Inactivation of Oxygen Evolution by Low pH-Treatment of Two Rice Photosystem II Preparations Containing One and Two Bound Ca2+

Abstract

Ca²⁺ is considered to function as a cofactor of PS II electron transport [1]. Oxygen-evolving PS II membranes prepared with Triton X-100 from higher plants contain two Ca²⁺ per PS II [2–4.] except for wheat preparations which have three Ca²⁺ per PS II [2]. The two Ca²⁺ are heterogeneous in terms of their binding stability; one Ca²⁺ can be readily extracted by various treatments while leaving another bound to the PS II membranes [2–5]. Ono and Inoue [5] have shown that a brief treatment of spinach PS II membranes at pH 3.0 results in a strong in-activation of oxygen evolution with a concomitant extraction of one of the two bound Ca²⁺. Because the activity was substantially restored upon addition of Ca²⁺, they suggested that the acid-extractable Ca²⁺ is essential for PS II electron transport.