NADP-Glutamate Dehydrogenase Activity Is Increased under Hyperosmotic Conditions in the Halotolerant Yeast Debaryomyces hansenii

Glutamate plays an important role in osmoprotection in various bacteria. In these cases, increased intracellular glutamate pools are not attributable to the NADP-dependent glutamate dehydrogenase (NADP-GDH) or the glutamate synthase, which do not increase their activities under hyperosmotic conditions, but rather to changes in other enzymes involved in glutamate metabolism. We performed a study which indicates that, as opposed to what happens in bacteria, the activity of NADP-GDH is fivefold higher when the halotolerant yeast Debaryomyces hansenii is grown in the presence of 1 M NaCl, compared with growth in media with no added salt. Since purified NADP-GDH activity in vitro was not enhanced by the presence of salt and was more sensitive to ionic strength than the two isoenzymes from S. cerevisiae, increased enzyme synthesis is the most plausible mechanism to explain our results. We discuss the possibility that increased NADP-GDH activity in D. hansenii plays a role in counteracting the inhibitory effect of high ionic strength on the activity of this enzyme.