COMPETITIVE INTERACTIONS IN THE COOPERATIVE OXIDATION OF EXOGENOUS NAD(P)H AND TRICARBOXYLIC-ACID CYCLE INTERMEDIATES IN THE MITOCHONDRIA OF SUGAR-BEET ROOT

The cooperative oxidation of exogenous NAD(P)H and succinate or substr ates related to NAD-dependent dehydrogenases of the tricarboxylic acid cycle was studied by spectrophotometric and polarographic methods in the mitochondria from sugar beet (Beta vulgaris L.) roots under variou s regimes of electron transport chain operation. Substrates which were superior donors of reducing equivalents to the electron transport cha in (succinate, malate) were shown to inhibit NAD(P)H oxidation, wherea s pyruvate, with a much lower oxidation rate, did not manifest such an effect. In addition to the activities of respective dehydrogenases th at controlled the rates of reducing equivalent transfer to the electro n transport chain, the phenomenon of substrate preference also depende d on the electron transport chain regime (states 3, 4 respiration, or uncoupled). The inhibition of exogenous NAD(P)H oxidation by succinate or malate was the highest under restricted electron transport chain a ctivity (state 4) and completely disappeared when respiration was unco upled.