OCCURRENCE OF THE CAROTENOID LACTUCAXANTHIN IN HIGHER-PLANT LHC-II

The pigment composition of the light-harvesting complexes of Photosystem II (LHC II) has been determined for lettuce (Lactuca sativa). In common with other members of the composite, the photosynthetic tissues of this species may contain large amounts of the carotenoid lactucaxanthin (epsilon, epsilon-carotene-3,3'-diol) in addition to their normal compliment of carotenoids. The occurrence and distribution of lactucaxanthin in LHC II has been examined using isoelectric focusing of BBY particles followed by reversed-phase HPLC analysis of the pigments. The major carotenoids detected in LHC IIb, LHC IIa (CP29) and LHC IIc (CP26) purified from dark-adapted lettuce were lutein, violaxanthin, neoxanthin and lactucaxanthin. Lactucaxanthin has been shown to be a major component of PS II, accounting for similar to 26% of total xanthophyll in both LHC IIb (similar to 23% total xanthophyll) and in the minor complexes (12-16%). In this study, LHC IIb was clearly resolved into four bands and their carotenoid composition determined. These four bands proved to be very similar in their pigment content and composition, although the relative amounts of neoxanthin and lutein in particular were found to increase from bands 1 to 4 (i.e. with increasing electrophoretic mobility). The operation of the xanthophyll cycle has also been examined in the LHC of L. sativa following light treatment. The conversion efficiency for violaxanthin-->zeaxanthin was nearly identical for each light-harvesting complex examined at 58-61%. Nearly half of the zeaxanthin formed in PS II was associated with LHC IIb, although the molar ratio of zeaxanthin:chlorophyll a was highest in the minor LHC.