SECTION 19.4

Bile Acids

423

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.

19.4

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