Energy-linked Adenosine Diphosphate Accumulation by Corn Mitochondria

Abstract

Corn mitochondria show respiration-linked net accumulation of [(3)H]ADP in the presence of phosphate and magnesium, especially if the formation of ATP is blocked with oligomycin. Inhibition of ADP-ATP exchange by carboxyatractyloside also activates ADP accumulation, and addition of carboxyatractyloside or palmitoyl-coenzyme A to oligomycin-blocked mitochondria produces additional ADP uptake. With carboxyatractyloside the accumulated ADP is phosphorylated to ATP. With oligomycin, only a little ATP is formed. Millimolar concentrations of ADP are required for maximum uptake, and the K(m) (3.77 millimolar) for ADP translocation is independent of whether oligomycin or carboxyatractyloside is used. This is not true for ADP concentrations in the 0.05 to 0.25 millimolar range. Accumulated [(3)H]ADP rapidly exchanges with unlabeled AMP, ADP, or ATP, but not with other diphosphate nucleotides or 2 millimolar substrate anions. [(3)H]AMP is not accumulated, but [(3)H]ATP is accumulated to about one-half the extent of [(3)H]ADP. Tricarboxylate substrates inhibit ADP net uptake, and inhibition by citrate is competitive with K(i) = 10 millimolar. The evidence suggests the presence of a pathway, carboxyatractyloside-insensitive and different from the translocase, which operates to maintain adenine nucleotides in the matrix.