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Unavailability of Cystine from Trypsin Inhibitors as a Factor Contributing to the Poor Nutritive Value of Navy Beans

University of Minnesota, St. Paul, Minnesota

A disproportionate fraction of the total cystine content of the navy bean is found in the isolated trypsin inhibitor. Experiments were undertaken to test the possibility that the poor availability of cystine from navy beans might be due to a failure to utilize the cystine which resides in this protein fraction. Chicks were fed the following diets: 1) basal cystine-deficient diet in which the only source of nitrogen was a mixture of amino acids; 2) basal diet plus 0.15% cystine; 3) basal diet plus 2% unheated navy bean trypsin inhibitor (NBTI); and 4) basal diet plus 2% heated NBTI. The replacement of unheated NBTI by heated NBTI produced essentially the same improvement in growth as the supplementation of the basal diet with cystine. The excretion of cystine, as well as several other representative amino acids (aspartic and glutamic acids, serine, threonine, and proline), was much greater on the diet containing unheated NBTI than on the one with heated NBTI; this difference, however, was confined to the protein-bound amino acids of the excreta. Unheated NBTI produced a pronounced depression in the level of trypsin in the pancrease and intestines with little change in the size of the pancreas. Unheated NBTI, but not heated NBTI, was markedly resistant to digestion by pepsin, trypsin and chymotrypsin. It is concluded that the cystine of NBTI is not readily available for the growth of chicks unless modified by heat; the resistance of unheated NBTI to attack by digestive enzymes appears to be the major factor involved.

² Department of Animal Science, Institute of Agriculture.

³ Present address: Department of Animal Science, South Dakota State University, Brookings, S. D.

Manuscript received 28 February 1969.