To ethylene glycol.

"nti
Automotive antifreeze/coolant is made of ethylene glycol, water, and a chemical additive package. For many years, there was only one type of antifreeze on the market, conventional green. Antifreeze technology, however, has become significantly more complex in the last several years with the introduction of Dexcool and other extended life antifreeze formulas. There are three main types of antifreeze on the market. They all contain an ethylene glycol base, water, dye and an additive package. Colored dye is added for leak detection and is not a reliable way of identifying coolant type. The additive package is what makes them different. "

And it is quite toxic to dogs and cats.

What to look for.

"Within the first few hours there may be vomiting and/or drooling, due to irritation of the stomach. There may also be stumbling and depression, resembling drunkenness (ethylene glycol is a type of alcohol). Within 24 to 48 hours, signs of kidney failure will develop, including loss of appetite, vomiting, dehydration, excessive urination, or no urination at all."

Hope the kitty recovers.

Aloha,
Weeze