Ornithine Serves Three Metabolically Important Roles, but Is Not Found

Ornithine serves three metabolically important roles, but is not found in proteins. What is one of the important roles of ornithine?

A) amine donor for many transamination reactions
B) an intermediate in the urea cycle
C) involved in the synthesis of serine, which serves as a precursor for glycine and cysteine
D) its reaction with methenyl tetrahydrofolate produces lysine
E) none of the above