We measured hepatic
cholesterol 7 alpha-hydroxylase activity, mass, and catalytic efficiency (activity/unit mass) in bile
fistula rats infused intraduodenally with
taurocholate and its 7 beta-hydroxy epimer, tauroursocholate, with or without
mevalonolactone to supply newly synthesized
cholesterol.
Enzyme activity was measured by an
isotope incorporation assay and
enzyme mass by densitometric scanning of immunoblots using rabbit anti-rat liver
cholesterol 7 alpha-hydroxylase antisera.
Cholesterol 7 alpha-hydroxylase activity increased 6-fold,
enzyme mass 34%, and catalytic efficiency 5-fold after interruption of the enterohepatic circulation for 48 h. When
taurocholate was infused to the
bile acid-depleted animals at a rate equivalent to the hepatic
bile acid flux (27 mumol/100-g rat/h),
cholesterol 7 alpha-hydroxylase activity and
enzyme mass declined 60 and 61%, respectively. Tauroursocholate did not significantly decrease
cholesterol 7 alpha-hydroxylase activity, mass and catalytic efficiency. The administration of
mevalonolactone, which is converted to
cholesterol, modestly increased
cholesterol 7 alpha-hydroxylase activity and
enzyme mass in the
bile acid-depleted rats. However, when
taurocholate was infused together with
mevalonolactone,
cholesterol 7 alpha-hydroxylase activity and catalytic efficiency were markedly depressed while
enzyme mass did not change as compared with
bile acid-depleted rats. These results show that (a) hepatic
bile acid depletion increases
bile acid synthesis mainly by activating
cholesterol 7 alpha-hydroxylase with only a small rise in
enzyme mass, (b) replacement with
taurocholate for 24 h decreases both
cholesterol 7 alpha-hydroxylase activity and mass proportionally, (c) when
cholesterol is available (
mevalonolactone supplementation), the infusion of
taurocholate results in the formation of a catalytically less active
cholesterol 7 alpha-hydroxylase, and (d) tauroursocholate, the 7 beta-hydroxy epimer of
taurocholate, does not inhibit
cholesterol 7 alpha-hydroxylase. Thus,
bile acid synthesis is modulated by the catalytic efficiency and mass of
cholesterol 7 alpha-hydroxylase. The enterohepatic flux of 7 alpha-hydroxylated
bile acids and the formation of hepatic
cholesterol apparently control
cholesterol 7 alpha-hydroxylase by different mechanisms.