Abstract
We explore the principles of pressure tolerance in enzymes of deep-sea fishes using lactate dehydrogenases (LDH) as a case study. We compared the effects of pressure on the activities of LDH from hadal snailfishes Notoliparis kermadecensis and Pseudoliparis swirei with those from a shallow-adapted Liparis florae and an abyssal grenadier Coryphaenoides armatus. We then quantified the LDH content in muscle homogenates using mass-spectrometric determination of the LDH-specific conserved peptide LNLVQR. Existing theory suggests that adaptation to high pressure requires a decrease in volume changes in enzymatic catalysis. Accordingly, evolved pressure tolerance must be accompanied with an important reduction in the volume change associated with pressure-promoted alteration of enzymatic activity ( ΔV∘PPΔVPP∘<math><mrow><mi>Δ</mi><msubsup><mi>V</mi><mtext>PP</mtext><msup><mrow></mrow><mo>∘</mo></msup></msubsup></mrow></math> ). Our results suggest an important revision to this paradigm. Here, we describe an opposite effect of pressure adaptation-a substantial increase in the absolute value of ΔV∘PPΔVPP∘<math><mrow><mi>Δ</mi><msubsup><mi>V</mi><mtext>PP</mtext><msup><mrow></mrow><mo>∘</mo></msup></msubsup></mrow></math> in deep-living species compared to shallow-water counterparts. With this change, the enzyme activities in abyssal and hadal species do not substantially decrease their activity with pressure increasing up to 1-2 kbar, well beyond full-ocean depth pressures. In contrast, the activity of the enzyme from the tidepool snailfish, L. florae, decreases nearly linearly from 1 to 2500 bar. The increased tolerance of LDH activity to pressure comes at the expense of decreased catalytic efficiency, which is compensated with increased enzyme contents in high-pressure-adapted species. The newly discovered strategy is presumably used when the enzyme mechanism involves the formation of potentially unstable excited transient states associated with substantial changes in enzyme-solvent interactions.