SUBCELLULAR-LOCALIZATION AND BIOCHEMICAL-PROPERTIES OF THE ENZYMES OFCARBAMOYL-PHOSPHATE AND UREA SYNTHESIS IN THE BATRACHOIDID FISHES OPSANUS-BETA, OPSANUS-TAU AND PORICHTHYS-NOTATUS

The subcellular localization and biochemical properties of the enzymes of carbamoyl phosphate and urea synthesis were examined in three repr esentatives of fishes of the family Batrachoididae, the gulf toadfish (Opsanus beta), the oyster toadfish (Opsanus tart) and the plainfm mid shipman (Porichthys notafus). The primary objective of the study was t o compare the biochemical characteristics of these fishes, which repre sent a range between ammoniotelism and ureotelism (O, beta being facul tatively ureotelic), with previous patterns observed for an ammoniotel ic teleost (Micropterus salmoides, the largemouth bass) and an obligat e ureogenic elasmobranch (Squalus acanthias, the dogfish shark), The p resent study documents the expression of mitochondrial carbamoyl phosp hate synthetase (CPSase) III and cytosolic CPSase II (and its associat ed enzymes of pyrimidine synthesis, dihydroorotase and aspartate carba moyltransferase) in the livers of all three batrachoidid species, Both mitochondrial and cytosolic activities of arginase were present in th e livers of all three species, as were cytosolic glutamine synthetase and argininosuccinate synthetase and lyase, However, O, beta also show ed mitochondrial glutamine synthetase activity and higher total hepati c levels of glutamine synthetase than either O. tau or P, notatus, Tak en together, these observations confirm that the arrangement of these enzymes in the batrachoidid fishes has greater similarity to that of M , salmoides than to that of S. acanthias. However, differences within the family appear to coincide with the different nitrogen excretion st rategies, O, tau and P. notatus are primarily ammoniotelic and most cl osely resemble the ammoniotelic M, salmoides, whereas ureotelism in O, beta is correlated with the presence of a mitochondrial glutamine syn thetase and the ability to induce higher total glutamine synthetase ac tivities than O. tau or P. notatus, Additionally, isolated mitochondri a from O, beta were able to generate citrulline from glutamine, wherea s those from O, tau were not, Also in contrast to S, acanthias, glutam ine synthetase activities in the mitochondria of O, beta are consisten tly lower than those of CPSase III, This and other kinetic observation s lend support to the hypothesis that glutamine synthetase may be an i mportant regulatory control point in determining rates of ureogenesis in O, beta.