Breakdown of thylakoid pigments by soluble proteins of developing chloroplasts

Abstract

In the presence of Triton X-100 (TX-100) or imazalil, plastidic pigments were degraded by a soluble enzyme extracted from developing chloroplasts. This bleaching was not photochemical and required oxygen; it was not inhibited by superoxide dismutase or catalase, but was strongly inhibited by benzoquinone, quinol, phenazine methosulphate and, more weakly, by other reagents. Synthetic intermediates of chlorophyll biosynthesis, e.g. Mg(II)-protoporphyrin IX monomethyl ester, was also degraded. This reaction was compared with the bleaching catalysed by soybean (Glycine max) lipoxygenase. The plastidic system required TX-100 and was inhibited by unsaturated fatty acids, whereas lipoxygenase required a polyunsaturated fatty acid and was inhibited by TX-100. The bleaching capability of the stromal extract decreased with age if the seedlings were placed in the greenhouse to allow further development of the chloroplasts. A direct relationship was observed between the promotion of pigment bleaching by TX-100 and the inhibition of the in vitro synthesis of divinylprotochlorophyllide. This bleaching reaction is discussed on the basis of interference by TX-100 with the normal O2-requiring anabolic processes of developing chloroplasts.