The effect of isoprene on the properties of spinach thylakoids and phosphatidylcholine liposomes

Isoprene is emitted from the leaves of some plants. It was recently reported that exogenous isoprene delays the onset of leaf damage during controlled increases in leaf temperature (Singsaas et al. Plant Physiology 115: 1413-1420 [1997]). Thylakoid membranes are presumed to be the site of action based upon isoprene's hydrophobicity, production in chloroplasts, and effect upon chlorophyll fluorescence at high temperatures. In an attempt to discern the mechanistic basis for isoprene's thermoprotective role, we studied the effect of exogenous isoprene on the peroxidation, permeability, and stability of spinach thylakoids and phosphatidylcholine liposomes. Isoprene, supplied at either 18 or 21 μL L^-1, had no effect upon the rate of liposome peroxidation in the presence of a hydroxyl radical-generating system. Isoprene also did not affect liposome peroxidation at high temperatures. Neither the proton permeability of thylakoids nor the leakage of a fluorescent probe from liposomes was influenced by exogenous isoprene, when measured at several temperatures. Isoprene did not affect the stability of thylakoid membrane proteins during a temperature increase, as shown by differential scanning calorimetry. Therefore, despite the use of a variety of techniques to investigate fundamental membrane parameters, we were unable to demonstrate an effect of isoprene.