MITOCHONDRIAL CITRULLINE SYNTHESIS IN THE UREAGENIC TOADFISH, OPSANUS-BETA, IS DEPENDENT ON CARBONIC-ANHYDRASE ACTIVITY AND GLUTAMINE TRANSPORT

Mitochondria isolated from the liver of the Gulf toadfish Opsanus beta produce scitrulline in the presence of 5 mM glutamine. Citrulline pro duction was inversely related to succinate concentration between 0 and 10 mM, with a maximal rate being achieved at 0.1 mM. When toadfish ar e induced to become ureagenic by crowding-associated stress, mitochond rial citrulline production increases by approximately 10-fold. Citrull ine synthesis is dependent on intramitochondrial carbonic anhydrase (C A) activity, being inhibited by about 75% by both acetazolamide and me thazolamide. The addition of exogenous CA did not increase mitochondri al citrulline production. Anesthetizing the fish with MS 222 prior to mitochondrial isolation resulted in the near elimination of the capaci ty for citrulline synthesis. Mitochondria were also shown to possess a n inducible glutamine transport system. The Vmax for glutamine uptake increased three-fold and the Km increased four-fold in ureagenic vs. a mmoniogenic toadfish. The transport system is the second labile compon ent of the overall ornithine-urea cycle to be identified, and it provi des a link between the production of glutamine via cytoplasmic glutami ne synthetase and its consumption via mitochondrial carbamoyl phosphat e synthetase III. (C) 1997 Wiley-Liss, Inc.