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Mitochondria isolated from various plant tissues (leaves, etiolated shoots and hypocotyls, and stem tubers) oxidize exogenous NADPH with respiratory control values and ADP:O ratios similar to those obtained with exogenous NADH as substrate. In all the mitochondria investigated, the electron-transfer inhibitors rotenone and amytal each had the same effect on the oxidation of NADPH as they had on the oxidation of NADH. The oxidation of exogenous NADPH by white potato tuber mitochondria was much more sensitive to inhibition by citrate or ethylene glycol bis-(beta-aminoethyl ether)-N,N-tetraacetic acid than was the oxidation of NADH. Mitochondria isolated from aged beetroot slices showed an increased capacity for the oxidation of exogenous NADH (compared with mitochondria from fresh tissue) but no such increase in the capacity to oxidize exogenous NADPH. These results suggest that exogenous NADPH and NADH are oxidized via different flavoproteins in plant mitochondria.

Potato tuber mitochondria oxidized exogenous NADH and exogenous NADPH at similar rates; the electron transfer inhibitor rotenone did not inhibit the oxidation of either substrate. Submitochondrial particles, prepared from potato tuber mitochondria, exhibited a greater capacity to oxidize NADH than NADPH; rotenone inhibited the oxidation of NADH by 29% and the oxidation of NADPH by 16%. The oxidation of both NADH and NADPH Mitochondria were isolated by the method of Laties (14), except that the wash resuspension medium contained 0.4 M mannitol, 25 mm Tes (pH 7.6), and 0.1% BSA. Submitochondrial particles were prepared by sonication (2 x 25 s sonications at 40 w power with a Heat Systems-Ultrasonics, Inc. sonicator) in a medium containing 0.25 M sucrose, 1 mM ATP, and 10 mm Tes (pH 7.2) at 10 mg protein/ml medium. Submitochondrial particles were pelleted at 100,000g for I h after an initial centrifugation at 10,000g for 10 min to remove whole mitochondria. The final pellet was resuspended in 0.25 M sucrose and 0.1% BSA.Mitochondrial Assays. Mitochondrial respiration was measured polarographically using a Rank 02 electrode (Rank Bros., Bottisham, Cambridge, U.K.) at 25 C in a standard reaction medium of 0.4 M mannitol, 25 mm Tes (pH 7.2), 2 mm MgSO4, 5 mM KH2PO4, and 0.1% BSA (14). The O2 concentration in air-saturated medium was taken as 240 IM (9)

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