19. Ornithine serves three metabolically important roles, but is not found in proteins. What is one of the important roles of ornithine? (A) precursor in the synthesis of aspartate, (B) an intermediate in the urea cycle, (C) formation of ornithine from serine, (D) it condenses with aspartate to form argininosuccinate, (E) it is derived from glutamate-5-phosphate.