Influence of Unsaturated Fatty Acids in Chloroplasts

Abstract

Linolenic acid (C_18:3) is the main endogenous unsaturated fatty acid of thylakoid membrane lipids, and seems in its free form to exert significant effects on the structure and function of photosynthetic membranes. In this investigation the effect of linolenic acid was studied at various pH values on the electron flow rate in isolated spinach chloroplasts and related to ΔpH, the proton pump and the pH of the inner thylakoid space (pH_i). The ΔpH and pHi were estimated from the extent of the fluorescence quenching of 9‐aminoacridine. Linolenic acid caused a shift (approximately one unit) of the pH optimum for electron flow toward acidity in the following systems: (a) photosystems II + I (from H₂O to NADP⁺ or to 2,6‐dichlorophenolindophenol) coupled or non‐coupled; (b) photosystem II (from H₂0 to 2,6‐dichloro‐phenolindophenol in the presence of dibromothymoquinone). In photosystem I conditions (phenazine methosulphate), the ΔpH of the control increased as a function of external pH_o with a maximum around pH 8.8. When linolenic acid was added, the ΔpH dropped, but its optimum was shifted toward more acidic pH_o. The same phenomena were also observed in photosystems II + I (from H₂O to ferricyanide) and in photosystem II conditions (from H₂O to ferricyanide in the presence of dibromothymoquinone). However, the ΔpH was smaller and the sensitivity of the proton gradient toward linolenic acid was eventually higher than for photosystem I electron flow activity. The proton pump which might be considered as a measure of the internal buffering capacity of thylakoids was optimum at pH_o 6.7 in the controls. An addition of linolenic acid diminished the proton pump and shifted its optimum toward higher pH_o. As a consequence, pH_i increased when pH_o was raised. At the optimal pH_o 8.6 to 9, pH_i were 5 to 5.5. Additions of increasing concentrations of linolenic acid displaced the curves toward higher pH_i. A decrease of pH_o was therefore required to maintain the pH_i in the range of 5 ‐ 5.5 for maximum electron flow. In conclusion, the electron flow activity seems to be delicately controlled by the proton pump (buffer capacity), ΔpH, pH_i and pH_o. Fatty acids damage the membrane integrity in such a way that the subtle equilibrium between the factors is disturbed.