AbstractIt is shown that the catalytic center of Thiocapsa roseopersicina remains active after prolonged treatment with cyanide. It was found that the incubation of cyanide-treated hydrogenase in the presence of beta-mercaptoethanol, ferric iron, and sodium sulfide restored the hydrogenase activity in the hydrogen-oxidation reaction in the presence of methylviologen. The process of activity reconstruction depended on time and reached its maximum value (~ 60%) within 30 min at room temperature. In this case, an absorption band at 420 nm appeared in the absorption spectrum of the hydrogenase, which was present in the native hydrogenase and disappeared after treatment with cyanide; this indicated the reconstruction of iron–sulfur clusters. Thus, instead of growing bacteria in the presence of an iron isotope, one can replace 56Fe with 57Fe in the isolated enzyme, which will allow the use of smaller amounts of 57Fe.