ADPG formation by the ADP-specific cleavage of sucrose-reassessment of sucrose synthase

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Abstract
The standardized enzyme coupling method for assaying sucrose synthase activities in the direction of sucrose cleavage was reexamined using enzyme preparations from cultured cells of sycamore (Acer pseudoplatanus L.) and spinach leaves (Spinacea oleracea). Both ATP and Tris, commonly utilized in assay systems to measure sucrose synthase, were found to inhibit non-competitively the ADPG-synthesizing activities of the enzyme. Upon substituting ATP by either GTP or UTP, and Tris by HEPES, we found that the sucrose synthase is capable of producing ADPG effectively, recognizing ADP as the principal substrate (Km = 5.3 microM (sycamore) and 16.8 microM (spinach]. The Vmax value for the synthesis of ADPG clearly surpasses the Vmax observed for the synthesis of UDPG by the enzyme. It was found that UDP is not inhibitory on the synthesis of ADPG by SS, which behaves allosterically with respect to the concentration level of sucrose.