Role of Serum Free Fatty Acids in L-Tryptophan Transport into the Liver

Abstract

A large part of L-tryptophan (Trp) present in the plasma or serum is bound to albumin (McMenamy et al., 1957). It is known (Curzon et al., 1974; Kragh-Hansen, 1983) that this binding of Trp to albumin is affected by various substances such as free fatty acids (FFA). It has been shown (Madras et al., 1974; Curzon et al., 1972) that in the plasma of rats under feeding of a high fat diet, starvation, and immobilization, free Trp levels are enhnaced by an increase in FFA levels. Madras et al. (1974) have demonstrated that in rats fed a protein-car-bonhydrate diet with various amounts of fat, liver Trp content dose not correlate with an increase in free serum Trp levels consequent to increased serum FFA levels. Recently, we have reported (Sasaki et al, 1993) that in rats fed a 40% lard-containing diet, there are little changes in the disappearance of blood Trp and the content of liver Trp under a marked increase in free serum Trp levels, and have suggested that under physiological levels of serum Trp and albumin, increased serum FFA levels do not have a large influence on Trp transport into the liver. However, it is still unclear whether or not serum FFA plays a facilitating role in Trp transport into the liver of rats, although increased serum FFA levels seem to have little effect on Trp transport into the liver under physiological levels of serum Trp and albumin. We, therefore, attempted to further clarify the role of serum FFA in Trp transport into the liver using analbuminemic rats, which are the mutant strain established from Sprague-Dawley rats and are genetically lacking in plasma albumin (Nagase et al., 1979).