Abstract
Glutamine metabolism was investigated in the little skate (Raja erinacea), spiny dogfish (Squalus acanthias), and hagfish (Myxine glutinosa). Amino acid analysis revealed that dogfish plasma glutamine concentration was 11.6 nmol/ml, whereas 48βfold higher levels, 0.56 ΞΌmol/gm, were found in the red muscle. Mitochondria were isolated from the red muscle of the skate and dogfish. The isolation procedure yielded wellβcoupled mitochondria, which oxidize glutamine and Ξ²βhydroxybutyrate more rapidly than any other substrate tested. Palmitoyl carnitine was not oxidized, confirming previous studies that elasmobranch red muscle has a negligible capacity for fatty acid oxidation. Glutaminase and glutamine synthetase were measured in the muscle of the hagfish, dogfish, as well as several tissues of the skate. The brain and red muscle have the highest activities of glutaminase, and the activity in dogfish muscle is the highest reported for any muscle. Glutamine synthetase activity is highest in the brain, liver, and kidney, whereas the activity of this enzyme is low or not detectable in elasmobranch and hagfish muscle. The high glutaminase/glutamine synthetase ratio in the muscle indicates that the metabolic organization of elasmobranch and agnathan muscle is geared toward glutamine catabolism. The present study shows that, in addition to Ξ²βhydroxybutyrate, glutamine is an important fuel for elasmobranch red muscle. Β© 1992 WileyβLiss, Inc.