Activity and functional interaction of alternative oxidase and uncoupling protein in mitochondria from tomato fruit

Cyanide-resistant alternative oxidase (AOX) is not limited to plant mitocho ndria and is widespread among several types of protists, The uncoupling pro tein (UCP) is much more widespread than previously believed, not only in ti ssues of higher animals but also in plants and in an amoeboid protozoan. Th e redox energy-dissipating pathway (AOX) and the proton electrochemical gra dient energy-dissipating pathway (UCP) lead to the same final effect, i.e., a decrease in ATP synthesis and an increase in heat production. Studies wi th green tomato fruit mitochondria show that both proteins are present simu ltaneously in the membrane. This raises the question of a specific physiolo gical role for each energy-dissipating system and of a possible functional connection between them (shared regulation). Linoleic acid, an abundant fre e fatty acid in plants which activates UCP, strongly inhibits cyanide-resis tant respiration mediated by AOX. Moreover, studies of the evolution of AOX and UCP protein expression and of their activities during post-harvest rip ening of tomato fruit show that AOX and plant UCP work sequentially: AOX ac tivity decreases in early post-growing stages and UCP activity is decreased in late ripening stages. Electron partitioning between the alternative oxi dase and the cytochrome pathway as well as H+ gradient partitioning between ATP synthase and UCP can be evaluated by the ADP/O method. This method fac ilitates description of the kinetics of energy-dissipating pathways and of ATP synthase when state 3 respiration is decreased by limitation of oxidiza ble substrate.