A test of biochemical symmorphosis in a heterothermic tissue: bluefin tunawhite muscle

To test predictions of biochemical symmorphosis, we measured the activity o f seven consecutive glycolytic enzymes at three positions along the heterot hermic white muscle of the bluefin tuna. Biochemical symmorphosis predicts that adjustments in sequential enzyme concentrations along a thermal gradie nt should occur as a function of the thermal sensitivity of the enzymes to ensure that no one enzyme in the pathway is in excess at any point along th e gradient. We found no evidence for adjustments in enzyme quantity or qual ity along the thermal gradient, as well as no evidence for the prediction t hat the more temperature-sensitive enzymes would exhibit more dramatic comp ensation. Conservation of glycolytic flux in the cold exterior and warm int erior muscle may be achieved by the near insensitivity of glyceraldehyde-3- phosphate dehydrogenase to temperature in this tissue. This may have the a dded benefit of moderating flux during seasonal or transient changes in the thermal gradient. According to the strictest application of biochemical sy mmorphosis, such a mechanism represents adequate, yet suboptimal design.