Preillumination of spinach thylakoid membranes under strictly anaerobic conditions (i.e. in the presence of glucose oxidase) and in the absence of an electron acceptor inactivates specifically photosystem (PS) II. Inhibition can be complete within 3 min depending on the glucose oxidase concentration and light intensity. Artificial donor or acceptor systems do not restore PS II. Atrazine binding affinity is not diminished. No degradation of peptide subunits is observed. Trypsinized PS II preparations, in which the QB-binding site has been destroyed, can also be inactivated by light. It is concluded that photoinhibition of thylakoid membranes under anaerobic conditions inactivates the reaction center of PS II. This photoinactivation does not depend on the degradation of a peptide subunit. Not only the QB site but also a modified (trypsinized) QA site can induce photoinhibition.
