Activation and Function of Mitochondrial Uncoupling Protein in Plants

Abstract

Plant mitochondrial uncoupling protein (UCP) is activated by superoxide suggesting that it may function to minimize mitochondrial reactive oxygen species (ROS) formation. However, the precise mechanism of superoxide activation and the exact function of UCP in plants are not known. We demonstrate that 4-hydroxy-2-nonenal (HNE), a product of lipid peroxidation, and a structurally related compound, trans-retinal, stimulate a proton conductance in potato mitochondria that is inhibitable by GTP (a characteristic of UCP). Proof that the effects of HNE and trans-retinal are mediated by UCP is provided by examination of proton conductance in transgenic plants overexpressing UCP. These experiments demonstrate that the mechanism of activation of UCP is conserved between animals and plants and imply a conservation of function. Mitochondria from transgenic plants overexpressing UCP were further studied to provide insight into function. Experimental conditions were designed to mimic a bioenergetic state that might be found in vivo (mitochondria were supplied with pyruvate as well as tricarboxylic cycle acids at in vivo cytosolic concentrations and an exogenous ATP sink was established). Under such conditions, an increase in UCP protein content resulted in a modest but significant decrease in the rate of superoxide production. In addition, 13 C-labeling experiments revealed an increase in the conversion of pyruvate to citrate as a result of increased UCP protein content. These results demonstrate that under simulated in vivo conditions, UCP is active and suggest that UCP may influence not only mitochondrial ROS production but also tricarboxylic acid cycle flux.

Hydroxynonenal, a lipid peroxidation end product, stimulates uncoupling protein activity in Acanthamoeba castellanii mitochondria; the sensitivity of the inducible activity to purine nucleotides depends on the membranous ubiquinone redox state

“…In animal mitochondria, the products of lipid peroxidation that arise from ROS, such as HNE and the structurally related compounds trans-retinoic acid, trans-retinal and other 2-alkenals, specifically induce the uncoupling of mitochondria through the uncoupling proteins UCP2 and UCP3 (Rial et al 1999;Echtay et al 2003). Similarly, the induction of mitochondrial uncoupling by HNE and trans-retinal has been observed in plant mitochondria (Smith et al 2004). Moreover, it has been shown that exogenously generated superoxide activates UCPs in both animal and plant mitochondria in the presence of FFAs (Considine et al 2003;Echtay et al 2002a).…”

“…Our results indicated that HNE induced the AcUCP-mediated proton conductance both in non-phophorylating and phosphorylating mitochondria, decreasing the yield of oxidative phosphorylation in the latter. Therefore, we can conclude that activation by alkenal lipid peroxidation products also affects the UCPs of unicellular organisms, such as that of protozoan A. castellanii, similar to plant (Smith et al 2004) and mammalian UCPs . In mammalian and plant mitochondria, the stimulation of UCPs by physiologically relevant concentrations of HNE and FFAs (Kowaltowski et al 1998) decreases mitochondrial ROS generation.…”

“…4-Hydroxy-2-nonenal (HNE) activates the uncoupling protein of A. castellanii mitochondria during non-phosphorylating respiration Because exogenously generated ROS did not increase the proton leak catalyzed by AcUCP, we used exogenous HNE, which has been reported to be a potent UCP activator in mammalian and plant mitochondria Smith et al 2004). It was necessary to incubate the amoeba mitochondria with HNE for 15 min before energization (Fig.…”

Plant respiration under low oxygen

“…To overcome this deficiency in the proton gradient, plants have a strategy developed from uncoupling proteins (UCP) embedded in the inner mitochondrial membrane (Smith et al, 2004;ArnholdtSchmitt et al, 2006). Uncoupling proteins activity can enhance the transfer of protons into the mitochondria by controlling the rate of production of superoxides, and may even have some control over the TCA cycle, because increases in the UCP content can increase the rate of conversion of pyruvate to citrate (Smith et al, 2004).…”

“…Furthermore, alternatives have also been found to the maintenance of the proton gradient in the mitochondrial matrix, which is performed through uncoupled proteins (UCP) that enables flows of protons to enter the matrix independent of ATP synthesis (ArnholdtSchmitt et al, 2006). In turn, UCP would participate in the reduction of reactive oxygen species (ROS), a function also contributed to by AOX (Smith et al, 2004).…”