Proline is a crucial part of breaking down arginine in yeast cells. Mutants that can't use proline excrete it when fed arginine or ornithine. Yeast cells have an enzyme called P5C dehydrogenase that breaks down proline and arginine. This enzyme is only activated by proline. When P5C comes from proline, it turns into glutamate. But when it comes from ornithine, it first becomes proline before turning into glutamate.