The endoplasmic reticulum-Golgi system is a major site of plastoquinone synthesis in spinach leaves.

Plastoquinone biosynthesis was recently found to be enriched in the microsomal fraction of spinach leaves. Since it is generally assumed that this lipid may predominantly be synthesized in the chloroplast envelope we have prepared total microsomes, containing both endoplasmic reticulum and Golgi membranes, and chloroplast envelopes. Marker enzymes and lipid content showed no significant cross-contamination. Nonaprenyl-2-methylquinol transferase, participating in plastoquinone synthesis, was found in the microsomes but was absent from the chloroplast envelopes and thylakoid membranes. Nonaprenyl-4-hydroxybenzoate transferase activity, an enzyme of ubiquinone biosynthetic system, in microsomes exceeded more than 10 times that found in chloroplast envelopes. The result indicates that in plants plastoquinone is synthesized in the endoplasmic reticulum-Golgi system and transported to the chloroplasts.