JAWS - THE INSIDE STORY - THE METABOLISM OF ELASMOBRANCH FISHES

Elasmobranchs are of metabolic interest for several reasons, including their primitive evolutionary position, their osmotic strategy and the ir Low incidence of neoplasia. Some aspects of the metabolism of elasm o-branch fishes are unique when compared with those of the other verte brates. Although many features of their metabolism can be attributed t o their primitive evolutionary position (e.g., fewer isoforms of enzym es and other proteins), some unique features appear to be related to t he unusual solute system (urea and methylamines) used by elasmobranchs . The solute system exerts widespread effects, which has an impact on the metabolism of lipids, ketone bodies and amino acids and the struct ure of proteins and membranes. Effects of urea on the transport of lip id may influence aspects of lipid metabolism reducing extrahepatic lip id catabolism via effects on nonesterified fatty acid transport and en hancing a need for reliance on ketone bodies. Amino acid metabolism of elasmobranchs is also heavily influenced by the need for continuous s ynthesis of urea with glutamine as the nitrogen donor. These effects, in rum, may play a role in their low incidence of cancer. Specifically , the reduced availability of glutamine (an important nutrient for rap idly growing cells) coupled with the low levels of nonesterified fatty acids in the blood reduces the availability of molecules essential fo r tumor growth. This metabolic design may thus provide marine elasmobr anchs with a ''systemic'' resistance to cancer. (C) 1997 Elsevier Scie nce Inc.