SEDOHEPTULOSE-1,7-BISPHOSPHATE PHOSPHATASE-ACTIVITY OF CHLOROPLAST FRUCTOSE-1,6-BISPHOSPHATASE - IDENTIFICATION OF ENZYMES HYDROLYZING FRUCTOSE-1,6-BISPHOSPHATE AND SEDOHEPTULOSE-1,7-BISPHOSPHATE IN STROMAL EXTRACTS FROM CHLOROPLASTS OF SPINACH (SPINACIA-OLERACEA)

The identity of enzymes present in soluble extracts of spinach (Spinac ia oleracea) chloroplasts that are capable of hydrolysing fructose-1,6 -bisphosphate and sedoheptulose-1,7-bisphosphate has been investigated using antibodies against purified spinach chloroplast fructose-1,6-bi sphosphatase (EC 3.1.3.11). The activity of purified fructose-1,6-bisp hosphatase, which can exist in a less active oxidised form or a more a ctive reduced form as well as total fructose-1,6-bisphosphatase in str omal extracts is inhibited completely by the antiserum. Apparently, on ly a single enzyme, which can exist in an oxidised or reduced form, is responsible for hydrolysis of fructose-1,6-bisphosphate in the chloro plast. Purified chloroplast fructose-1,6-bisphosphatase can also exhib it sedoheptulose-1,7-bisphosphatase activity, but only when reduced. O xidised chloroplast stromal extracts contain little or no sedoheptulos e-1,7-bisphosphatase activity whereas reduced extracts contain sedohep tulose-1,7-bisphosphatase activity. Antiserum against fructose-1,6-bis phosphatase does not inhibit sedoheptulose-1,7-bisphosphatase activity detectable at pH 8 or less with 2 mM Mg2+ bur substantially inhibits (up to 60%) the sedoheptulose-1,7-bisphosphatase activity at higher pH or Mg2+ concentration, i.e. conditions under which the chloroplast fr uctose-1,6-bisphosphatase exhibits sedoheptulose-1,7-bisphosphatase ac tivity. Apparently, the chloroplast stroma contains at least two enzym e species capable of hydrolysing sedoheptulose-1,7-bisphosphate, a spe cific sedoheptulose-1,7-bisphosphatase (EC 3.1.3.37) and the chloropla st fructose-1,6-bisphosphatase.