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Treatment of Dietary Casein with Formaldehyde Reduces its Hypercholesterolemic Effect in Rabbits

C. E. West, A. C. Beynen, K. E. Scholz^{1, 2,}, A. H. M. Terpstra³, J. B. Schutte^*, K. Deuring^* and L. G. M. Van Gils

Department of Human Nutrition, Agricultural University, 6703 BC Wageningen ^* Institute of Animal Nutrition Research (ILOB-TNO), 6709 PJ Wageningen Trouw & Co. B. V. International, Research and Development, 3881 LA Putten, The Netherlands

Rabbits were fed cholesterol-free, semipurified diets containing 42% (wt/wt) casein or 21% casein plus one of the following nitrogen sources: soy isolate, amino acid mixture simulating casein, amino acid mixture simulating soy isolate, formaldehyde-treated casein or formaldehyde-treated soy isolate. Two additional groups of rabbits were fed the 42% casein diet and the diet containing casein plus soy isolate to which 0.4% (wt/wt) pure formaldehyde was added, this amount being identical to the amount of formaldehyde present in the diets with formaldehyde-treated proteins. Growth was somewhat reduced on the three diets containing 42% casein. The diet containing 42% casein to which no formaldehyde had been added induced severe hypercholesterolemia, the level of serum cholesterol after 8 weeks being about 10 mmol/L. The hypercholesterolemia was markedly reduced by the replacement of half of the casein by soy isolate, formaldehyde-treated soy isolate or formaldehyde-treated casein. No significant reduction of the concentration of serum cholesterol was seen when half of the 42% casein was replaced by an amino acid mixture imitating either casein or soy isolate. Formaldehyde per se did not significantly influence the level of serum cholesterol. We conclude that the differential tertiary structure of intact casein and soy isolate is an important factor in determining the cholesterolemic responses in rabbits to these proteins.

KEY WORDS: • rabbits • serum cholesterol • dietary casein • dietary soybean protein • protein structure • formaldehyde

¹ KES was supported by the Netherlands Nutrition Foundation and by an International Agriculture Centre fellowship from the Dutch Ministry of Agriculture and Fisheries.

² Present address: Institut fuer Physiologie und Biochemie der Ernährung, Bundesanstalt fuer Milchforschung, D-2300 Kiel, West Germany.

³ Present address: Department of Nutrition and Metabolism, The Miriam Hospital, 164 Summit Avenue, Providence, RI 02906.

Manuscript received 20 May 1983.