Interaction of dietary protein differing in sulfur amino acid content and pectin on bile acid conjugation in immature and mature rats.

Rapidly growing immature (4-wk-old) and slowly growing mature (15-wk-old) rats were fed fiber-free or 10 g/100 g pectin diets containing various proteins differing in the sulfur amino acid content for 30-32 d. Soybean protein, casein, whole egg protein and egg albumen were used at the nitrogen level of 2.7 g/100 g diet. These experimental diets contained 0.354, 0.540, 0.945 and 1.22 g sulfur amino acids/100 g, respectively. In the rats fed fiber-free diets, a substantial quantity of glycine-conjugated bile acids was detected in the bile of immature rats fed soybean protein and casein (73 and 25% of total bile acids, respectively), but not in the other groups (less than 13%). Dietary pectin increased bile acid excretion both in immature (48-77%) and mature (34-114%) rats irrespective of the protein source, except in immature rats fed egg albumen and mature rats fed whole egg protein. Because a pectin-dependent increase in bile acid excretion was essentially attributed to the increase in glycine-conjugates, this dietary fiber significantly increased the ratio of glycine-conjugates to taurine-conjugates (2.4- to 6.5-fold). This increase was accompanied by a 40-50% decrease in the concentration of liver taurine, except in immature rats fed soybean protein and egg albumen. However, there was no consistent relationship between the extent of taurine conjugation and the activity of liver cysteine dioxygenase, one of the rate-limiting enzymes in taurine synthesis.