T HE transfer of a reactive group involving substrate and enzyme was postulated by Dou-doroff, et at.(1) for sucrose phosphorylase. This was based on the interchange of phos-phate between glucose-i-phosphate and radioactive inorganic phosphate in the presence of the enzyme. Jagannathan and Luck (2) later suggested a similar transfer mechanism for phosphoglucomutase, based on the exchange of radioactive phosphate between glucose-i-phosphate and a proposed enzyme-phos-phate. They suggested the following mechanism: transfer of phosphate from crystalline phosphoglu-comutase (4) to glucose-l-phosphate and glucose-6-phosphate with a net synthesis of glucose-1, 6-diphosphate in relatively substantial amounts. The following two-step mechanism was therefore postulated: Glucose-i-phosphate+ phospho-enzyme= glueose-1, 6-diphosphate+ dephospho-enzyme;(1) Glucose-1, 6-diphosphate+ dephospho-enzymeglucose-6-phosphate+ phospho-enzyme.(2)'This mechanism differs from that proposed by Jagannathan and Luck in three respects.(i) Glucose-Glucose 1 phosphate