Effect of pyridine homologues on the basal rate of electron transport and H+/e- ratio in chloroplasts.

Low concentrations of hydrophobic pyridine homologues (1 mM) were found to increase the rate of the Hill reaction in chloroplasts without significantly affecting either the steady-state proton uptake or the rate of proton leakage in the dark. By assuming that the organic base can be bound to two types of independent binding sites in the thylakoid membrane with dissociation constants K1 and K2 respectively, the kinetic data can be treated quantitatively. The values of K1 and K2 determined by the treatment are in the same relative order as the hydrophobicities of the pyridine homologues: K1 - 1.16 mM and K2 = 54 mM for pyridine; 0.6 and 38 mM for 4-picoline; 0.27 and 31 mM for 4-ethylpyridine, 0.10 and 4.2 mM for 4-t-butylpyridine; 0.08 and 3.2 mM for 4-n-butylpyridine. The rates of oxygen generation and proton uptake by illuminated chloroplasts with either ferricyanide or 1,4-benzoquinone as the electron acceptor were also measured in the presence of various pyridine homologues. Low concentration of pyridine homologues were found to decrease the H+/e- ratio. This last observation seems to substantiate an indirect coupling mechanism between electron transport and proton translocation.