[Interaction of ribulose-1,5-diphosphate carboxylase with ribulose-1,5-diphosphate and with the magnesium ion by the spin label method].

: A selective modification of external SH-groups of riboluse 1,5-diphosphate carboxylase (EC 4.1.1.39) from spinach by a spin label on the basis of maleimide was performed. From the EPR spectra of spin labels of three types of SH-groups were classified. It is shown that part of spin labels pre-reduced with dithiothreitol are reoxidized by the enzyme after the removal of dithiothreitol from the reaction mixture. The reoxidation is connected with the fact that one of the spin labels is bound to the SH-group at the enzyme active site and can act as its substrate, which results in alteration of the electron structure of the greater than N--O fragment in the spin label upon interaction of the spin-labeled enzyme with the substrate and Mg2+.