Oscillatory phenomena in chloroplasts from spinach cotyledons

Abstract: A suspensionofchloroplasts, isolated from spinach cotyledons, was found to be an oscillatory system, i.e. a system in which one parameter (or more) varies over time in a fairly regular way.The oscillations, with a period of fifteen to twenty minutes,occured spontaneously in the light as well as in the dark and were self-sustained for several hours.Catalase activity was the parameter on which the oscillations in the chi oroplastic system were first observed and was also the parameter most intensely studied. Some other parameters investigated were the amount of glutathione, oxidation-reduction potential and activities of glucose-6-phosphate dehydrogenase and NADP-1inked malate dehydrogenase.Oscillations could be induced in previously non-oscillating systems (e.g. in dialysed chloroplast extracts) by addition of ascorbate and glutathione. A mechanism is suggested, built upon a respiratory chain in which the ascorbate and glutathione redox couples are involved and which is known to occur in young plants. The mechanism involves cyclic changes in the redox state of sulphur groups (-SH^^-SS- transitions) in some enzymes, causing changes in the activities of the enzyme. The proposed feed-back mechanism thus implies covalent modification of enzymes. The enzymes involved were found to be localized mainly in the stroma space of the chloroplast.