Theoretically, I'm sure with a perfect shot in exactly the right place, you could kill a large bear. Practically, you wouldn't have a chance in hell. The .45 ACP does not have the penetration to do much damage against THICK layers of fur, skin, fat, and bone. Where I live 10mm Buffalo Bore heavy loads are the minimum for black bears, and .44 mag for browns. Since there's plenty of both wandering around here, nobody carries anything less than a .44. The revolver vs auto argument is another aspect that I won't go in to here.
Speaking from experience, if you are going to be around bears, use good judgment first, bear spray second, and a firearm third.