Glucose-6-phosphatase activity in a soluble fraction from cotyledon tissue of Brassica nigra

Abstract

A soluble fraction from cotyledon tissue of black mustard ( Brassica nigra) was found to catalyze the hydrolysis of glucose 6-phosphate. In an attempt to determine whether this reaction was catalyzed by a distinct glucose-6-phosphate ( d-glucose 6-phosphate phosphohydrolase, EC 3.1.3.9) or by an acid phosphatase (orthophosphoric monoester phosphohydrolase, EC 3.1.3.2), various characteristics of glucose 6-phosphate and p- nitrophenyl phosphate hydrolysis were compared. Both phosphatase activities exhibited a similar distribution pattern in subcellular fractions and in fractions obtained by precipitation with (NH 4) 2SO 4. The activities also were compared with respect to the effects of reaction mixture pH in the absence and presence of 1 mM KF-4 mM EDTA and the effect of partial inactivation by various enzyme pretreatments. Glucose 6-phosphate and p- nitrophenyl phosphate were mutually competitive inhibitors for the hydrolysis of the other compound. The K m and K f values were, respectively, 4.6 mM and 4.6 mM for glucose 6-phosphate and 0.72 mM and 0.79 mM for p- nitrophenyl phosphate. A substrate-specificity study indicated that p- nitrophenyl phosphate and phenyl phosphate were hydrolyzed more rapidly than glucose 6-phosphate. The soluble enzyme preparation did not exhibit PP 1-glucose phosphotransferase activity. Glucose did not inhibit the hydrolysis of glucose 6-phosphate. It is concluded that the observed hydrolysis of glucose 6-phosphate was catalyzed by an acid phosphatase. The possible existence in plants of a distinct glucose-6-phosphatase is discussed.