Hepatic and extrahepatic distribution of ornithine urea cycle enzymes in holocephalan elephant fish (Callorhinchus milii).
Cartilaginous fish comprise two subclasses, the Holocephali (chimaeras) and Elasmobranchii (sharks, skates and rays). Little is known about osmoregulatory mechanisms in holocephalan fishes except that they conduct urea-based osmoregulation, as in elasmobranchs. In the present study, we examined the ornithine urea cycle (OUC) enzymes that play a role in urea biosynthesis in the holocephalan elephant fish, Callorhinchus milii (cm). We obtained a single mRNA encoding carbamoyl phosphate synthetase III (cmCPSIII) and ornithine transcarbamylase (cmOTC), and two mRNAs encoding glutamine synthetases (cmGSs) and two arginases (cmARGs), respectively. The two cmGSs were structurally and functionally separated into two types: brain/liver/kidney-type cmGS1 and muscle-type cmGS2. Furthermore, two alternatively spliced transcripts with different sizes were found for cmgs1 gene. The longer transcript has a putative mitochondrial targeting signal (MTS) and was predominantly expressed in the liver and kidney. MTS was not found in the short form of cmGS1 and cmGS2. A high mRNA expression and enzyme activities were found in the liver and muscle. Furthermore, in various tissues examined, mRNA levels of all the enzymes except cmCPSIII were significantly increased after hatching. The data show that the liver is the important organ for urea biosynthesis in elephant fish, but, extrahepatic tissues such as the kidney and muscle may also contribute to the urea production. In addition to the role of the extrahepatic tissues and nitrogen metabolism, the molecular and functional characteristics of multiple isoforms of GSs and ARGs are discussed.
Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564, Japan. wtakagi@aori.u-tokyo.ac.jp
Wataru
W
Takagi
Makiko
M
Kajimura
Justin D
JD
Bell
Tes
T
Toop
John A
JA
Donald
Susumu
S
Hyodo
D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D001119:Arginase / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D001499:Bayes Theorem; D019731:Carbon-Nitrogen Ligases / Q000235:genetics / Q000378:metabolism*; D003001:Cloning, Molecular; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D005399:Fishes / Q000378:metabolism / Q000502:physiology*; D005974:Glutamate-Ammonia Ligase / Q000235:genetics / Q000378:metabolism; D007668:Kidney / Q000378:metabolism; D008099:Liver / Q000201:enzymology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000378:metabolism; D009954:Ornithine Carbamoyltransferase / Q000235:genetics / Q000378:metabolism*; D010802:Phylogeny; D020033:Protein Isoforms / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D018709:Statistics, Nonparametric; D014508:Urea / Q000378:metabolism*; D014739:Victoria; D014882:Water-Electrolyte Balance / Q000502:physiology*
D017931:DNA Primers; D018076:DNA, Complementary; D020033:Protein Isoforms; D014508:Urea; D009954:Ornithine Carbamoyltransferase; D001119:Arginase; D019731:Carbon-Nitrogen Ligases; D005974:Glutamate-Ammonia Ligase; C024233:carbamoyl-phosphate synthetase (N-acetylglutamate)