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Plant and Cell Physiology, 1986, Vol. 27, No. 2 337-348
© 1986

Article

Quantitative Analysis of ATP-Dependent H+ Efflux and Pump Current Driven by an Electrogenic Pump in Nitellopsis obtusa1

Kazuhiko Takeshige, Teruo Shimmen and Masashi Tazawa

Department of Botany, Faculty of Science, University of Tokyo Hongo, Tokyo 113, Japan

Internodal cells of Nitellopsis were made tonoplast-free by perfusion with a medium containing EGTA. Cytoplasmic concentrations of solutes were controlled by a second perfusion with media of known composition. The electrogenic pump current (Ip), which was calculated from electrical data obtained from cells with and without ATP, was compared with the current carried by H+ (IH+) across the plasma membrane. A close correlation between Ip and IH+ was found under various internal and external conditions. (1) Ip and IH+ depended on the internal ATP and showed Michaelis-Menten type saturation curves. For Ip, Km was 120 µM and the maximum current Vmax was 15.1 mA m–2, while for IH+, Km was 160 µM and Vmax was 16.6 mA m–2. (2) Ip and IH+ showed almost the same IH2+ dependence. The Mg2+-dependent Ip was 19.5 mA m–2, while the Mg2+-dependent IH2+ was 17.7 mA m–2. (3) IH2+ was maximal at an external pH of 8 and decreased both in acidic and alkaline pH ranges. Ip was nearly equal to IH+ in the pH range between 8 and 5. (4) IH+ became maximal at an internal pH of 7.3, which is nearly the same as the pH for maximal electrogenecity found by Mimura and Tazawa (1984).

All these facts support the idea proposed in our previous paper (Takeshige et al. 1985) that the electrogenic ion pump located in the plasma membrane of Nitellopsis is the H+ pump.

1 Dedicated to Professor Dr. Erwin Bünning on the occasion of his 80th birthday.

(Received June 21, 1985; Accepted December 20, 1985)
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