The proteolytic inhibiting substance in the extract from unheated soy bean meal and its effect upon growth in chicks.

The nutritional significance of enzymatic inhibitors which modify the action of enzymes acting in the digestive tract has not been heretofore studied, although the possibilities of such significance have been mentioned in some of the recent publications upon naturally occurring inhibitors (l-3). It has been suggested that they may be of importance in bacterial nutrition also (4, 5). It has been known for many years that the growth-promoting value of soy beans is greatly enhanced by heating (6). This increase in nutritional value of the heated soy beans was attributed by Hayward and Hafner (7) and by Almquist, IMecchi, Kratzer, and Grau (8) to the general raising of the level of the sulfur-bearing amino acids (cystine and methionine) by heat treatment. They have pointed out, however, that this amino acid deficiency may not have been the only limiting growth factor eliminated by autoclaving. It was also found by Almquist et al. that supplemental methionine resulted in a stimulation of growth when fed with heated soy bean meal. The explanation that heat enhances the value of proteins is inconsistent with the generally accepted belief that elevated temperatures tend to lower the nutritional value of most proteins (g-11). In addition, Thayer and Mussehll demonstrated that a growth-inhibiting property of raw soy beans was primarily due to a substance present in the raw soy beans which could be extracted with water, leaving a residue which compared favorably with autoclaved meal in nutritional value. It was likewise shown by them that the addition of methionine and cystine increased the nutritional value of both heated and unheated soy bean meal. Because of the similarity of properties of the growth-retarding factor extractable from unheated soy beans and the trypsin-inhibiting substance also found in these extracts, it appeared highly probable that the two were identical (3).