The metabolism of PQ by MAO-A showed a marked enantioselectivity, with (−)-PQ being depleted four times faster than (+)-PQ (Fig. 2b). This depletion was not significantly affected by CQ or QN with any of the substrates until 1 h, after which a very modest inhibition was observed (Fig. 5a–c). The only major metabolite from MAO-dependent metabolism was the alcohol formed by oxidative deamination of the terminal amine. It is formed twice as much from (−)-PQ compared to (+)-PQ. By 2 h, CQ and QN caused about 60 % inhibition of the formation of PQ alcohol with both PQ enantiomers (Fig. 5d–f).