Abstract
Experiments were conducted to determine if the observed reduced influx of sugars following mannose treatment could be attributed to changes in ATPase activity. Mannose was supplied to Beta vulgaris L. (sugar beet) leaves via the transpiration stream for either a 16- or 24-h light period. The plasma membrane was then isolated using the aqueous polymer two-phase partitioning technique. Analysis of ATP hydrolytic activity showed that mannose pretreatment had a progressive inhibitory effect on the H +-ATPase activity. These results are consistent with previous reports that mannose can dramatically influence membrane transport events in sugar beet leaves. Measurement of Mg ATP-dependent, pH-gradient formation by plasma-membrane vesicles indicated that mannose pretreatment shifted the maximal initial rate of proton pumping to more alkaline regions compared with control membranes. At pH 6.5, mannose pretreatment also inhibited the total quench. We cannot attribute this to increased leakage of protons out of the vesicles as mannose pretreatment had no effect on the passive backflow of protons out of the vesicles. Mannose appears to elicit a general perturbation of membrane transport processes which is not limited to sugars and amino acids. Collectively, these studies indicate that mannose may inhibit sugar transport, at least in part, by affecting the activity of the plasma-membrane ATPase.
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