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Activities of enzymes for glucose catabolism in the swimbladder of the European eel Anguilla anguilla

Gas secretion into the swimbladder of the eel relies on the production of CO 2 and lactic acid from glucose in the swimbladder epithelium. The activities of the enzymes involved in glucose catabolism have been measured and compared with those in the rete mirabile, the liver and white skeletal muscle to evaluate whether the pentose phosphate shunt may contribute to glucose metabolism in the swimbladder tissue. The activities of enzymes of the pentose phosphate shunt were higher in the swimbladder epithelium than in white muscle, and close to those in the liver. The activities of the enzymes of anaerobic glycolysis were 2–5 times higher in the swimbladder epithelium than in the rete mirabile, reaching or even exceeding the levels in liver and white muscle, whereas the activities of the enzymes of oxidative metabolism were extremely low. Compared to enzymes of the other tissues, swimbladder phosphofructokinase and glucose-6-phosphate dehydrogenase showed no special adaptation to low pH values. The results show that the swimbladder epithelium is equipped with enzymes that produce CO 2 from glucose without the removal of O 2 , which is particularly advantageous for creating the high gas partial pressures needed for filling the swimbladder at great depth.