Metabolism of [5-h]proline by barley leaves and its use in measuring the effects of water stress on proline oxidation.

The objective of these experiments was to determine the fate of tritium from the 5 position of proline and to assess the validity of its loss to H(2)O as a measure of proline oxidation. When [5-(3)H]proline was fed to barley (Hordeum vulgare) leaves, tritium was recovered in H(2)O and metabolites such as glutamate, glutamine, organic acids, aspartate, asparagine, and gamma-aminobutyrate. Collectively these metabolites, which are oxidation products of proline, accounted for 8% of the (3)H recovered after 5 hours. In spite of the amount recovered in metabolites, the rates of proline oxidation estimated by measuring (3)H(2)O recovery from [5-(3)H]proline were only slightly lower than rates estimated by incorporation of (14)C into oxidized products and loss of (14)C from total proline. Therefore, (3)H(2)O recovery from [5-(3)H]proline is useful in assessing the effects of stress on proline metabolism.Water stress inhibited proline oxidation, as reported previously. In addition, a reconversion of proline oxidation products to proline occurred in stressed leaves. This observation probably indicates a breakdown in cellular compartmentation of proline synthesis and proline oxidation.