Proton-pumping activities of soybean (Glycine max L.) root microsomes: Localization and sensitivity to nitrate and vanadate

Soybean (Glycine max L. cv. Williams '79) root microsomal suspensions exhibit two proton-pumping activities. The largest proportion of activity, localized at a low density on sucrose gradients and inhibited by nitrate, does not correspond to markers for Golgi, endoplasmic reticulum, plasma membrane, or mitochondria and presumably originates from the tonoplast. There is a small shoulder of proton-pumping activity, which is sensitive to vanadate, localized at a higher density on gradients (37%, w/w). This activity coresponds fairly well with vanadate-sensitive ATPase activity (39%, w/w) and a shoulder of glucan synthetase II activity (39%, w/w). The main peak of glucan synthetase II activity co-equilibriates with the mitochondrial marker cytochrome c oxidase at 43% (w/w). It is likely that this activity originates from the plasma membrane.