Uncoupler-induced ATPase activity of plant mitochondria.

The ATPase activity of mammalian mitochondria can be readily stimulated by a variety of protonophoric uncoupling agents, the extent being dependent on the assay conditions and the isolation medium used 111. The situation in plant mitochondria is less straight-forward. Jung and Laties [2] classified plant mitochondria into three types: (i) those where uncoupler readily stimulates activity, as in corn [3,9]. (ii) those where an 'ATP transport defect' exists and ATPase is induced only after a short burst of respiration in the presence of Mg*+ and Pi, as in cauliflower [4,9], (iii) those where a putative inhibitor protein limits activity; this latter class was subdivided into (a) those where electron transport is sufficient to cause inhibitor dissociation, as in castor bean endosperm [5], and (b) those where only sonication or trypsin treatment releases the inhibitor, as in potato [2]. Using isolated intact mung bean (Phaseolus aureus) mitochondria we decided to investigate further the factors controling uncoupler-induced ATPase activity. In agreement with previous findings [6], osmotic support markedly affected activity, with isoosmolar sucrose completely abolishing activity. Removal of sucrose led to the appearance of uncoupler-induced activity, but had no effect on endogenous activity. These results are consistent with the existence of an ATP transport defect; removal of osmotic support would lead to swelling, resulting in greater permeability to molecules such as ATP. Cereijo-Santalo [7] argued that inhibition of ATPase activity in rat liver mitochondria by sucrose was due to depression of the water content of the matrix, i.e., an osmotic effect, rather than one involving ATP/ADP transport [8]. Furthermore, this 'sucrose effect' could be relieved by addition of a permeant salt. However, potassium acetate plus valinomycin did not significantly release the inhibition of activity by iso-osmolar sucrose in mung bean mitochondria. Further evidence for an ATP transport defect was obtained from experiments involving 'respiratory priming': A short burst of respiration resulted in a significant increase in activity, this effect being most marked in a medium containing iso-osmolar sucrose. The effect could be inhibited 82% by the electron transport inhibitor, antimycin A. Furthermore, in agreement with results for cauliflower mitochondria (41, when 1 mM succinate