On the mechanism of activation of phosphoglucomutase by metal ions.

4. Tetrahydropteroylglutamate is a competitive inhibitor of the cofactor activity of extract of heated E. coli, and of tetrahydropteroyltriglutamate and its N5-formyl derivative. It also inhibits the activity of the N10-formyl compound. In all cases the inhibition is overcome by the addition of cobalamin. 5. The cofactor activity of tetrahydropteroylglutamate and tetrahydropteroyltriglutamate is unstable to exposure to air at 300 for 18 hr., whereas the activities of extract of heated E. coli and of N10and N5-formyltetrahydropteroyltriglutamate are relatively stable under these conditions. 6. Cobalamin is required for optimum formation of methionine by ultrasonic extracts of E. coli 121/176 (a cobalaminor methionine-requiring auxotroph) when the derivatives of pteroyltri glutamate, tetrahydropteroylglutamate or an extract of heated organisms supply the necessary folic acid cofactor.