ADPglucose pyrophosphorylase from the Crassulacean acid metabolism plants Hoya carnosa and Xerosicyos danguyi were partially purified to study their regulatory and kinetic properties. The molecular weight of the native enzymes from both plants was determined to be about 209,000. The enzyme from both plants was found to be activated by glycerate 3-phosphate and inhibited by inorganic phosphate. The kinetic constants for the substrates and Mg(2+) are reported. The significance of the activation by glycerate 3-phosphate and inhibition by inorganic phosphate of ADPglucose synthesis catalyzed by the H. carnosa and X, danguyi enzymes is discussed. ADPglucose synthesized by the above enzymes was found to be the most effective donor of the glucosyl portion to alpha-glucan primer in the starch synthase reaction observed in CAM plants.