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 live r. Earlier studies dealing with H-3-tryptophan binding to rat hepatic nuclei in vitro revealed that some amino acids, such as L-alanine, L-p henylalanine, and L-tyrosine, but not others, such as L-leucine, compe ted for such binding. This study evaluated the influence of L-leucine together with unlabeled L-tryptophan on H-3-tryptophan binding to hepa tic nuclei in vitro. Whereas L-leucine alone had little effect, L-leuc ine together with unlabeled L-tryptophan (excess, 10(-4) M) caused sig nificantly less inhibition of H-3-tryptophan binding to hepatic nuclei that did unlabeled L-tryptophan (10(-7) to 10(-4) M), when added toge ther with unlabeled L-tryptophan (10(-)7 to 10(-4) M), abrogated the i nhibition of binding because of the unlabeled L-tryptophan (10(-7) to 10(-4) M), abrogated the inhibition of binding because of the unlabele d L-tryptophan alone. Other in vitro H-3-tryptophan binding effects we re: 1) L-isoleucine and L-valine revealed binding effects similar to t hat with L-leucine; 2) the dipeptide, TRP.LEU, acted similarly (but so mewhat less) to that of L-leucine; and 3) L-alanine and sodium selenit e, each of which was inhibitory to H-3-tryptophan binding, showed less inhibition when L-leucine was also added. In vivo effects were invest igated after tube-feeding water, L-leucine or/and L-tryptophan to rats 10 min before killing and revealed the following results: 1) using is olated hepatic nuclei for in vitro H-3-tryptophan binding revealed les s 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 microsom es for in vitro C-14-leucine incorporation into protein revealed great er stimulation by L-tryptophan alone (+61%) than by L-tryptophan plus L-leucine (+8%) over control (water alone). Thus, the addition of L-le ucine appears to have an inhibitory effect on actions attributable to L-tryptophan alone. (C) Elsevier Science Inc. 1997.