Relationship between the level of serum L-tryptophan and its hepatic uptake and metabolism in rats with carbontetrachloride-induced liver cirrhosis

In the serum of rats with liver cirrhosis induced by 12-week intermittent carbon tetrachloride (CCl₄,) injection, free L-tryptophan (Trp) levels increased with decreases in total Trp, albumin-bound Trp, and albumin levels. In the serum of the cirrhotic rats, there were no changes in the ratio of albumin-bound Trp to albumin and the level of free fatty acids which are known to weaken the binding of Trp to albumin. In the liver of the cirrhotic rats, there were increases in protein and free Trp (i.e., non-protein Trp) contents and a decrease in total tryptophan 2,3-dioxygenase (TDO) activity. The decreased TDO activity was mainly due to the reduction of apo-TDO activity. When [³H]Trp was injected into the portal vein of the cirrhotic and control rats, radioactivity derived from the injected [³H]Trp in the liver was higher in the cirrhotic rats than in the control rats at 10 min after the injection, while the radioactivity in the serum was lower in the former rats than in the latter rats. These results indicate that the increased Trp is easily taken up into the cirrhotic liver, and suggest that the Trp taken up into the cirrhotic liver could be utilized for the maintenance of synthesis of proteins in the tissue through the reduction of Trp metabolism due to reduced TDO activity in the tissue.