Alteration of hepatic bile acid pool composition and enzymes regulating bile acid metabolism by dietary fiber

Semipurified diets containing 5% pectin or psyllium hydrocolloid were compared to cellulose. As a dietary fiber source, pectin and psyllium increased bile acid pool size 309% and 266% (1.8 $\pm$ 0.4; 5.5 $\pm$ 1.2; 4.7 $\pm$ 0.7 $\mu$mol/100 g body weight), liver secretion of bile acids 223% and 285% (0.57 $\pm$ 0.09; 1.23 $\pm$ 0.50; 1.55 $\pm$ 0.25 $\mu$mol/h $\cdot$ 100g body weight), and liver synthesis of bile acids 148% and 145% (0.09 $\pm$ 0.02; 0.13 $\pm$ 0.03; 0.13 $\pm$ 0.04 nmol/h $\cdot$ 100g body weight), compared with cellulose, respectively. Both soluble fibers caused a decrease in 12$\alpha$-hydroxylated bile acids/non 12$\alpha$-hydroxylated bile acids and caused a decrease in the hydrophobicity of the circulating bile acids. A second study compared the effects of 5% pectin, psyllium or oat bran with those of 5% cellulose or 2% cholestyramine on hepatic microsomal cholesterol 7$\alpha$-hydroxylase activity. Cholestyramine increased enzyme activity to the greatest extent but both pectin and psyllium caused a two-fold increase in activity over that found with cellulose (95.4 $\pm$ 3.6; 36.0 $\pm$ 4.5; 35.5 $\pm$ 3.5; 16.9 $\pm$ 1.9; 12.3 $\pm$ 2.0 pmol min$\sp{-1}$ mg$\sp{-1}$protein). Adding cholesterol (0.25%) to cellulose, oat bran or rice bran caused a (165%, 218%, 161%) increase in enzyme activity however when added to the psyllium diet cholesterol did not increase activity above levels caused by psyllium alone (34.2 $\pm$ 5.0; 34.6 $\pm$ 4.9 pmol min$\sp{-1}$ mg$\sp{-1}$protein). Subsequent experiments measured the activity of steroid 12$\alpha$-hydroxylase and steroid 6$\beta$-hydroxylase in microsomes isolated from livers of rats fed 5% of pectin, psyllium, oat bran, cellulose or 2% cholestyramine. The soluble fibers examined did not cause an alteration in 12$\alpha$-hydroxylase activity however there was a significant increase in 6$\beta$-hydroxylase activity when cholestyramine, pectin or psyllium was fed, compared to cellulose (595 $\pm$ 59; 573 $\pm$ 50; 600 $\pm$ 49; 428 $\pm$ 36 pmol min$\sp{-1}$ mg$\sp{-1}$protein). Oat bran did not alter the activity of either enzyme compared to cellulose. It is concluded that the rat can maintain a relatively hydrophilic circulating bile acid pool through modulation of steroid 6$\beta$-hydroxylase and an increased synthesis of bile acids by increases in the activity of cholesterol 7$\alpha$-hydroxylase. When fed soluble dietary fiber such as pectin or psyllium, changes in composition and therefore hydrophobicity of circulating bile acids may be responsible for changes in enzyme activities