Dietary sulfur-containing amino acids and glycine as determinant factors in plasma cholesterol regulation in growing rats.

Key Words sulfur-containing amino acids, glycine, plasma cholesterolA number of studies on the influence of dietary proteins on plasma cholesterol levels in experimental animals and humans have revealed that plant proteins generally have a greater cholesterol-lowering or anti-hypercholesterolemic effect than animal proteins (1, 2).The mechanism of action by which dietary protein exerts its hypo or hypercholesterolemic effect has not been fully elucidated, but it is possible that the differences in the amino acid composition of dietary protein may be responsible for the effect (3).Katan et al. (4) demonstrated the serum cholesterol -lowering effect of dietary-supplemented glycine in rabbits and rats.On the other hand, Sautier et al. (5) reported that in rats, serum cholesterol levels had a significant positive correlation with those of tyrosine and glutamic acid, and a negative correlation with cystine and alanine regardless of the source of dietary protein.We recently reported the plasma cholesterol-elevating and cholesterol -lowering effects of diet-supplemented methionine and cystine, respectively, in rats fed on a cholesterol-containing casein diet (6).Furthermore, we observed that when methionine was added to the diet together with sufficient glycine, plasma cholesterol levels were markedly decreased (manuscript in preparation).These findings suggest that methionine metabolism is involved in the regulation of plasma cholesterol levels, and hence that sulfur-containing amino acids (SAA) and glycine in dietary protein might be responsible for the effect wielded.This communication describes the successful manipulation of plasma cholesterol levels in rats fed on amino acid mixture diets by altering the content of SAA and glycine, supporting the above hypothesis.Male albino rats of Wistar strain with an average weight of about 100g were fed on the experimental diets for 15days in an air-conditioned room with a 12h