[3H]norharman ([3H]beta-carboline) binds reversibly and with high affinity to a specific binding site in rat liver.

In addition to the known binding of norharman (NH) to monoamine oxidase (MAO) and benzodiazepine (BZ) binding sites (at microM concentrations), a distinct class of high-affinity NH binding sites was discovered in rat brain. Investigations of several organs of the rat led to the discovery of high affinity binding sites in the liver, which successfully could be solubilized from P2 membrane homogenate (0.25% w/v Triton X-100). Scatchard analysis revealed an apparent KD value of 26 +/- 8 nM and a maximum number of binding sites of 11 +/- 3 pmol/mg protein (n = 14). Association kinetics showed that equilibrium was nearly reached after two hours. Dissociation was totally complete only after more than 16 hours. The MAO-inhibitors examined did not influence the binding characteristics. No displacement of specific binding could be found by haloperidol.