The present study was aimed at determining which rat cytochrome P450 (CYP) isoforms are involved in the hydroxylation of tyramine to dopamine and at determining whether the reaction can take place in the brain. Moreover, we examined the relative distribution of the CYP2D subfamily's activity in the rat brain, focusing our attention on dopaminergic structures. The study was conducted with cDNA-expressed CYP isoforms (rat CYP1A1, 2A2, 2B1, 2C6/11/13, 2D1/2/4/18, 2E1, 3A2 and human CYP2D6) and with rat brain microsomes. Of the rat CYP isoforms tested, only CYP2D2, 2D4 and 2D18 (but not CYP2D1) were capable of forming dopamine from tyramine. The rat CYP2D isoforms were less efficient than the human CYP2D6 and the efficiency of both human and rat enzymes was higher for m-tyramine (K(m)=256, 143 and 87 microM; V(max)=0.47, 0.23 and 9.55 pmol/pmolCYP/min for CYP2D4, 2D18 and 2D6, respectively) than for p-tyramine (K(m)=433 and 688 microM, V(max)=0.12 and 0.19 pmol/pmolCYP/min for CYP2D4 and 2D18, respectively). Brain microsomes were able to metabolize tyramine to dopamine. The reaction was inhibited by the CYP2D inhibitor quinine and by anti-CYP2D4 antibodies, which suggests that CYP2D4 is the isoform governing tyramine hydroxylation to dopamine in the rat brain. A relatively high level of CYP2D activity (bufuralol 1'-hydroxylation) was found in the substantia nigra, the cerebellum, the nucleus accumbens and the truncus cerebri. The results are discussed in the context of the likelihood of CYP2D-mediated dopamine synthesis in vivo, the implications for Parkinson's disease and the addiction process.