The phosphorylation state of casein and the species-dependency of its hypercholesterolaemic effect.

1. The present study concerns the question whether the hypercholesterolaemic effect of casein, a phosphorylated protein, is modified in species differing both in glycine-taurine conjugation of bile acids and in intestinal alkaline phosphatase (EC 3.1.3.1). 2. Since these two variables are entirely different in rabbits and rats, identical (cholesterol-free) semi-purified diets containing either casein or soya-bean protein were given to both species. 3. In rabbits casein, as compared with soya-bean protein, did not affect calcium absorption but immediately increased phosphate absorption and decreased faecal excretion of bile acids. These effects preceded the accumulation of apo B-cholesterol in serum, which indicates a cause-and-effect relation. 4. In contrast, none of these casein-specific effects were observed in rats. 5. These results suggest that the hypercholesterolaemic potential of casein is expressed mainly in species (like the rabbit) with a low activity of intestinal phosphatase and with a high glycine conjugation of bile acids. This might explain why species (like rat and man) are rather insensitive to dietary casein.