| Abstract: | Microsomal membranes isolated from red beet (Beta vulgaris L.) storage tissue were found to contain high levels of ionophore-stimulated ATPase activity. The distribution of this ATPase activity on a continuous sucrose gradient showed a low density peak (1.09 grams per cubic centimeter) that was stimulated over 400% by gramicidin and coincided with a peak of NO3−-sensitive ATPase activity. At higher densities (1.16-1.18 grams per cubic centimeter) a shoulder of gramicidin-stimulated ATPase that coincided with a peak of vanadate-sensitive ATPase was apparent. A discontinuous sucrose gradient of 16/26/34/40% sucrose (w/w) was effective in routinely separating the NO3−-sensitive ATPase (16/26% interface) from the vanadate-sensitive ATPase (34/40% interface). Both membrane fractions were shown to catalyze ATP-dependent H+ transport, with the transport process showing the same differential sensitivity to NO3− and vanadate as the ATPase activity. Characterization of the lower density ATPase (16/26% interface) indicated that it was highly stimulated by gramicidin, inhibited by KNO3, stimulated by anions (Cl− > Br− > acetate > HCO3− > SO42−), and largely insensitive to monovalent cations. These characteristics are very similar to those reported for tonoplast ATPase activity and a tonoplast origin for the low density membrane vesicles was supported by comparison with isolated red beet vacuoles. The membranes isolated from the vacuole preparation were found to possess an ATPase with characteristics identical to those of the low density membrane vesicles, and were shown to have a peak density of 1.09 grams per cubic centimeter. Furthermore, following osmotic lysis the vacuolar membranes apparently resealed and ATP-dependent H+ transport could be demonstrated in these vacuole-derived membrane vesicles. This report, thus, strongly supports a tonoplast origin for the low density, anion-sensitive H+-ATPase and further indicates the presence of a higher density, vanadate-sensitive, H+-ATPase in the red beet microsomal membrane fraction, which is presumably of plasma membrane origin. |
