibitssedoheptulose-1,7-bisphosphatase activity. Apparently, the chloroplast stromacontains at least two enzyme species capable of hydrolysingsedoheptulose-1,7-bisphosphate, a specific sedoheptulose-1,7-bispho-sphatase(EC 3.1.3.37) and the chloroplast fructose-1,6-bisphosphatase.