Alkaloids isolated from Hortia superba (Rutaceae) interact with spinach thylakoids inhibiting the electron transport chain

Flindersine (1) and N-methyl-flindersine (2) were isolated from Hortia superba. Both compounds inhibited the synthesis of ATP and non-cyclic electron transport, i.e. they behaved as Hill reaction inhibitors. Both alkaloids inhibited electron flow through PSII. The effect of the alkaloids on the partial PSII reactions was measured as under: from water to oxidized DCPIP, water to sodium silicomolybdate and from reduced DPC to oxidized DCPIP. The results demonstrated that their inhibition site was at Q(B), since the ectron flow from water to Pheophytin was not affected. Furthermore, alkaloid 2 had another site of inhibition located at PQH(2) oxidation site, the b(6)f complex. The polarographic results were corroborated by Chl a fluorescence measurements; in thylakoids, the alkaloids changed the shape of the Kautsky curve. The OJIP test indicated that the behavior of alkaloids 1 and 2 was similar to DCMU. In addition, alkaloid 2 was less active than 1. The main inhibition site of 2 was located at the PQH(2) site of oxidation within the b(6)f complex.