Studies on Herbicide Binding in Photosystem II Membrane Fragments from Spinach

The mechanism of atrazine binding and its modification by Chelex-100-induced Ca2+ deple­ tion and proteolytic degradation by trypsin, was analyzed in PS II membrane fragments from spinach. It was found: 1) Chelex-100 treatment leads in a comparatively slow process (/, 2 = 5 — 10 min) to Ca2+ re­ moval from a site that is characterized by a high affinity as reflected by Ku values of the order of 10 7 M. The number of these binding sites was found to be almost one per PS II in samples washed twice with Ca2+-free buffer. 2) Chelex-100 treatment does not affect the affinity of atrazine binding but increases the susceptibility to proteolytic attack by trypsin. 3) The electron transport activity is only slightly affected by Chelex-100 treatment. 4) The atrazine binding exhibits a rather small T-dependence within the physiological range of 7 °C to 27 °C. The implications of these findings for herbicide binding are discussed.