SWIMMER, Y; BRILL, R; MAILLOUX, L; MOYES, C: Cardiac Biochemistry in Three Species of Tuna with Widely Divergent Temperature Tolerances

Tracking of pelagic tunas reveals strikingly different depth and temperature distributions among three congeners–yellowfin (Thunnus albacares), skipjack (Katsuwonus pelamis), and bigeye (T. obesus). The former spend 60-80% in relatively warm water (~20 to 28^oC) and do not subject themselves to more than an 8^oC temperature change. Bigeye tuna, however, often reach 500m where ambient temperatures are ca. 5^oC. We seek to define adaptations potentially responsible for limiting vertical movements of yellowfin and skipjack tunas, but that allow bigeye tuna to withstand rapid and extreme changes in temperature. We investigated thermal sensitivities of flux-regulating maximal in vitro cardiac enzyme activities to test the idea that enzyme activities are increased in bigeye to compensate for reduced catalysis at low temperatures. We measured total myofibrillar ATPase, pyruvate kinase, lactate dehydrogenase (LDH), citrate synthase, and carnitine palmitoyl transferase. We also compared species-specific enzyme substrate affinities (K_m) of LDH for pyruvate in both cardiac and white muscle. In general, cardiac enzyme activity levels,and K_m values of LDH for pyruvate are similar in the three tuna species at at 12, 17 and 25^oC. The only significant findings are higher K_m values of white muscle LDH to pyruvate in bigeye tuna at 17 and 25^oC. Our data do not explain the apparent disparate depth distributions; rather, our results suggest that all three tuna species have similar enzymatic tolerances to decreases in ambient temperature.