Mechanism of enzymic isomerization and epimerization of D-erythrose 4-phosphate

The mechanism of the enzymic isomerization and epimerization of D-erythrose 4-phosphate (Ery4P) by an enzyme preparation from bovine liver was investigated with the use of 2H2O. The incorporation of 2H was quantitatively determined by a procedure using gas chromatography-mass spectrometry. About one atom of 2H was incorporated per molecule of the enzymic epimerization reaction product of Ery4P (D-threose 4-phosphate) or that of D-ribulose 5-phosphate. Computer simulation of the Ery4P isomerization reaction indicated that the 2H of 2H2O was not directly incorporated into the enzymic reaction product (D-erythrulose 4-phosphate). Instead, intramolecular transfer of hydrogen atoms had occurred.