Catalyzes the reversible interconversion of fructoselysine with its C-3 epimer, psicoselysine. May allow S.flexneri to utilize psicoselysine for growth. N(6)-(D-psicosyl)-L-lysine = N(6)-(D-fructosyl)-L-lysine Homooctamer. Belongs to the FrlC family.