Possible Linkage between NADH-Oxidation and Proton Secretion inZea maysL. Roots

Abstract

A possible involvement of two different systems in proton translocation and the correlation of this factor to growth rates were measured simultaneously by means of a pH stat and an optical system. Ferricyanide, which can accept electrons at the plasmalemma, led to an immediate increase of net H⁺ -efflux but also decreased root growth rate. The reduced form, ferrocyanide, inhibited net H⁺ -effluxwithout changing the growth rate. Thus, corn root growth was not determined by proton secretion exclusively. Vanadate strongly inhibited net H⁺ -efflux by the roots but did not prevent the stimulating effect of fcrricyanide. Moreover, the extent of enhancement of net H⁺ -effluxby ferricyanide was exactly the same in vanadate pretreated as in untreated roots. Alcohols were used to try to increase the intracellular NADH level through the action of the cytoplasmic alcohol dehydrogenase present in the roots and coleoptiles. Alcohols, known to be substrates for alcohol dehydrogenase such as propan- 1-ol, ethanol and butan-l -ol increased net H⁺ -effluximmediately but methanol and secondary alcohols which are not substrates had no effect on proton secretion. The K_m values of alcohol dehydrogenase for the alcohols correspond only partly to their effects on proton secretion. However, the specificlty observed suggests that increased H⁺ -efflux arose from reduction of endogenous NAD by ADH and consequent increased membrane NADH-oxidasc activity trans locating protons and electrons out of the cells. Decreased oxygen concentrations slowed proton secretion at values far higher than are necessary to saturate cytochrome c oxidase. The results of these experiments suggest two distinct systems contributing to proton efflux.