| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 2000: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 1999: ¥1,000,000 (Direct Cost: ¥1,000,000)
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| Research Abstract |
To examine the transition to ureogenesis, M.abei was immersed in 2 mM NH_4HCO_3 or ^<15>N-ammonia solution (1 mM ^<15>N-(NH_4)_2 SO_4 (99.7 atom%) + 2 mM NH_4HCO_3, pH 8.0) for 4-8 days. In 20% sea water (control ), urea exeretion rates were low, and the pereentage of nitrogen excreted as urea was 14% of the total nitrogen (ammonia +urea) excretion. When exposed to 2 mM NH_4HCO_3 or ^<15>N-ammonia solution for 4 days, the urea excretion rate increased to 7-fold that of the control, and sustained that level over 4 days. The percentage of nitrogen excreted as urea reached to 62% of the total nitrogen excretion (Ammonia-N+Urea-N). ^<15>N-enrichment (atom% excess) of excrered urea-N increased with time and reached a plateau on the third day following exposure to ^<15>N-ammonia .After exposure to ^<15>N-ammonia for 4 days, ^<15>N-enrichment of the amide-N in glutamine in the tissues was virtually the same as that of ammonia-N, which was about 2-fold that of urea-N in the excreta as well as
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in the tissues. Glutamine contents and glutamine synthetase activities in the liver and muscle increased greatly following exposure to ammonia. In accord with enhancement of urea excretion, urea and citrulline contents in the muscle and whole body of the exposed fish significantly increased, whereas uric acid contents in these tissues remained unchanged. CPSaseIII mRNA expression and CPSaseIII activities could be detected in muscle, skin and gill, but were negligible in liver, although CPSaseIII activities in these tissues alone did not account for the observed rates of urea excretion following exposure to ammonia. All other ornithine-urea cycle (OUC) enzyme activities were present however in muscle, skin and gill at levels that were sufficient to account for the observed rate of urea excretion. This is the first report of the presence of all OUC enzymes not only in muscle, but also in the skin and gill of fish. Thus, M.abei clearly shows the transition from ammoniotely to ureotely under ammonia loading conditions, and is able to produce urea mainly via OUC operating in multiple tissues as a means for ammonia detoxitication. Less
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