ENZYME ADAPTATION ALONG A HETEROTHERMIC TISSUE - THE VISCERAL RETIA MIRABILIA OF THE BLUEFIN TUNA

We measured enzyme activities along a heterothermic tissue, the viscer al retia mirabilia of the bluefin tuna, to test current theories of en zyme temperature adaptation. The heterothermic tissue model is ideal f or the study of fundamental temperature adaptation because it eliminat es confounding effects of whole animal acclimation. Enzymes were measu red at six positions along the rete at four temperatures (15, 20, 25, and 30 degrees C). Five enzymes (aspartate aminotransferase, citrate s ynthase, glucose-6-phosphate dehydrogenase, glutamate dehydrogenase, a nd pyruvate kinase) exhibited a significant positive compensatory effe ct, with activity at the cold end of the rete 1.2-3.1 times higher tha n at the warm end. Two enzymes (alanine aminotransferase and lactate d ehydrogenase) exhibited no significant compensation. On the basis of a ctivation energies of enzymes along the rete, differences in activity were due to differences in enzyme concentration and not isozymes or en zyme modification. Analysis of the compensatory responses of the enzym es in light of their thermal sensitivities leads us to conclude that t he pentose phosphate shunt is especially enhanced at the cold end of t he rete.