Fatty Acid Oxidation and Chlorophyll Bleaching

Lipoxygenase (linoleate: oxygen oxidoreductase ; E.C. 1.13. 1.13: also called lipoxidase) catalyzes the oxidation of certain polyunsaturated fatty acids with cis, cis-1,4-pentadiene unit such as linoleic and linolenic acids. Fatty acids are oxidized to their respective conjugated diene hydroperoxides in the presence of molecular oxygen. Lipoxygenase activity has been reported in many higher plants, mainly leguminous seed, cereal grains, and oil seeds. Potato tubers and leaves of some plants were also found to be active. Recently, Zimmerman and Vick observed the activity of lipoxygenase in Chlorella pyrenoidosa when cells were homogenized under anaerobic conditions. The bleaching of carotene in the presence of polyunsaturated fatty acids in vitro has also been known for many years. While Sumner and Sumner reported that lipoxygenase and "carotene oxidase" were identical, Kies et al. observed that homogeneous crystalline lipoxygenase of soybean was essentially ineffective in bleaching carotene. Presence of a heat-sensitive entity which was distinct from Theorell enzyme was required for bleaching carotene under their e xperimental conditions. In 1965, Holden reported the enzymatic bleaching of Chl in the presence of long chain fatty acids with leguminous seed extracts. She confirmed that Chl bleaching was coupled with a chain reaction involving peroxidation of fatty acid by lipoxygenase and subsequent breakdown of hydroperoxide by a heat-labile factor. Later on, a new enzyme which catalyzes the isomerization of hydroperoxides formed by lipoxygenase was isolated from flaxseed by Zimmerman and Vick, and named as hydroperoxide isomerase. Since Chl was not bleached when this enzyme was inactivated by heating, they proposed that the heat-labile factor described by Holden corresponded to hydroperoxide isomerase. Little is known about the mechanism of Chl degradation in vivo during senescence. The present study was initiated regarding the lipoxygenase-catalyzed bleaching of Chl as one of the model system of Chl degradation. A possible involvement of lipoxygenase-linoleate system in the Chl degradation in vivo is presumable from the following findings: (i) During senescence, a rapid decline in some complex lipid levels, particularly those characteristic of chloroplasts, is observed. (ii) Chloroplasts are rich source of unsaturated fatty acids, especia lly linolenic acid. (iii) The 1st step of disappearance of cellular lipid during senescence might involve the degradation of endogenous lipid by lipase to yield free fatty acids and water soluble components, and the former are broken down catabolically to CO_2. Possible involvement of lipoxygenase in this catabolism is supposed by the observation that linolenic acid levels fall much faster than those of other acids during senescence. Preliminary experiments were attempted to determine the involvement of any factors such as hydroperoxide isomerase besides lipoxygenase in the bleaching of Chl and some attempts were made to cause senescence in wheat and barley leaves and the changes in their lipoxygenase activity were measured