Date on Master's Thesis/Doctoral Dissertation

12-2003

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Biochemistry and Molecular Biology

Committee Chair

Dean, William L.

Author's Keywords

Tyrosine; Phosphorylation; Lens cells; Sodium; Potassium; ATPase; Cataracts

Subject

Phosphorylation; Eye--Physiology

Abstract

Na,K-ATPase is essential for the regulation of cytoplasmic Na+ and K+ levels in lens cells. Insufficient Na,K-ATPase activity is associated with cataract formation. Based on earlier studies in which Src-tyrosine kinase inhibitors were found to suppress Na,K-ATPase activity changes that occur in response to either thrombin, endothelin, or dopamine, I hypothesize that regulation of Na,K-ATPase activity might occur through phosphorylation of the Na,K-ATPase a1 (catalytic) subunit by Src-family tyrosine kinases. Here, I tested the influence of Src-kinase family members on tyrosine phosphorylation and Na,K-ATPase activity in membrane material isolated from porcine lens epithelium. Western blot studies indicated the expression of Src-kinase family members, Lyn, Fyn, Src, and Lck in lens cells. When membrane material was incubated in ATP-containing solution containing partially purified Lyn kinase, Na,K-ATPase activity was reduced by ~38%. Similarly, Fyn kinase inhibited Na,K-ATPase activity by 25%. Na,K-ATPase a1-catalytic subunit is abundant in both lens epithelium and fibers. Lens epithelium exhibits high Na,K-ATPase activity while Na,K-ATPase activity in the fibers is low. Lens fiber cells show an abundance of Na,K-ATPase a1, however, Na,K-ATPase activity is low. Incubation with PTP-1B caused a ~50% increase of Na,K-ATPase activity in fiber membrane material. In contrast, Na,K-ATPase activity in lens epithelium membrane material was not significantly altered by PTP-1B treatment. While endogenous PTP-1B was detected in both cell types, endogenous tyrosine phosphatase activity was low in both epithelium and fiber membrane material. Taken together, PTP-1B and Src-family kinases may be important mechanisms for regulating Na,K-ATPase activity in lens cells.

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