Cytoplasmic regions of the alpha subunit of the sodium pump involved in modulating the Na,K-ATPase reaction

Carotenoid oxidative degradation products inhibit Na+-K+-ATPase

This study investigates the biological significance of carotenoid oxidation products using inhibition of Na+-K+-ATPase activity as an index. Beta-carotene was completely oxidized by hypochlorous acid and the oxidation products were analyzed by capillary gas-liquid chromatography and high performance liquid chromatography. The Na+-K+-ATPase activity was assayed in the presence of these oxidized carotenoids and was rapidly and potently inhibited. This was demonstrated for a mixture of beta-carotene oxidative breakdown products, beta-Apo-10'-carotenal and retinal. Most of the beta-carotene oxidation products were identified as aldehydic. The concentration of the oxidized carotenoid mixture that inhibited Na+-K+-ATPase activity by 50% (IC50) was equivalent to 10 microM non-degraded beta-carotene, whereas the IC50 for 4-hydroxy-2-nonenal, a major lipid peroxidation product, was 120 microM. Carotenoid oxidation products are more potent inhibitors of Na+-K+-ATPase than 4-hydroxy-2-nonenal. Enzyme activity was only partially restored with hydroxylamine and/or beta-mercaptoethanol. Thus, in vitro binding of carotenoid oxidation products results in strong enzyme inhibition. These data indicate the potential toxicity of oxidative carotenoid metabolites and their activity on key enzyme regulators and signal modulators.

Modification of the N-terminal polyserine cluster alters stability of the plasma membrane H(+)-ATPase from Saccharomyces cerevisiae

The N-terminus of the H(+)-ATPase from Saccharomyces cerevisiae contains a serine-rich cluster of 11 serine residues in the first 17 amino acids, including a stretch of eight consecutive serine residues. This cluster is conserved in the weakly expressed PMA2 gene from the same organism, but it is not present in PMA genes from other organisms suggesting that it is not likely to represent a conserved functional motif. To better understand whether this region plays a regulatory role, a series of mutant enzymes were generated in which the serine tract was systematically converted to alanine or deleted. Conversion of the first six serine residues to alanine or deletion of the entire serine tract had little effect on cell growth phenotypes. However, when eight or more serines were converted, the mutant cells displayed prominent hygromycin B-resistant and low pH-sensitive phenotypes indicative of reduced H(+)-ATPase function. The mutant enzymes were found to display relatively normal kinetic properties for ATP hydrolysis, but showed significantly decreased abundance in the plasma membrane under stress conditions when eight or more serine residues were converted to alanine. The reduced abundance of the enzyme appeared to be due to degradative turnover, as mutant enzymes with multiple alanine substitutions showed an accelerated rate of turnover relative to wild-type. The polyserine tract in the H(+)-ATPase does not appear to be important for catalysis, but may contribute to overall protein stability.