FUNCTIONAL-EFFECTS OF CHEMICAL MODIFICATION OF UNSTACKED AND STACKED CHLOROPLASTS WITH GLUTARALDEHYDE

Although glutaraldehyde alkylates protein NH2 groups to the same extent in unstacked and stacked thylakoids, the photosynthetic electron transport of the stacked membranes is always more inhibited. Inhibition of photosystem II electron transport, measured in the presence of lipophilic Hill oxidants, is 20-30% in unstacked and 60-70% in stacked thylakoids. Photosystem I electron transport is nearly completely inhibited in both preparations, but in the case of stacked thylakoids maximal inhibition occurs at a lower glutaraldehyde level than in unstacked thylakoids. In contrast, the photooxidation of the reaction center chromophore of photosystem I (P700) is unaffected by the glutaraldehyde treatment of either stacked or unstacked chloroplasts. The results are discussed with regard to the accessibility of membrane sites to exogenous electron transport cofactors, in view of the observation that N-methylphenazonium methosulfate, a quencher of electronically excited chlorophyll a, partitions more easily into the pigment domains of the glutaraldehyde-fixed unstacked thylakoids. © 1979.