Before revision:  We simplify the study of steady states of these systems by explicit elimination of intermediate complexes  (inspired by Feliu-Wiuf'12 , 13 and Thomson-Gunawardena'09)and we  define an important subclass of MESSI systems with toric steady states .
After revision:  We simplify the study of steady states of these systems by explicit elimination of intermediate complexes and we give conditions to ensure an explicit rational parametrization of the variety of steady states (inspired by Feliu-Wiuf'12 , 13 and Thomson-Gunawardena'09)and we define an important subclass of MESSI systems with toric steady states .