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