When a nutritionally complete basal diet containing 10% protein from casein was supplemented with 20% protein from unheated casein, wheat gluten or soy protein isolate, weanling mice exhibited significantly increased weight gains. In contrast, weight gains were markedly reduced compared to those with the basal diet; that is, growth was inhibited, when the supplement was soy protein or gluten that had been heated at 200 or 215 degrees C for 72 min in the dry state to stimulate crust baking. Addition of various carbohydrates to the gluten during heating prevented such growth inhibition. After heating with sodium ascorbate (but not L-ascorbic acid), soy protein (at 200 degrees C) and gluten (at 215 degrees C) completely prevented growth when added to the basal diet. Growth inhibition also occurred with a heated casein-ascorbate mixture, but was less than with the other proteins. The extent of growth inhibition increased sharply with temperature of heating in the range 180-215 degrees C and with sodium ascorbate concentration in the range 1-20%. Possible physical and chemical changes during heating of protein-ascorbate mixtures are discussed, as are possible mechanisms for the growth inhibition.

Treatment of raw soy flour at 75°C with 0.03M sodium sulfite for 1 hr completely inactivated trypsin inhibitors leaving no sulfite residues in the soy proteins. Rat feeding studies showed that the protein efficiency ratio (PER) increased from 1.55 for raw flour to 2.11 for heated flour and 2.49 for flour heated in the presence of sulfite. This nutritional improvement was accompanied by enhancement in in viva nitrogen digestibilities. Pancreas weights were elevated in rats fed raw soy flour, but not in those consuming heated soy flour, with or without sodium sulfite. Knowledge of the factors that decrease the stability of potentially toxic trypsin inhibitors may be useful for improving the quality of certain foods.

Growth assays on a synthetic amino acid diet fed to mice showed that substituting D-methionine for the L-isomer resulted in a dose-dependent relative weight gain reaching approximately 76% when D-methionine was fed at a level equivalent to that optimal for the L-form. L-Cysteine and L-cystine stimulated growth in the presence of suboptimal levels of L-methionine, but D-cystine was growth depressing. L-Cystine is at least equally efficient in stimulating growth in the presence of D-methionine as in the presence of the L-isomer. Compared to a suboptimal (25% of maximum) level of L-methionine alone, supplements to this level L-methionine by N-acetyl-L-cysteine produced a weight gain of 214%; L-cysteine, 178%; L-cysteic acid, 154%; DL- + meso-lanthionine, 127%; L-cysteine sulfinic acid, 113%; D-cysteine, 76%; and S-methyl-L-cysteine, 13%. The observed growth-depressing effect of D-cysteine, D-cystine and S-methyl-L-cysteine at a concentration no greater than that optimal for methionine implies that these three sulfur-containing amino acids may be toxic. The results are discussed in terms of known and postulated transamination and transsulfuration pathways that govern the biological utilization of isomeric sulfur-containing amino acids and derivatives.

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