Effects of soybean lipoxygenase on Na+/K+-ATPase activity in vitro

Oxidized metabolites of polyunsaturated fatty acids produced by lipoxygenas e are among the endogenous regulators of Na+/K+-ATPase. The direct effect o f lipoxygenase on Na+/K+-ATPase activity was assessed in vitro using soybea n lipoxygenase. Treatment of 4.2 mu g/mL Na+/K+-ATPase (from dog kidneys) w ith 4.2 mu g/mL of soybean lipoxygenase caused 20 +/- 2% inhibition of ATPa se activity. A 10-fold increase in lipoxygenase concentration (41.6 mu g/mL ) led to 30 +/- 0.3% inhibition. In the presence of 12 mu g/mL phenidone (a lipoxygenase inhibitor) and 15.4 mu g/mL glutathione (a tripeptide contain ing a cysteine residue) inhibition of Na+/K+-ATPase activity was blocked an d an increase in ATPase activity was observed. The presence of lipoxygenase enhanced the inhibition of Na+/K+-ATPase activity caused by 20 ng/mL ouaba in (31 +/- 2 vs. 19 +/- 2) but had little or no effect with higher concentr ations of ouabain. These findings suggest that lipoxygenase may regulate Na +/K+-ATPase by acting directly on the enzyme.