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Properties of a Membrane-bound Phosphatase from the Thylakoids of Spinach Chloroplasts ¹

Department of Biochemistry, Michigan State University, East Lansing, Michigan 48824

A 3-phosphoglycerate phosphatase activity of about 2 micromoles per minute per milligram chlorophyll is associated with the thylakoid membranes of spinach chloroplasts. The K_m for 3-phosphoglycerate is 3 millimolar. The enzyme can be solubilized from thylakoid membranes by treatment with 0.33 molar MgCl₂ or sodium deoxycholate. The activity is not stimulated by sulfhydryl reagents or the addition of 10 millimolar MgCl₂. The enzymic activity is insensitive to ethylenediaminetetraacetate. The pH optimum is broad, between 5.5 to 7.5. Although the substrate specificity is broad, 3-phosphoglycerate is the best substrate of those tested at neutral pH. However, p-nitrophenyl phosphate was a more effective substrate at pH 5.5. The enzyme exhibits the general characteristics of an acid phosphatase.