The cholesterol-lowering effects of soy protein may by located in the liver, suggests new research from Korea University and Nestlé Research Center that fills in the gaps in our understanding of the heart healthy benefits of soy.
Soybean protein hydrolysate (SPH) was found to stimulate the low-density lipoprotein receptor (LDL-R) transcription in liver cells, report the researchers in the Journal of Agricultural and Food Chemistry.
The study helps fill in the gaps in our understanding of the mechanism of how soy protein may help improve blood lipid levels, an association that led to the Food and Drug Administration (FDA) approval of a CVD reduction claim for soybean protein in 1999.
Hypercholesterolaemia has a long association with many diseases, particularly cardiovascular disease (CVD), the cause of almost 50 per cent of deaths in Europe, and reported to cost the EU economy an estimated €169bn ($202bn) per year.
The researchers, led by Cherl-Ho Lee, incubated liver cells (Hep T9A4) with hydrolysates of isolated soy protein (Supro 1500, Solae) and found that LDL-R transcription was strongly stimulated.
The hydrolysates were prepared using the proteases from Bacillus amyloliquefaciens FSE-68 at different enzyme to soy protein ratios of 0.1 per cent (NPR68) or 0.15 per cent (APR68). Three types of soy protein hydrolysate (SPH) were prepared from each of the two enzyme to soy protein ratios, giving six SPHs. These were classified depending on their degree of hydrolysis: five, 10 or 15 per cent.
According to Lee and co-workers, the most effective LDL-R transcription stimulator was the 15 per cent SPH obtained by NPR68.
On the other hand, other mechanisms previously suggested to play a role in cholesterol-lowering, such as inhibition of cholesterol synthesis, and blockage of bile acid and/or cholesterol absorption, were not affected, said the researchers.
"The bioactivity is due to soybean peptides because the ethanol extract of soybean protein which contains isoflavones does not stimulate LDL-R transcription," wrote the researchers.
This supports previous reports that soy protein isolates (SPI), and not soy isoflavones, are associated with lowering blood fat levels (The American Journal of Clinical Nutrition, Vol. 83, pp. 244-251).
"Therefore, we suggest that soy peptides can effectively stimulate LDL-R transcription in the human liver cell line and reduce blood cholesterol level," wrote the researchers.
However, they stated that work was still needed to properly nail down the mechanism.
"Further experiments are required to demonstrate the exact target of these soybean peptides in the hepatic cells," they concluded.
Source: Journal of Agricultural and Food Chemistry
Volume 55, Number 26, Pages 10599-10604, doi: 10.1021/jf071903f
"Cholesterol Lowering Mechanism of Soybean Protein Hydrolysate"
Authors: S.-J. Cho, M.A. Juillerat, C.-H. Lee