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High concentrations of proline in the cell increase reactive oxygen species (ROS) formation in the mitochondrion. Proline catabolism is repressed under osmotic stress but once the stress is withdrawn, proline is oxidized to ?1-pyrroline-5-carboxylic acid (P5C) by proline dehydrogenase (PDH; EC 1.5.99.8), also known as proline oxidase, the first enzyme in the proline degradation pathway. P5C is then converted back to glutamate by the enzyme P5C dehydrogenase (P5CDH; 1.5.1.12) (Forlani et al., 1997). Thus, both PDH and P5CDH form two important enzymes in the degradation of proline to glutamate in higher organisms. During the recovery from stress, this short catabolic pathway provides energy, reducing power and precursors for nucleotide synthesis to allow the resumption of cell division (Hare and Cress, 1997).
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