Urea (NH2)2CO carries amine (ammonia) generated by the catabolism of amino acids. Assume the natural abundance of 14N is 100%. Citrulline (Exam Images Sheet) is synthesized with 15N in both nitrogen atoms (assume 100%). The citrulline enters into the urea cycle. Approximately what percentage of nitrogen in the urea produced in one turn of the cycle contains a 15N, assuming the naturally occurring nitrogen isotope is 100% 14N?