Lots of us have heard of taurine as a critical part of a cat's diet, but another essential amino acid that's less of a household word is arginine. Arginine is used in large amounts in a cat's "urea cycle."
What's hyperammonemia? That's when too much ammonia in the blood gets to the brain and potentially triggers neurologic disorders. Cats can't synthesize enough of this amino acid from ornithine or citrulline - the process that converts it to arginine - so it must be in their diet.
The good news is that arginine is available in ample amounts in animal tissues, so cats on meat-based diets aren't at risk.
References: http://1.usa.gov/1dgSAAF & http://bit.ly/17gbhOJ