Is it OK to purchase cat food?

Forgive a (possibly) silly question - but I have a cat as a pet, and of course I purchase cat food for him and feed it to him. Being a pretty recent participant of Buddhist thought and practice, I also understand that eating meat (something I admittedly still do myself) is frowned upon, but the thought occurred to me the other day - is it also frowned upon to buy my pet meat-based products? By doing so I participate in the killing of animals. On the other hand, cats are pretty much carnivores by nature, and trying to get a cat to go vegetarian would likely harm him.