Proline in yeast is turned into glutamate by two enzymes, proline oxidase and P5C dehydrogenase. The levels of these enzymes depend on the amount of proline in the cell. Yeast can transport proline best when grown on poor nitrogen sources like proline or urea. Mutants unable to use proline as a nitrogen source have been found, with some lacking proline oxidase (put1 mutants) and others lacking P5C dehydrogenase (put2 mutants). Proline triggers the production of both enzymes. The breakdown of arginine or ornithine can also lead to the production of P5C, which can induce the proline-degradative enzymes.