Sheree L. Rybak, former Graduate Student in Biological Sciences, in collaboration with Dr. Fred Lanni, Associate Professor of Biological Sciences

Introduction and Goals

The Na⁺,K⁺-ATPase has been observed to partially inhibit acidification of early endosomes, while chloride channels have been observed to enhance acidification in endosomes and lysosomes. However, little theoretical analysis has been done of the expected pH resulting from different combinations of pumps, such as the H⁺-ATPase and the Na⁺,K⁺-ATPase, and channels, such as the chloride channel. We therefore developed quantitative theoretical models of endosomal pH regulation based on thermodynamic considerations embodied in the Nernst equation, which defines the free energy change associated with transporting a hydrogen ion. Our goal was to explore the limits of the pH gradient established at steady state when different combinations of pumps and channels are present in the endosomal membrane. Since we were interested in steady-state values, the absolute number or concentration of ion pumps or channels present in the membrane was not considered as a factor. We similarly did not consider the rate of ion pumping (or ion leakage) since pumps were assumed to act until limited by thermodynamics (e.g., the amount of free energy available via ATP hydrolysis). This requires that passive fluxes of any pumped ions be insignificant or that leaks (deviations from the steady state) be sufficiently slow that they can be corrected by additional pumping.

Conclusions

We conclude that: (1) both size and shape of endosomes will influence steady-state endosomal pH whenever membrane potential due to the pH gradient limits proton pumping, (2) steady-state pH values similar to those observed in early endosomes of living cells can occur in endosomes containing just H⁺-ATPases and Na⁺,K⁺-ATPases when low endosomal buffering capacities are present, and (3) inclusion of active chloride channels results in predicted pH values well below those observed in vivo. The results support the separation of endocytic compartments into two classes, those (such as early endosomes) whose acidification is limited by attainment of a certain membrane potential and those (such as lysosomes) whose acidification is limited by the attainment of a certain pH. The theoretical framework and conclusions described are potentially applicable to other membrane-enclosed compartments that are acidified, such as elements of the Golgi apparatus.

Distribution of Models

A Microsoft Excel for Macintosh Version 5 Workbook containing the models is available.

A paper describing this work has been published: Sheree L. Rybak, Frederick Lanni, and Robert F. Murphy (1997) Theoretical Considerations on the Role of Membrane Potential in the Regulation of Endosomal pH, Biophysical Journal 73: 674-687.

Last Updated: 01 Dec 2004

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