Requirement for plastoquinone a in the hill reaction of isolated chloroplasts

In order to avoid the problem of an uncoupling effect in stimulation of potential Hill reaction activity during extraction with heptane. Hill reaction activity was measured with uncoupled spinach chloroplasts. The reactions were also run under aerobic conditions. Under these conditions a correction for a 3-(p-chlorophenyl)-1, 1-dimethylurea insensitive, photostimulated O₂ uptake reaction is necessary, especially with extracted chloroplasts. The reaction displayed proper Hill reaction stoichiometry in untreated and restored systems if these corrections were made. 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 plastoquinone A in the Hill reaction but the restoration of maximum activity depends upon additional factors. The variability of response to plastoquinone A which has been reported in various laboratories may be determined by the degree of extraction of the other materials.