Slice egg in half lengthwise, and remove yolk; reserve egg yolk for another use. Dice egg white halves; set aside.
Prepare grill or broiler.
Sprinkle the lemon juice and pepper over tuna. Place the tuna on a grill or broiler rack coated with cooking spray; cook 4 minutes on each side until tuna is medium-rare or desired degree of doneness. Coarsely chop tuna.
Combine tuna, diced egg white, celery, and the next 5 ingredients (celery through pineapple) in a bowl. Line each pita half with 1/3 cup lettuce and 2 tomato slices. Divide the tuna mixture evenly among pita halves.