THE EFFECTS OF SELECTED CELL OSMOLYTES ON THE ACTIVITY OF LACTATE DEHYDROGENASE FROM THE EURYHALINE POLYCHAETE,NEREIS SUCCINEA

Abstract

1. In searching for an explanation for the high intracellular concentrations of nitrogenous organic osmolytes found in all marine invertebrates, the similarity of their functional groups to those neutral salts known to stabilize native macromolecular conformations was noted. This led to the hypothesis that such osmolytes are innocuous and substitute for more deleterious salts at high osmotic pressures. The hypothesis was tested on a representative macromolecule, the enzyme lactate dehydrogenase (LDH), from the euryhaline polychaete, Nereis succinea. 2. The activity of LDH from worms adapted to a range of salinities was measured at several pH values and in the presence of various concentrations of neutral salts and of low molecular weight nitrogenous solutes characteristically found intracellularly in marine and euryhaline invertebrates. 3. Differences in LDH activity in homogenates from worms adapted to osmoconforming (100%-35% SW) and osmoregulating (20% SW) salinities were observed. Enzyme activity from osmoregulating worms ("20%" LDH) occurred at nearly twice the concentration in tissue, and was less inhibited by neutral salts than was "100%" LDH. These differences may reflect the synthesis of a new isozyme during osmoregulatory adaptation, designed to divert pyruvate from entering the free amino acid pool by more readily converting it to lactate. 4. Low molecular weight nitrogenous solutes are generally far less inhibiting to LDH activity than are intracellular ions at similar osmolarities. The more nonpolar the compound added, the more innocuous its effect on the enzyme. This may be due either to suppression of charge-initiated dissociation of subunits, or the nonpolar groups may stabilize the macromolecule-solvent interface. 5. Although glycine is unable to protect LDH from inhibition by neutral salts, 5% bovine serum albumin is effective in this respect. This suggests that soluble cytoplasmic protein and perhaps less polar nitrogenous solutes as well play an important role in protecting macromolecules from functional inactivation by ions normally occurring in the cell.