Effects of Proline and Carbohydrates on the Metabolism of Exogenous Proline by Excised Bean Leaves in the Dark

Proline was metabolized when vacuum infiltrated into starved bean (Phaseolus vulgaris L.) leaves from plants previously in the dark for 48 hours, but an equivalent increase in protein proline was not observed. When (14)C-proline was infiltrated into starved leaves, a large percentage of the (14)C was recovered in other amino acids, organic acids, and CO(2), in addition to that recovered as protein proline. However, extensive oxidation of proline was observed only if enough proline was added to increase substantially the endogenous concentration of proline. Increasing the endogenous concentration did not affect the amount of proline that was incorporated into protein.When added to leaves from plants previously in the light of near saturating intensity for more than 16 hr, very little (14)C-proline was oxidized, even when a sufficient amount of proline was added to increase the endogenous concentration. Adding sucrose to starved leaves along with the proline slowed down the oxidation of proline. Thus, it appears that some carbohydrate or intermediate of carbohydrate metabolism may inhibit the oxidation of proline in leaves.Based on kinetics of labeling, the pathway of proline oxidation was by conversion to glutamic acid and subsequent metabolism to intermediates in the Krebs cycle and to CO(2).