Chlorophyll degradation by free radicals derived from linolenic acid in incubated barley thylakoids

Using chloroplasts isolated from the senescent primary leaves of barley seedlings, the linolenic acid-dependent degradation of chlorophyll was studied in incubated thylakoids. This reaction, measured by the decrease of absorbance at 672 nm, was produced by free radicals from linolenic acid oxidation and, thus, was inhibited by M-propyl-gallale and a-tocopherol, general scavengers of free radicals. Although the beginning of the reaction was delayed by heating the thylakoids, this process must be of a non-enzymatic nature since it is not impeded by thermal treatment and it proved impossible to isolate a crude extract from thylakoids with a degradative enzymatic activity using a mixture of solubilized chlorophylls as substrate. The enzyme chlorophyll oxidase, described by others for thylakoids, was not found. In addition, the reaction of chlorophyll with the peroxidation products of linolenic acid seems to occur with the cooxidation of one or more carotenoids. Due to the high content of esterified linolenic acid in the thylakoid galactolipids, the degradation of chlorophyll studied here may have a physiological significance.