arginine, creating urea. Therefore, ornithine is a central part of the urea cycle, which allows for the disposal of excess nitrogen.
Ornithine is not an amino acid coded for by DNA, and in that sense, is not involved in protein synthesis. However, in mammalian non-hepatic tissues, the main use of the urea cycle is in arginine biosynthesis, so as an intermediate in metabolic processes, ornithine is quite important.
In bacteria, such as E. coli, ornithine can be synthesized from L-glutamate (see reference below).