Regulation of proline metabolism in the marine centric diatom Cyclotella cryptica

Proline synthesis from both glutamate and arginine is much lower in the dark than in the light. The reduction of proline synthesis in the dark is more pronounced from glutamate than from arginine in starved cells under water-stress conditions. Addition of glucose to such cells stimulates the synthesis of proline from both amino acids. There is no appreciable effect of light on the catabolism of proline. Light seems to be particularly required for the formation of proline from glutamate or arginine in C. cryptica. Other pathways of amino acid metabolism are less, and differently, affected by light conditions. The synthesis of asparagine is stimulated by starving cells in the absence of light, while the synthesis of macromolecules from glutamate is strongly depressed under these conditions. The capacity for amino acid uptake is strongly stimulated by dark incubation in the absence of glucose (energy deficiency).Feedback regulation of proline biosynthesis is very effective in normal cells with either amino acids or glucose as precursors. However, such regulation of proline synthesis is considerably decreased when the cells are subject to water stress. Glutamate strongly inhibits arginine conversion to glutamate and aspartate, and to a smaller degree its conversion to proline.Several enzymes involved in proline synthesis have been examined. Arginase (EC 3.5.3.1) and Δ1-pyrroline-5-carboxylate reductase (EC 1.5.1.2) occur in cell free extracts of C. cryptica. There is no significant difference in the enzymatic activities from normal and water-stressed cells. These enzymes are not inhibited by proline. Glutamate kinase and ornithine transaminase (EC 2.6.1.13) were not detected in cell-free extracts, although results from radiotracer experiments imply the presence of both of these enzymes at high levels in the cells.