Relationship between dietary proteins, their in vitro digestion products, and serum cholesterol in rats

A relationship was assessed between the amino acid composition of 9 protein sources or of their in vitro digestion products and total serum cholesterol in rats. Three animal proteins (casein, beef, fish) and 6 vegetable proteins (soy, pea, peanut meal, rapeseed, oatmeal, wheat gluten) were tested. The intact protein sources were submitted to an enzymatic proteolysis according to a new in vitro digestion method. Each protein source was hydrolyzed for 30 min with pepsin at pH 1.9, then with 10 mg pancreatin at basic pH in a dialysis cell. The digestion products diffused through the dialysis membrane of the cell and were collected by a circulating sodium phosphate buffer over a 6-h period. They were likely to correspond to end products luminal in vivo digestion. The aromatic and the basic amino acids were present in higher proportions in the digestion products than in the intact protein sources, reflecting the specificity of the proteolytic enzymes. Total serum cholesterol was measured on male Sprague-Dawley rats fed cholesterol-free or cholesterol-enriched (1% cholesterol, 0.5% cholic acid) semipurified diets containing protein sources. Total serum cholesterol ranged from 70 mg/dl with the pea diet to 98 mg/dl with the peanut meal diet in rats fed cholesterol-free diets and from 163 mg/dl with the wheat gluten diet to 313 mg/dl with the casein diet in rats fed the cholesterol-enriched diets. These results suggested no specific effect of protein from animal or vegetable origin on total serum cholesterol in rats. In rats fed cholesterol-enriched diets, significant correlations were observed between total serum cholesterol and tyrosine content or leucine/isoleucine ratio of digestion products. These correlations were stronger than those observed with intact protein sources.