BIOCHEMICAL ADAPTATION OF DEEP-SEA FISHES: SUSCEPTIBILITY OF DEHYDROGENASES TO PRESSURE-INACTIVATION AND PROTEOLYSIS

Muscle-type lactate dehydrogenase (M(,4)-LDH; EC 1.1.1.27), cytoplasmic malate dehydrogenase (MDH; EC 1.1.1.37) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12) from several shallow- and deep-living marine teleost fishes were examined for susceptibility to inactivation by hydrostatic pressure and proteolysis. The molecular mass and subunit aggregation state of these enzymes are identical to those of other vertebrate homologues. In general, LDH homologues of deeper-living species tend to require greater pressures for inactivation than do homologues of shallow-occurring species. Fish MDH and GAPDH homologues are more resistant to inactivation by pressure than are LDH homologues from the same species. Pressures required to inactivate dehydrogenases in vitro are greater than pressure normally encountered in the habitat of the species. Inactivation by hydrostatic pressure appears to be determined by the subunit aggregation state of the enzyme and the number of subunit interactions. Enzymes with more subunits and more subunit interactions tend to be less stable at high pressures. This is consistent with the model of Paladini and Weber (Biochemistry, 20:2587-2593, 1981) which predicts that susceptibility to dissociation of oligomeric proteins is controlled by the small "free volumes" at the intersubunit boundaries. Hydrostatic pressure, by affecting protein aggregation state and conformation, may increase susceptibility to proteolysis. LDH homologues of shallow-living fishes are inactivated faster at atmospheric pressure than are homologues of deep-living fishes. Pressures as low as 200 atm have a greater influence on proteolytic inactivation of LDH homologues of shallow-living fishes than for homologues of deep-living fishes. Fish MDH and GAPDH homologues are less susceptible to proteolytic inactivation than LDH homologues from the same species. Enzymes that are less susceptible to inactivation by proteases. The structural integrity of MDH and GAPDH homologues of both shallow- and deep-living fishes appear to be pre-adapted to the hydrostatic pressures of the deep-sea.