Very briefly, the present state of knowledge on the latent, lumen orie
nted polyphenol oxidase (PPO) of the chloroplast is reviewed. The loca
tion of PPO in the thylakoid membrane was described by D. Arnon 46 yea
rs ago. The N-terminus sequence of the spinach enzyme is reported. A h
istorical sketch is given of the discovery of photophosphorylation and
Amen's visit to the admired O. Warburg.