Quantitation of photosystem II in spinach chloroplasts

Abstract

The concentration of photochemical reaction centers in spinach chloroplasts was measured with direct spectrophotometric techniques. Quantitation of Photosystem II (PS II) was implemented (a) from the light-induced absorbance change in the near-ultraviolet ( ΔA 320), monitoring the primary quinone acceptor Q A of PS II; and (b) from the light-induced absorbance change in the green region ( ΔA 540–550), monitoring the band-shift C550 of the PS II reaction center pheophytin molecule. Both measurements yielded identical chlorophyll (Chl) per PS II ratios ( Chl PS II = 380:1 ). Hydroxylamine treatment and the loss of Mn from the oxygen-evolving enzyme of photosynthesis had no effect on the quantitation of Q A by ΔA 320. It is concluded that, under the experimental conditions employed, measurements of Q A photoreduction are not overlapped by state transitions of the water-splitting enzyme. Quantitation of Photosystem I (PS I) was implemented from the light-induced absorbance change in the red region of the spectrum ( ΔA 700), monitoring photooxidation of the PS I reaction center P-700. A Chl PS I = 650:1 was derived. Thus, a PS II PS I stoichiometric ratio of 1.7 was estimated in spinach chloroplasts.