Secretion of pyruvate. An antioxidant defense of mammalian cells.

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Abstract
Cells in culture are exposed to marked oxidative stress, H2O2 being one of the predominant agents. Pyruvate and other alpha-ketoacids reacted rapidly, stoichiometrically, and nonenzymatically with H2O2, and they protected cells from its cytolytic effects. All five human and murine cell types studied, both malignant and nonmalignant, released pyruvate at an initial rate of 35-60 microM/h/2.5 X 10(6) cells when placed in 1 ml pyruvate-free medium. After 6-12 h a plateau of 60-150 microM pyruvate was attained, corresponding to concentrations reported for normal human serum and plasma. The rate of pyruvate accumulation was almost doubled in the presence of exogenous catalase, suggesting that released pyruvate functions as an antioxidant. The rate of pyruvate accumulation was dependent on cell number. Succinate, fumarate, citrate, oxaloacetate, alpha-ketoglutarate, and malate were not secreted in significant amounts from P815 cells; export was specific for pyruvate and lactate among the metabolites tested. Extracellular pyruvate was in equilibrium with intracellular stores. Thus, cells conditioned the extracellular medium with pyruvate at the expense of intracellular pyruvate, until homeostatic levels were attained in both compartments. We propose that cells plated at low density in the absence of exogenous pyruvate fail to thrive for two reasons: prolonged depletion of intracellular pyruvate and prolonged vulnerability to oxidant stress.