We have performed transport measurements on Al and Nb based metal-oxide-metal tunnel junctions with an AlOx tunnel barrier and found a strong dependence of the effective potential barrier height on the oxide-metal interface properties. Our estimations of the barrier height based on a phenomenological Simmons' model are consistent with the values obtained from the first-principle calculations. The calculations clearly show that the barrier height is strongly affected by the formation of metal induced gap states originating from the hybridization between metallic bands and Al2O3 conduction band. These findings are important for nanoelectronic devices containing tunnel junctions with a thin insulating layer.