Effect of mixing ratio in lecithin/bile acid mixed micelles on Na(+)-K+ ATPase

Lecithin and bile acid form mixed micelles in the bile. We found that the mixing ratio in lecithin/bile acid mixed micelles directly changes the activity of Na(+)-K+ ATPase. Na(+)-K+ ATPase activity was suppressed to 13.3% of that in the buffer alone at a lecithin/bile acid mixing ratio of 0.1 in the presence of 10(-2) M bile acid (that is, a concentration level close to that of the hepatic bile). With increase in the mixing ratio in the presence of 10(-2) M bile acid, the activity of the enzyme was found to be augmented accordingly. Thus addition of lecithin to an extent such that the mixing ratio reached 0.6 led to an enhancement of enzymatic activity to 193.8%. If lecithin addition is made in the presence of bile acid at the near gallbladder concentration of 10(-1) M, however, the Na(+)-K+ ATPase activity can be observed to increase with the increase in the lecithin/bile acid ratio. Yet, at the same mixing ratio of 0.6, the enzyme activity was arrested at only 25.7%. However, this change in mixing ratio had no effect on ouabain-insensitive ATPase. The state of mixed micelles may exert the same effect on Na(+)-K+ ATPase activity in vivo.