Characteristics of a β-Galactosidase Associated with the Stroma of Chloroplasts Prepared from Mesophyll Protoplasts of the Primary Leaf of Wheat

Chloroplasts prepared from mesophyll protoplasts of the primary leaf of wheat (Triticum aestivum L. cv Egret) contain about 50% of the cellular beta-galactosidase (EC 3.2.1.23) activity. More than 80% of this activity is associated with the stroma and most of the remainder, although tightly bound to the thylakoids, can be washed free with sodium pyrophosphate. The vacuole contained about 20% and the remaining enzyme was presumed to be cytoplasmic or associated with one of the other organelles. Both the vacuolar and chloroplast enzymes were capable of releasing galactose from the galactolipid monogalactosyldiacylglycerol. Apart from their distinct locations within the cells, we conclude that the enzymes are different because they differed with respect to assay pH-optimum, comparative activity against the synthetic substrates phenyl-beta-d-galactoside, 4-methylumbelliferyl-beta-d-galactoside, 6-bromo-2-naphthyl-beta-d-galactoside, the disaccharide lactose, and the inhibitors d-galactose and d-galactono-1,4-lactone.