Discovery of plastoquinones: a personal perspective

The discovery and the rediscovery of plastoquinone (PQ) are described together with the definition of its structure as a 2,3-dimethyl 5 solanosyl benzoquinone. The discovery, by M. Kofler, was a result of a search for Vitamin K. Its rediscovery was made by me, when I was at The Enzyme Institute of the University of Wisconsin, analyzing animals and plants for the newly discovered coenzyme Q. In green plants, I found another lipophilic quinone in addition to coenzyme Q. Some misleading evidence suggested as if the new quinone had coenzyme Q activity in mitochondria, but improved methods gave negative results. When I found that the quinone was concentrated in chloroplasts, I considered a role for it in photosynthesis analogous to the role of coenzyme Q in mitochondria. After moving to the Chemistry Department, University of Texas at Austin, I used a plain light bulb and some spinach chloroplasts to show that PQ could be involved in photosynthetic redox reactions. This effect was supported by Norman Bishop's restoration of chloroplast electron transport after solvent extraction, with PQ and photoreduction studies by E. R. Redfern and J. Friend in R. A. Morton's laboratory in Liverpool, UK. We also found an additional analog of PQ in addition to a second analog found in Wisconsin. We called the new analogs PQB and PQC. Although we found some restoration effects with PQC, the discovery by W. T. Griffiths in Morton's laboratory, that PQB and PQC consisted of six forms of PQ each, made it more likely that the new analogs were breakdown products. Morton's group established the structure of the PQCs as a series of PQs, with a hydroxyl group on the prenyl side chain, and the PQB series as having fatty acids esterified to the hydroxyl groups of PQC. Possible functions of the analogs are also discussed in this article.