AJP - Gastrointestinal and Liver Physiology, Vol 251, Issue 1 103-G110, Copyright © 1986 by American Physiological Society
Development of arginine-synthesizing enzymes in mouse intestine
R. Hurwitz and N. Kretchmer
The urea biosynthetic pathway functions in mammalian liver to convert
excess ammonia to urea and to maintain the concentration of ammonia in
blood at nontoxic levels. This action is accomplished by enzymatic
adaptation to quantitative changes in dietary protein. The first two
enzymes of the pathway are found in the intestine of the adult mouse, but
they do not adapt to dietary change. The enzymes in the intestine produce
citrulline, which is carried by the bloodstream to the kidney, where it is
converted by the next two enzymes of the pathway to arginine. This
mechanism serves as the major source of circulating arginine. We have
demonstrated that, at birth, the arginine-synthesizing enzymes in the
kidney of the C57Bl/6 mouse are minimally developed, whereas in the
intestine activity of carbamoyl-phosphate synthase is elevated and
argininosuccinate synthase and lyase, usually present only in trace
quantities in the adult intestine, are markedly increased in the newborn.
The arginine formed cannot be converted to urea, since arginase does not
appear in intestinal cells of the mouse until the age of 15 days. Except
for liver, intestine has the most rapid protein turnover of any normal
tissue. Our study indicates that, at a time when no other endogenous source
of arginine for protein synthesis is available, the intestine of the
newborn C57Bl mouse is capable of synthesizing arginine from either
citrulline or NH3 and CO2.
