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

Hugh Baird Matheson, Purdue University

Abstract

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.

Degree

Ph.D.

Advisors

Story, Purdue University.

Subject Area

Nutrition|Biochemistry

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