Proline Synthesis During Osmotic Stress in Megalopa Stage Larvae of the Blue Crab, Callinectes sapidus

The free amino acid (FAA) pool of individual Callinectes sapidus megalopas acclimated to 100% seawater averaged over 56% larger than that of 50% seawater acclimated megalopas. Most of the difference was due to a four-fold increase in proline concentration at the higher salinity. In 100% seawater, proline comprises 64% of the total FAA pool in megalopas; this contrasts with the role of proline in adult tissues where it never exceeds 25% of the total FAA pool. Metabolic tracer studies using 14C-glucose and 14C-glutamate as radiolabelled precursors showed that de novo synthesis of proline was very low unless induced by hyperosmotic stress. The induction of the synthetic pathway was inhibited by cycloheximide, a protein synthesis inhibitor. These results suggest that the induction of proline synthesis is regulated by the synthesis of either one of the enzymes catalyzing the three steps in the glutamate to proline pathway or a protein acting to stimulate the activity of one of those enzymes.