Asparagine metabolism and asparaginase activity in a euryhaline Chlamydomonas species

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Abstract
A Chlamydomonas species isolated from a marine environment possesses an L-asparaginase, an enzyme not yet reported in the microalgae. This enzyme enabled the organism to grow as well with asparagine as sole nitrogen source as with inorganic nitrogen sources (NO3−, NH4+). Only the amide nitrogen was used for growth since growth did not occur on aspartate and aspartate accumulated in the media when cells were either grown on asparagine or during short-term incubations with L-[U-14C]asparagine. Cells grown on NO3−, NH4+. or L-asparagine in batch culture possessed equivalent asparaginase activities. However, nitrogen-limited cells possessed four times the activity of cells grown with sufficient nitrogen for normal growth, regardless of the source of nitrogen (NO3−, NH4+ or L-asparagine). In batch culture experiments, cells in log phase possessed the lowest activity per cell, while lag phase and stationary phase cells possessed greater activity. The enzyme behaved like a periplasmic space enzyme since (1) breaking the cells did not release into solution more activity than was shown by whole cells and (2) whole cells converted L-[U-14C]asparagine to [14C]aspartate with little intracellular accumulation of radioactivity. Cell-free preparations of the enzyme possessed a Km value for asparagine of 1.1 × 10−4 M, with no glutaminase activity.