Dogmas and controversies in the handling of nitrogenous wastes:Expression of arginase Type I and II genes in rainbow trout: influence of fasting on liver enzyme activity and mRNA levels in juveniles

Abstract

SUMMARY Through analysis of a cDNA library and third-party annotation of available database sequences, we characterized the full-length coding regions of rainbow trout (Oncorhynchus mykiss) Type I, Onmy-ARG01, and Type II, Onmy-ARG02, arginase genes. Two partial related arginase sequences, Onmy-ARG01b and Onmy-ARG02b, and a full-length zebrafish arginase coding region (Danio rerio), Dare-ARG02, are also reported. Comparison of vertebrate arginase sequences shows that both Type I and Type II genes in bony fishes contain a mitochondrial targeting N-terminal domain. This suggests that the cytosolic Type I arginase found in ureotelic vertebrates arose in the common ancestor of amphibia and mammals. Onmy-ARG01 and Onmy-ARG02 mRNA was detected in liver,kidney, gill, intestine, red muscle and heart tissues. Onmy-ARG01 was expressed at a significantly higher level relative to Onmy-ARG02 in liver and red muscle tissue. To investigate whether there was differential regulation of Onmy-ARG01 and Onmy-ARG02, juvenile trout were fasted for 6 weeks and hepatic enzyme activities and mRNA levels were compared with those of fed control fish. There was a 3-fold increase in liver arginase activity and a 2-fold increase in Onmy-ARG02 mRNA levels but no change in Onmy-ARG01 mRNA levels in fasted fish relative to fed fish. These findings indicate that both types of arginase genes are present and expressed in rainbow trout and that the pattern of expression varies between tissues. The increase in liver arginase activity after a 6-week fast is due,in part, to an increase in the expression of Onmy-ARG02 mRNA levels.

Effects of crowding on ornithine–urea cycle enzyme mRNA expression and activity in gulf toadfish (Opsanus beta)

“…Similar to OCT, there was a dichotomy between ARG mRNA expression and activity where transcript levels were not affected by crowding but enzyme activity was; however, unlike OCT, ARG mRNA expression and activity were not correlated. This result could be explained by the fact that there are at least two different ARG genes described in mammals and in teleost fishes that arose from a duplication event (Joerink et al, 2006;Morris, 2002;Wright et al, 2004;Yu et al, 2001). Polyploid fish have undergone further duplications of the ARG genes, resulting in multiple copies of the ARG isoforms (Joerink et al, 2006;Wright et al, 2004).…”

“…This result could be explained by the fact that there are at least two different ARG genes described in mammals and in teleost fishes that arose from a duplication event (Joerink et al, 2006;Morris, 2002;Wright et al, 2004;Yu et al, 2001). Polyploid fish have undergone further duplications of the ARG genes, resulting in multiple copies of the ARG isoforms (Joerink et al, 2006;Wright et al, 2004). ARG I functions to detoxify ammonia by participating in the O-UC whereas ARG II is thought to play a role in ornithine production (Wright et al, 2004;Yu et al, 2001).…”

“…Polyploid fish have undergone further duplications of the ARG genes, resulting in multiple copies of the ARG isoforms (Joerink et al, 2006;Wright et al, 2004). ARG I functions to detoxify ammonia by participating in the O-UC whereas ARG II is thought to play a role in ornithine production (Wright et al, 2004;Yu et al, 2001). In fish, both ARG I and ARG II are found in the liver (Joerink et al, 2006;Wright et al, 2004); thus, total ARG enzyme activity can reflect either ARG I, ARG II or both isozymes (Morris, 2002).…”

“…ARG I functions to detoxify ammonia by participating in the O-UC whereas ARG II is thought to play a role in ornithine production (Wright et al, 2004;Yu et al, 2001). In fish, both ARG I and ARG II are found in the liver (Joerink et al, 2006;Wright et al, 2004); thus, total ARG enzyme activity can reflect either ARG I, ARG II or both isozymes (Morris, 2002). However, the predominant isozyme has been shown to differ among teleost fish, i.e.…”

“…However, the predominant isozyme has been shown to differ among teleost fish, i.e. in rainbow trout, the activity of ARG I makes up most of the total ARG enzyme activity whereas in carp, ARG II is the main isozyme (Joerink et al, 2006;Wright et al, 2004). In the toadfish, measurement of liver ARG mRNA expression was specific for ARG I but both ARG I and ARG II may have contributed to the measured ARG activity, which may partially explain why ARG mRNA levels are not correlated to ARG activity.…”