Response of Acer pseudoplatanus Cells to Pyruvate and 2 , 4 - Dinitrophenol ' Curtis

The endogenous respiration rate of unstarved cultured cells of Acer pseudoplatanus L. is markedly stimulated by 2,4-dinitrophenol. Pyruvate is also stimulatory but to a lesser degree than dinitrophenol. Exogenously supplied sugars cause no short-term stimulation. Pyruvate does not enhance the elevated rate of 0, uptake in the presence of dinitrophenol but does cause additionatl CO2 evolution. The endogenous concenitration of pyruvate is elevated in the presence of dinitrophenol. These observations suggest that the rate of O., uptake by the unstarved intact cells is limited by the rate of glycolysis and that rate of glycolysis is regulated by the intracellular concentration of adenine nucleotides or inorganic phosphate. Dinitrophenol stimulation of endogenous respiration is due in part to an indirect acceleration of glycolysis but also to a more direct faoilitation of oxidation in the presence of excess mitochondrial substrate. Desp.te much recent progress in elucidating the chemica.l pathways involved in the respiration of higher planits, relatively little information is avaiilable concerninig the control of rates of oxygen consumption by intact plant cells. Altihough the respiration rate of many no-ngreen plant tissues fallls off when they iare deprived of a carbon source for long periods of time, and the reduced respiratioIn raites resulting from su-ch starvation can often be elevated by supplying appropriate sugars (13), it is nevertheless clear that the respiriatory rate of plant cells is frequen.tly not liimiteid by the availability of carbohydrates. For example, in the presence of excess substrate, the respiration rate of many intact cells or of isolated mitochondria can be markedly stimulated by compounds which uncou,ple oxidative 'phosphorylation from respiratory electron flow (8). Although the most direct effect of an uncoupling conimpound such as 2,4-dinitrophenol (DNP) is to disrupt the normall linkage between electron flow and oxidative phosphorylation (15), a DNP-induiced stimulati,on of 0., conisumption bx initact celils need not be wholly the result of removing a constraint upon electron flow in the mitochonidrial cytochrome chain. Uncoupling electron flow from the synthesis of ATP not only facil,itates elecitron flow but may also produce alterations in intracelluIlar concentrations of adenine nucleotides and Pi or may effect a redistrilbution of these compounds between 1 This research was supported in part by a National Institutes of Health pre-doctoral fell-owship to C. V. Givan and by Research Grant RG-08145 from the Nationial Institutes of Healtlh, Public Health Service. the intraanid extra-mitoohondrial compartmenits (6). Because increases in ADP or Pi or decreases in ATP are kniown to speecd up the rate of glycolysis (16), it has been suggested that at least part of the respiraltory stimu.lation evoked by uncoupling agents may stem from additional mitochondrial substrate (pyruivate) which is thereby made available (2). If the amount of pyruivate l)ro(luced by glycolysis is insufficient to satturate the respiratory capacity of the miitochonidria, indirect stimulation of glycolysis brought about by DNP treatment could lead to increased 0., u,ptake sim,ply by supplying the mitochondria with additional substrate. If, on the other hand, the mitochondria are normally respiring in the presence of excess substrate, any stimulation of 02 uiptake can be ascribed to a removal of restraint on respiratory electron flow or to the activation of tricarboxvlic acid cycle enzymes. Previous studies on the effecits of uncoupling agents have suiggested that uincouiplers stimulate both glycolysis and respirationi in higher plant tissules, julst as in yeast cellts or animal tissues. Newcomb (17), working on tobacco calliuls, found that at certain concentrations, DNP led not only to increased 0., uptake but also to an even greater sti,mulation o,f CO., evolution; i.e., there was an increase in respiratory quotient (R. Q.). This rise in R. Q. indicated a stimulation of glycolysis proportionally greater than the respiratory increase. The intracellullar pyruivate concentration prestumably increased to the point where it could not be accommodated by the mitochondrial oxidizing systenm so that the excess was deca,rboxylated. This interpretation wals corroborated by Beevers (2) and by Bianchetti and MAlarre (4) who likewise found that DNP treatment cauised a rise in R. Q. When R. Q. 635 www.plantphysiol.org on September 22, 2017 Published by Downloaded from Copyright © 1968 American Society of Plant Biologists. All rights reserved.