ALL‐trans β‐CAROTENE‐5,6‐EPOXIDE IN THYLAKOID MEMBRANES

Abstract

All‐transβ‐carotene‐5,6‐epoxide has been found in the thylakoid membranes of spinach and of the cyanobacterium Synechococcus vulcanus Copeland. The epoxide was extracted from the thylakoid membranes with acetone, and was isolated by high‐performance liquid chromatography (HPLC). The structure of the epoxide was identified by means of mass, Raman, and electronic absorption spectroscopy. Changes in the amount of the epoxide, as a result of epoxidation and (apparent) de‐epoxidation reactions in the membranes, were traced by analysis of extracts on HPLC. In isolated thylakoid membranes, only the epoxidation reaction took place. The reaction was caused by irradiation or by the addition of ferricyanide, suggesting that electron transport reactions in the membranes are involved in the epoxidation. In intact spinach leaves, however, both epoxidation and de‐epoxidation took place; the extent of epoxidation correlated with the intensity of light incident on the leaves. The epoxidation and de‐epoxidation of all‐transβ‐carotene are contrasted with those of xanthophylls (in the violaxanthin cycle).

Photodynamics of the Bacteriochlorophyll-Carotenoid System. 1. Bacteriochlorophyll-photosensitized Oxygenation of β-Carotene in Acetone¶

“…The fragmentation pattern (data not shown) further supports this assignment. Moreover, the same compound was found among Car oxidation products in vivo (47) and appears in chlorophyll-sensitized Car oxidation in vitro as one of the main products (27).…”

“…The chlorophyll-sensitized oxidation reactions of Car may have a biogenetic/evolutionary relevance to in vivo systems such as the enzymatic formation of ␤-carotene-5,6-epoxide, observed in leaves under high light (47,49) and more generally to the formation of epoxycarotenoids leading to the evolution of the xanthophyll cycle (8). If epoxides are indeed the early products, one might then even speculate about the recycling of such products outside the xanthophyll cycle.…”

Lycopene cyclase paralog CruP protects against reactive oxygen species in oxygenic photosynthetic organisms

“…7), a condition known to up-regulate At cruP transcripts (Table 1). β-carotene-5,6-epoxide has been identified in intact and isolated thylakoid membranes of spinach and T. elongatus (21). The level of β-carotene-5,6-epoxide in thylakoid membranes of spinach increased in proportion to light intensity.…”

“…5E) suggested that this compound had fewer conjugated double bonds than β-carotene. A study of the literature for rare carotenoids found in plants provided many possibilities (21)(22)(23)(24), but none seemed more likely than β-carotene-5,6-epoxide, a carotenoid that has previously been observed in photosynthetic tissues (21). β-carotene-5,6-epoxide was synthesized (25) and subjected to HPLC analysis, where it eluted at the same time and with the same spectrum as the unknown carotenoid ( Fig.…”

“…Considering published reports of β-carotene-5,6-epoxide formation via reactive oxygen species (ROS)-mediated degradation of β-carotene in photosynthetic tissues exposed to high light stress (21), together with our own observations of β-carotene-5,6-epoxide accumulation in At cruP KO plants and up-regulation of cruP in ROS-producing conditions (cold stress and dark anoxic treatment), we considered that CruP may play a role in reducing the accumulation of ROS. Cotyledons from Columbia (WT), At cruP KO, and 35S: Zm cruP lines exposed to anoxic conditions for 1 wk were stained with nitrotetrazolium blue (NTB) to screen for levels of ROS (Fig.…”