Influence of L-leucine on L-tryptophan binding to rat hepatic nuclei

This study was concerned with the effect of L-leucine on L-tryptophan's actions (nuclear receptor binding and protein synthesis) on rat liver. Earlier studies dealing with 3H-tryptophan binding to rat hepatic nuclei in vitro revealed that some amino acids, such as L-alanine, L-phenylalanine, and L-tyrosine, but not others, such as L-leucine, competed for such binding. This study evaluated the influence of L-leucine together with unlabeled L-tryptophan on 3H-tryptophan binding to hepatic nuclei in vitro. Whereas L-leucine alone had little effect, L-leucine together with unlabeled L-tryptophan (excess, 10(-4)M) caused significantly less inhibition of 3H-tryptophan binding to hepatic nuclei than did unlabeled L-tryptophan alone (9% vs. 68% inhibition, respectively). L-leucine (10(-10) to 10(-4) M), when added together with unlabeled L-tryptophan (10(-7) to 10(-4) M), abrogated the inhibition of binding because of the unlabeled L-tryptophan alone. Other in vitro 3H-tryptophan binding effects were: 1) L-isoleucine and L-valine revealed binding effects similar to that with L-leucine; 2) the dipeptide, TRP.LEU, acted similarly (but somewhat less) to that of L-leucine; and 3) L-alanine and sodium selenite, each of which was inhibitory to 3H-tryptophan binding, showed less inhibition when L-leucine was also added. In vivo effects were investigated after tube-feeding water, L-leucine or/and L-tryptophan to rats 10 min before killing and revealed the following results: 1) using isolated hepatic nuclei for in vitro 3H-tryptophan binding revealed less specific binding to nuclei of rats tube-fed L-tryptophan alone than to nuclei of those tube-fed L-tryptophan plus L-leucine, suggesting an in vivo inhibitory effect of L-leucine; and 2) using hepatic microsomes for in vitro 14C-leucine incorporation into protein revealed greater stimulation by L-tryptophan alone (+61%) than by L-tryptophan plus L-leucine (+8%) over control (water alone). Thus, the addition of L-leucine appears to have an inhibitory effect on actions attributable to L-tryptophan alone.