1-Trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo) and related derivatives: chemistry and biochemical effects on catecholamine biosynthesis Original Research Article

1-Trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo, 2) is a mammalian alkaloid that readily originates in the human organism, by Pictet–Spengler condensation of endogenously present tryptamine (Ta) and the non-natural hypnotic agent trichloroacetaldehyde (chloral, Clo). Due to its structural analogy to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 1), TaClo is discussed to possibly contribute to the pathogenesis of Parkinsonʹs disease acting as an environmental toxin. Previous investigations on rats and neuronal cell cultures revealed 2 to be capable of inducing severe disturbances on the dopamine metabolism. In this paper, we report on the effects of 2 on the activity of tyrosine hydroxylase [l-tyrosine, tetrayhydropteridine/oxygen oxidoreductase (3-hydroxylating), EC 1.14,16.2; TH] in vitro using rat brain homogenates prepared from the TH-rich nucleus accumbens. TaClo (2) dose-dependently inhibited basal TH activity (IC50=3 μM), and after enzyme activation by pituitary adenylate cyclase-activating polypeptide (PACAP-27), it also reduced l-DOPA formation (IC50=15 μM). Moreover, two presumable TaClo metabolites, 2-methyl-TaClo (N-Me-TaClo, 3) and 1-dichloromethylene-1,2,3,4-tetrahydro-β-carboline (1-CCl2-THβC, 4), which were synthesized in good yields, also proved to be potent inhibitors of TH, with the strongest effect on basal activity (similar to TaClo) being observed for 3 (IC50=3 μM). In contrast to TaClo, however, 3 and 4 showed biphasic effects after TH activation with PACAP-27, inducing a marked increase of enzyme activity in the nanomolar range (<0.1 μM), while TH activity was nearly completely blocked at high concentrations (IC100=0.1 mm). An X-ray diffraction investigation on the 3-dimensional structure of the 1-CCl2-THβC-derived trifluoroacetamide 7 revealed the voluminous and quite rigid dichloromethylene substituent to be only moderately twisted out of the β-carboline ring ‘plane’, thus resulting in an inreased ring strain of the partially hydrogenated pyrido moiety accompanied by a strong steric hindrance of Cl(1), Cl(2), C(13), and N(2), which pushes the N-trifluoroacetyl group upwards to an even higher extent than for the TaClo-related trifluoroacetamide 8.