Mix the last seven ingredients in a bowl. Pour 1/2 cup marinade into a large resealable plastic bag and add tuna steaks. Seal bag and turn to coat, then refrigerate for 2 hours. Cover remaining marinade and refrigerate. Drain and discard marinade from tuna. Use nonstick cooking spray on the grill rack. Grill tuna, uncovered, for 5-7 minutes on each side or until fish flakes easily with a fork. Serve with the rest of the marinade.