Metabolically Driven Self-Restoration of Energy-Linked Functions by Avocado Mitochondria

Abstract

To assess the restorative capacity of isolated avocado (Persea americana) fruit mitochondria, the organelles were first aged in the absence of an energy source at 250C for several hours until respiratory control and oxidative phosphorylation were greatly diminished or totally lost. Energy-linked functions were then gradually restored over a period of several hours after the addition of substrate. Restoration of respiratory control resulted from both an increase in state 3 and a decrease in state 4 respiratory rates. Either a-ketoglutarate or succinate served as restorants, each with distinctive rates of recovery in state 3 and state 4 respiration. ATP also served as a restorative agent but not as effectively as metabolizable substrate. ATP synthase activity was modulated by stress and restoration but neither the extent nor the rate of change was sufficient to constrain state 3 rates. Orthophosphate was released from the mitochondria during substrate-deprived stress. Restoration of phosphorylation preceded that of RC with phosphate uptake and phosphorylation being evident immediately upon the addition of substrate. During restoration [32P]orthophosphate was incorporated into several organic fractions: phospholipid, ATP, a trichloroacetic acid-precipitable mitochondrial fraction, and an organophosphate that accumulated in the medium in relatively large amounts. The organophosphate was tentatively identified as a hexosephosphate. Incorporation into ATP and the putative hexosephosphate continued unabated beyond the point of maximum restoration. Phosphate metabolism thus appears to be a necessary but not sufficient precondition for mitochondrial restoration and maintenance. Based on the recovery kinetics of the various phosphorylated components, the mitochondrial-bound fraction appears to be most directly linked with restoration. Results are discussed with reference to specific characteristics and components of selfrestoration and to possible underlying mechanisms. We suggest that a degree of self-restoration is consistent with the quasiautonomous nature of mitochondria and that this intrinsic capacity may be pivotal to the respiratory climacteric in senescent fruit cells and to cellular homeostasis in general.of mitochondria in the climacteric respiratory rise that accompanies the senescence of many fruit and other plant organs. Decades later it is still generally held that the respiratory climacteric has mitochondrial origins but causal relationships remain unresolved. Early postulates were premised on the notion that mitochondrial respiration increased during the climacteric in compensation for declining mitochondrial efficiency, with the latter attributed to either phosphorylative uncoupling (20) or the interposition of the energy-inefficient alternative electron transport pathway (37). These postulates gave way to subsequent evidence that mitochondria remain energy-efficient throughout the respiratory climacteric (16), and that the alternative pathway, though present, remains unengaged (39).Based on the ...

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