Protein is toxic because it increases levels of ammonia, which is toxic to the body.
Yes, ammonia is toxic, but a well-nourished body is quite good at turning it into harmless urea. Carnitine found in meat and other animal products helps to quickly detoxify the ammonia that each of us naturally produces in response to a high-protein meal. The toxicity of protein and the ammonia it temporarily generates is greatly exaggerated.
From: The Carnitine Miracle: Top Ten Protein Myths
By: Mark Hegsted of New England Regional Primate Research Center, Harvard University