Effects of alkali-treated proteins

Nutritional and Physiological Effects of Alkali-Treated Proteins. The first effect of the alkaline treatment of food proteins is a reduction in the nutritive value of the protein due to the decrease in (a) the availability of the essential amino acids chemically modified (cystine, lysine, isoleucine) and in (b) the digestibility of the protein because of the presence of cross-links (lysinoalanine, lanthionine, and ornithinoalanine) and of unnatural amino acids (ornithine, alloisoleucine, / -aminoalanine, and D-amino acids). The racemization reaction occurring during alkaline treatments has an effect on the nitrogen digestibility and the use of the amino acids involved. 

De Groot, A. P., Slurp, P., Feron, V. J. and Van Beek, L. (1976). Effects of alkali-treated proteins feeding studies with free and protein-bound lysinoalanine in rats and other animals. J. Nutr. 106, 1527-1538. 

Gould, D. H. and MacGregor, J. T. (1977). Biological effects of alkali-treated protein and lysinoalanine an overview. In "Protein Crosslinking Nutritional and Medical Consequences," M. Friedman, Ed., Part B,... 

Since lysinoalanine and at least one D-amino acid are toxic to some animals (35), we wished to distinguish their effects in alkali-treated proteins. Such discrimination is possible, in principle, since we have found that acylating the e-amino group of lysine proteins seems to prevent lysinoalanine formation. Since lysinoalanine formation from lysine requires participation of the e-amino group of lysine side chains, acylation of the amino group with acetic anhydride is expected to prevent lysinoalanine formation under alkaline conditions if the protective effect survives the treatment. This is indeed the case (16). 

L. F. (1979). Biological effects of alkali-treated soy protein and lactalbunin in the rat and mouse. 

Karaylannis, N. I., MacGregor, J. T. and Bjeldanes, L. F. (1979b). Biological effects of alkali-treated soy protein and lactalbumln In the rat and mouse. Food and Cosm. Toxicol., 17, 509-604. 

The most outstanding effect of the alkali-treated proteins that has been reported is due to the presence of lysinoalanine, which induces renal alterations in rats, designated nephrocytomegaly, which consists of enlarged nuclei and increased amounts of cytoplasm in epithelial cells of the straight portion of the proximal tubules (101,102,103,105,106). This effect has never been found in other species such as mice, hamsters, quails, rabbits, dogs, and monkeys (103). 

Karayiannis, N. (1976). Lysinoalanine Formation in Alkali Treated Proteins and their Biological Effects, Ph.D. Thesis. University of California, Berkeley. 

Effects of Alkali. Although alkali had been used to treat certain foods for many years, only recently has it been used widely by the texturized protein industry. Alkali-mediated degradation of proteins has long been known (13, 39-44). Some of the main initial reactions are apparently / -eliminations of cystines and substituted serines and threonines. The products (or their intermediates) then alkylate various other amino acid side chains to form substances like lanthionine and lysino-alanine [N -(DL-2-amino-2-carboxyethyl)-L-lysine]. Possible toxicities are currently under investigation (45, 46), but nutritional losses could also be important. 

Tovar (14) performed an experiment designed to evaluate the in vivo effect of D-amino acids in alkali-treated protein without the presence of lysinoalanine. In addition, either lime or caustic soda were used to investigate whether these alkalis had different effects vivo Zein was exposed to O.IN alkali for 4 hours at 85 C. Because lysine is absent from zein, no lysinoalanine formation was observed. Diets were prepared using untreated or alkali-treated zein and were supplemented with casein and free amino acids to meet the nutritional requirements of the... 

It is Important to know the separate effects of racemization and crosslinking for several proteins, especially those important in food systems. Therefore, the purpose of this study was to isolate racemization from crosslinking, examine the effects of racemization on i vitro digestibility of alkali-treated zein and in vivo accumulation of alkali-trated protein by Isolated rat jejunum. 

Table 2. Effect of time of treatment on LAL content of alkali-treated soy protein.1...

Table 4. Effect of glucose on amino acid composition of alkali -treated soy protein (Promi ne -D).l...

Tables 6 and 1 compare the effects of several organic and inorganic compounds on the lysinoalanine and lysine contents of alkali-treated wheat gluten and soybean protein. These results show that all these compounds partly inhibit lysinoalanine formation. The extent of inhibition may vary from protein to protein and should be related to both the content and reducibility of the disulfide bonds (Friedman, 1978a Finley et al., 1978 a,b Masri and Friedman, 1982).

Figure 17. The effect of disulfide bond modification of turkey ovomucoid by alkali on inhibitory activity against trypsin (T), a-chymotrypsin, (C), and subtilisin (S). Turkey ovomucoid (O.lOmM) was treated with alkali (lOOmM NaOH) at 23°C. Sulfhydryl content (moles per mole of protein)...

The nutritional and physiological effects of the alkali-treated food proteins have been studied extensively. Efforts have been concentrated mainly on the effects of lysinoalanine. 

Adverse effects of exposing proteins to alkaline conditions are known. As early as 1913, it was shown that severely alkali-treated casein fed to dogs was eliminated unchanged in the feces, that it was not attacked by putrefactive bacteria and that trypsin or pepsin was unable to hydrolyze it (9). Ten Broeck reported that egg albumin treated with 0.5 N. NaOH for 3 weeks at 37° had no immunological properties (10). The nitrogen digestibility values of 0.2 M and 0.5 M NaOH-treated casein (80°C, 1 hr), as determined in rats, was 71 and 47%, respectively, as... 

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