In addition to the three known forms of vitamin K (vitamin K quinone, vitamin K hydroquinone, and vitamin K epoxide), a fourth metabolite, hydroxyvitamin K, was found in reaction mixtures containing a vitamin K-dependent carboxylating enzyme system. When sulfite was added to such reaction mixtures, the formation of hydroxyvitamin K was substantially enhanced, whereas no epoxide was formed anymore. The vitamin K-dependent carboxylation was stimulated at these sulfite concentrations. Vitamin K hydroquinone could be replaced by t-butylhydroperoxide and also under these conditions the carboxylation was enhanced by sulfite. In the presence of peroxidase, the carboxylation reaction was blocked, whereas hydroxyvitamin K could still be detected in the reaction mixtures, even in the absence of sulfite. These observations lead us to the hypothesis that the carboxylation of glutamic acid residues is coupled to the heterolytic cleavage of a peroxide bond with the concurrent formation of vitamin epoxide.