A Study of Plastoquinones in Photochemical Reactions in the Chloroplasts of Euglena gracilis Strain Z.

Plastoquinone-9 (PQ-9) was isolated from the chloroplasts of Euglena gracilis Strain Z and spinach. The functional involvement and the structural specificity of PQ-9 in photochemical reactions was investigated in the isolated chloroplasts of Euglena gracilis. It was found that PQ-9 was required for both photoreduction of ferricyanide and photosynthetic phosphorylation in Euglena chloroplasts. The structural integrity of PQ-9 was not required to the same degree in the 2 photochemical reactions. Photosynthetic phosphorylation seemed to require the entire molecular structure of PQ-9 for the activity, whereas shortening in isoprenoid chain and modification of quinoid nucleus of PQ-9 do not seem to alter the photoreduction activity significantly. Addition of PQ-9 to the lyophilized Euglena chloroplasts inhibited the photoreduction of ferricyanide significantly, while it stimulated photosynthetic phosphorylation activity.