Plastoquinone A in the Hill Reaction of Isolated Chloroplasts'

SimnlIrv(11'. 1lii order to avoid the l)roblelll of ani uncouplinig effect in stimiiulatioln of potential Hill reaction activity durinig extractioln with heptane, Hill reaction activity was measured with uncouple(d spinach chloroplasts. TI'he reactions were also runi ulnder aerobic coniditionis. Under these conditions a correctioni for a 3(p-chloroph'enyl) -1. 1-dimethylurea insensitive, photostimiiulated O. uptake reaction is necessary, especially wvith extracted chlori oplasts. The reaction displayed l)roper Hill reaction stoiohionmetrv in untreate(d and restored systems if these corrections were mlade. Plastoquinone A always restored some activity to the extracted chloroplasts but never restored activity to the original level. Additional activity could be restored by the crude extract. It appears that there is a clear requirement for p)lastoquinone A in the Hill reaction l)ut the restoratioln of maximuimi activity dlepelnds upon additional factors. The variability of response to plastoquinone A which has been reported in various lalboratories may be determined by the degree of extraction of the other mlaterials.