Acid-base induced redox changes of the chloroplast cytochrome b-559

Abstract

At least 80-90% of the cytochrome b-559 in dark adapted, well coupled spinach chloroplasts has been found to be in a high potential, hydroquinone-reducible state [ f ,2] , whose midpoint is approximately +350 mV [3-61. It has been shown that illumination with actinic light preferentially absorbed by photosystem I can ini duce oxidation of cytochrome b-559 in the presence of either 4-trifluoromethoxyphenylhydrazone [ 1,7,8] or low concentrations of N-methyl phenazonium methosulphate [9], and after pre-illumination with high intensity red light [2]. The inhibition of photooxidation by the plastoquinone antagonist 2,5-dibromo-3-methyl6-isopropyl-p-benzoquinone (DBMIB) in each case implies that a low potential cytochrome b-559 species is being oxidised [ 1,2,8,9] . The very positive dark midpoint potential of cytochrome b-559 is very unusual for a b cytochrome. Because of the known lability of the high potential &ate, it would appear to be the consequence of a positive ionic environment in the neighborhood of the heme, created by an organised membrane structure. The mechanism of a reversible light-induced decrease in cytochrome b-559 potential may involve a change in the charge environment in-the membrane near the heme. ( See [lo] for a detailed discussion). During illumination a large proton flux occurs into or across the thylakoid membrane [ 1 l] . The possibility arises that local membrane pH changes occurring during illumination are responsible for the decrease in midpoint potential of cytochrome b-559. In this paper dark, acid-base induced redox changes of cytochrome b-559 are described, which can be interpreted in terms of a reversible decrease in midpoint potential at low