SOME UNIQUE CHARACTERISTICS OF THYLAKOID UNISITE ATPASE

Under unisite conditions (ratio of ATP to chloroplast coupling factor (CF0CF1), approximately 1:2.8), spinach thylakoid ATPase depends on pr ior reductive activation of CF1, just as multisite ATPase does, and is sensitive to removal of CF1, by EDTA, Faster rates in room light than in semidarkness and up to 80% inhibition by uncouplers only in room l ight indicate a strong effect of protonmotive force, which can be prov ided by room light. In addition, unisite ATPase is inhibited by azide as long as some ADP is bound to the CF1. Several differences were foun d between unisite and multisite ATPase, 1) The unisite activities of b oth membrane-bound and free enzyme were stimulated up to 3-fold by 4 m M free MgCl2 (a strong inhibitor of multisite ATPase). 2) Thylakoid un isite ATPase was inhibited by sulfite (50% inhibition at 5 mM), a powe rful activator of multisite ATPase. This inhibition is attributed to a nonspecific ionic strength effect. 3) Unisite ATPase was inhibited by trypsin treatment, which increases multisite ATPase severalfold. 4) T he pH profile of thylakoid unisite ATPase is somewhat different from t hat of mul tisite. 5) Alkylation of Cys-89 of the gamma subunit by N-e thylmaleimide did not affect the unisite activity, but inhibited multi site activity more than 90%.