Regulation of proline synthesis during osmotic stress in the copepod Tigriopus californicus

The marine copepod Tigriopus californicus shows rapid adjustment of intracellular proline and alanine pools in response to salinity stress. Small increases in salinity (on the order of 2 ppt or 50 mOsmoles) are sufficient to elicit proline synthesis in animals acclimated to 50% seawater. Increases in osmolarity achieved by adding organic solutes to a 50% seawater medium also elicited proline (but not alanine) synthesis. In constant 50% seawater, label from 14C‐(U)‐glucose is incorporated into the free alanine and glutamate (but not proline) pools; only the proline pool shows a monotonic increase in specific activity following transfer to 100% seawater. To demonstrate that glutamate is a proline precursor, 14C‐(U)‐glutamate was provided to animals maintained in 50% seawater (control) and animals transferred to 100% seawater; salinity change resulted in the rapid increase in alanine and proline concentrations, with proline (and not alanine) achieving specific activities similar to that of the glutamate pool. Taken together, these results indicate that small hyperosmotic stimuli result in the induction of proline synthesis from glutamate. The regulation of proline synthesis involves mechanisms discrete from those regulating the concentrations of other FAA functioning in osmotic response.