Preferential inhibition of the lycopene ε-cyclase by the substituted triethylamine compound MPTA in higher plants

In addition to the usual complement of carotenoids found in the plant leaf tissues, lettuce (Lactuca sativa), unusually, possesses large amounts of the diol lactucaxanthin. This carotenoid possesses two epsilon-end-groups and its presence provides a good model in which to study the effects of the substituted triethytamine compound 2-(4-methylphenoxy)triethytamine (MPTA) on the cyclisation of beta- and epsilon-end-groups during the biosynthesis of carotenoids. Treatment with 10 or 20 mu M MPTA significantly reduced levels of both beta-carotene and neoxanthin (up to 18-fold), whilst levels of violaxanthin and lutein were less affected (4-fold reduction). In contrast, levels of lactucaxanthin were not reduced even at the highest inhibitor concentration, and at 10 mu M MPTA levels of this xanthophyll doubled. The pigment stoichiometry of the bulk light-harvesting complex (LHCIIb) isolated from treated plants shows that tactucaxanthin successfully substituted for lutein and neoxanthin in two of the xanthophyll binding sites, namely L2 and N1. Inhibition of cyclisation was accompanied by the accumulation of lycopene and trace amounts of delta-carotene and a number of oxygenated derivatives of these precursors. Two forms of mono-hydroxy Lycopene were identified together with mono-epoxy delta-carotene. (c) 2005 Elsevier GrnbH. All rights reserved.