[en] Tryptophan (TRP), an essential amino acid, has been demonstrated to affect certain cellular processes including transcriptional and translational events in the liver. These events are presumed to be mediated at the nuclear level possibly via binding of TRP to nuclei. In an effort to delineate the role of TRP on these metabolic processes, the nuclear location of these binding sites was investigated. Incubation of isolated, intact, hepatic nuclei with [³H]TRP followed by fractionation revealed the presence of about 60% of specific TRP binding to nuclear membranes. This binding reached equilibrium by 2 hours after incubation at room temperature. Scatchard analysis revealed two classes of binding sites: (1) high affinity (K/sub D/ of about 10^-10M) and (2) low affinity (K/sub D/ of about 10^-8M). The inhibition of binding by treatment with either β-galactosidase or concanavalin A suggested that the binding entity was a glycoprotein. However, treatment with neuraminidase resulted in an increase in binding which suggested that terminal sialic acid residues play a role, possibly an inhibitory one, on TRP binding. The function of these binding sites on the mechanism of TRP action is being investigated