What is the by-product of amino acid catabolism in liver?

The by-product of amino acid catabolism in the liver is urea. Urea is synthesized in the liver through a process called the urea cycle, also known as the ornithine cycle. The urea cycle involves several steps and enzymes that convert ammonia, a toxic waste product of protein metabolism, into urea, which is less toxic and can be safely excreted by the kidneys in urine.