Inhibition of electron transport on the oxygen-evolving side of photosystem II by an antiserum to a polypeptide isolated from the thylakoid membrane

A polypeptide fraction with the apparent molecular weight 11 000 was isolated from stroma-freed chloroplasts from Anthirrhinum majus. An antiserum to this polypeptide fraction inhibits photosynthetic electron transport in chloroplasts from Nicotiana tabacum. The relative degree of inhibition is pH dependent and has its maximum at pH 7.4. The maximal inhibition observed was 93%. The dependence of the inhibition on the amount of antiserum yields a sigmoidal curve which hints at a cooperative effect. A calculation of the Hill interaction coefficient gave the value of 10. The inhibition occurs on the water splitting side of photosystem II between the sites of electron donation of tetramethyl benzidine and diphenylcarbazide. Tetramethyl benzidine donates its electrons before the site where diphenylcarbazide feeds in its electrons. Analysis of the steady state level of the variable fluorescence also indicates that the inhibition site is on the water splitting side of photosystem II. Tris-washed chloroplasts are equally inhibited by the antiserum and the inhibition is also observed in the presence of an inhibitor of photophosphorylation like dicyclohexyl carbodiimide and in the presence of the uncoupler carbonylcyanide m-chlorophenyl hydrazone (CCCP) which means that the inhibitory action is directed towards the electron transport chain. Valinomycin which is supposed to affect the cation permeability of the thylakoid membrane has no influence on the inhibitory action of the antiserum. The same is valid for gramicidin. Methylamine on the other hand can induce a state in the thylakoids in which the antiserum is not effective. If the antibodies are already adsorbed prior to the methylamine addition then the high inhibitory effect by the antiserum remains unchanged upon addition of methylamine. From the experiments it follows that a component from the vicinity of photosystem II is accessible to antibodies that is, the component is located in the outer surface of the thylakoid membrane. It appears that the inhibitory effect is produced in the course of the light reaction.

Electron microscopic and kinetic studies dealing with an artificial enzyme membrane. Application to a cytochemical model with the horseradish peroxidase-3,3'-diaminobenzidine system

A new colorimetric method for measuring horseradish peroxidase (HRP) activity, in solution, with 3,3'-diaminobenzidine (DAB) as the hydrogen donor is presented. The same method was also used to test HRP activity when immobilized in an artificial proteic membrane. The reaction was shown to be of first order in both cases. Direct spectrophotometric measurements dealing with HRP membrane show that the total amount of insoluble polymer produced does not increase linearly with the quantity of enzyme introduced in the membrane. Moreover, a study of the DAB permeability through membranes with or without peroxidase activity was performed to allow the study of diffusion limitations. Membranes have also been studied by electron microscopy. We have especially visualized the existence of product concentration profiles inside the membrane. The results show that in the membrane there is no geometrical similarity between the distribution profiles of the enzyme and of the insoluble product.