Bile Acids
7 a,4-C h o lesten -3-o n e
C h olyl-C oA
C h enod eo xych o lyl-C o A
FIGURE 19-17
Formation of bile acids.
(about 80 mg/day) is also lost through shedding of the
outer layers of the skin.
Bile Acids
Bile acids are 24-carbon steroid compounds. Primary bile
acids (cholic and chenodeoxycholic) are synthesized in
the liver from cholesterol (Figure 19-17). In human bile,
about 45% is chenodeoxycholic acid, 31% cholic acid and
24% deoxycholic acid (a secondary bile acid formed in
the intestine). Early studies in rodents showed the pre-
ferred substrate to be newly synthesized cholesterol. How-
ever, whole-body turnover studies in humans using ra-
dioactive markers indicate that approximately two thirds
of bile acid is derived from HDL cholesterol delivered to
the liver. Formation is initiated by 7a-hydroxylation, the
committed and rate-limiting step catalyzed by microsomal
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