Effects of Optically Active 1-(α-Methylbenzyl)-3-(3,4-dichlorophenyl)urea on Reactions of Mitochondria and Chloroplasts

Effects of the R- and S-isomers and racemate of 1-(alpha-methylbenzyl)-3-(3,4-dichlorophenyl)urea (MBPU) were measured on phosphorylation and electron transport in mung bean (Phaseolus aureus L.) mitochondria and spinach (Spinacia oleracea L.) chloroplasts.In chloroplasts, S-MBPU inhibited basal and methylamine-uncoupled electron transport with ferricyanide as the oxidant, both photoreduction and coupled photophosphorylation with water as the electron donor and with ferricyanide and nicotinamide adenine dinucleotide phosphate (NADP) as oxidants, and cyclic photophosphorylation with phenazine methosulfate as the electron mediator under an argon gas phase. With ascorbate 2,6-dichloro-phenolindophenol as the electron donor, phosphorylation coupled to NADP reduction was inhibited, but the reduction of NADP was not inhibited. The R-isomer of MBPU, like the S-isomer, inhibited all of the photophosphorylation reactions studied. However, unlike the S-isomer, the R-isomer either did not inhibit or was a very weak inhibitor of all photoreduction reactions. The effects of the MBPUs on the chloroplast reactions can be explained by action at two different sites: an optically specific site near photosystem II and the oxygen evolution pathway, and a second optically nonspecific site associated with the generation of ATP.In mitochondria, both the R- and S-isomers stimulated state 4 respiration, inhibited state 3 respiration, and released oligomycin-inhibited respiration with malate, succinate, and NADH as substrates. Both enantiomers were equally active in all studies with malate and succinate as substrates. However, with NADH as substrate, R-MBPU was a stronger inhibitor of state 3 respiration and a weaker stimulator of state 4 respiration than S-MBPU.