Chlorophyllase and Photsystem

Abstract

Preliminary experiments showed that Triton X‐100 at low concentrations (appr. 0.01%) induces reversible changes in some photosynthetic reactions in Phaeodactylum cells without influencing their absorption spectrum; presumably membrane permeability is effects. Although DC PIP can pass through the cell membrane in the presence of 0.01% Triton X‐100, only a very low PS I activity, measured as MV photoreduction by DCPIP‐ascorbate, is observed with whole cells. MV photoreduction sensitized by membrane fragments isolated from Phaetdactylum, Euglena, Porphyridium and Synechococcus after French Press treatment is much higher with Phaeodactylum than with the other organisms. Spinach shroma membrane fragments show higher photosensitizing activity than grana fragments. MV photoreduction in the above mentioned experiments is not influenced by DCMU (2 × 10⁻⁶M) or sodium azide (0.01–0.05 M); KCN (4 × 10⁻³M) has an inhibiting effect only with the blue‐green alga Synechococcus. The reaction mechanism of chlorophyll‐sensitized MV photoreduction is discussed. Chlorophyll in an aqueous medium containing Triton X‐100 (JsO.01%) sensitizes the photoreduction of MV by DCPIP ascorbate. A similar reaction is observed with chlorophyll combined with solubilized Phaeocdactylum chlorophyllase: in the latter case the presence of both MgCl₂ and DTT is required. MV‐photoreduction is concluded to be a very unreliable procedure for determining PS I activity in membranes if these membranes have been prepared in (the presence of detergents. The results support the hypothesis that (prochlorophyllase is a PS I protein. The capacity of PS I to reduce MV in vivo is discussed.