C. Doblander et R. Lackner, METABOLISM AND DETOXIFICATION OF NITRITE BY TROUT HEPATOCYTES, Biochimica et biophysica acta (G). General subjects, 1289(2), 1996, pp. 270-274
Nitrite (NO2-) is one of the most important toxicants to fish. Freshwa
ter fish are especially sensitive, particularly salmonids. Nitrite upt
ake is thought to occur via the HCO3- Cl--exchanger at the gill epithe
lia with nitrite substituting for chloride. In this way freshwater fis
h accumulate nitrite in the blood up to 100-fold from the surrounding
water. Another source, endogenous nitrite as a degradation product of
nitric oxide, rarely leads to pharmacologically relevant concentration
s. We developed a new method for the determination of nitrate (NO3-) i
n biological samples and used it to measure nitrite oxidation in isola
ted rainbow trout (Oncorhynchus mykiss) hepatocytes which were found t
o detoxify nitrite to the almost non-toxic nitrate. Detoxification is
inhibited by 0.05 mM bumetanide and 0.1 mM furosemide but not by SITS
and DITS, suggesting the involvement of the Na+, K+, 2Cl(-)-cotranspor
ter with nitrite or nitrate substituting for chloride. Oxidation of ni
trite is strongly accelerated by 0.05 mM uric acid. The efficacy of th
is antioxidant suggests that similar reactions are involved as known f
or haemoglobin [33]. However, in the case of trout liver also membrane
bound detoxificating activity can be observed which is also enhanced
by uric acid. ATP concentrations remained constant in the hepatocytes
during all experiments demonstrating that hepatocyte energy status was
not influenced by nitrite oxidation. Thus nitrite resistance in fish
is governed by at least two mechanisms, nitrite uptake and the rate of
detoxification. It is unknown whether fish actually differ in their a
bility to distinguish between chloride and nitrite during branchial up
take, but evidence presented in this paper suggests a significant cont
ribution of detoxification pathways to a possible nitrite tolerance of
fish.