On the mechanism of 2'-deoxyuridylate hydroxymethylase

dUMP hydroxymethylase from SP01-infected Bacillus subtilis has been purified 160-fold by chromatography on DEAE-cellulose and ethylagarose. The enzyme catalyzes exchange of the 5-hydrogen of dUMP for protons of water in the presence or absence of the cofactor CH2-H4folate. Upon treatment with FdUMP and CH2-H4folate, an isolable covalent complex is formed which is believed to be structurally similar to a steady-state intermediate of the normal reaction. The FdUMP-CH2-H4folate-dUMP hydroxymethylase complex is stable toward denaturation with sodium dodecyl sulfate and shows a subunit molecular weight of 46 000. By analogy with chemical models and studies of dTMP synthetase, a mechanism is proposed for the reaction catalyzed by dUMP hydroxymethylase.