Lysine nutritional availability

Acidic pH conditions caused the removal of dtraconyl groups from the citraconylated yeast proteins, so the environmental conditions of the stomach, i.e., pH and temperature, may cause the deacylation to occur following consumption of citraconylated proteins. This then would make lysine nutritionally available. The peptic digestibility of the citraconylated and decitraconylated yeast proteins was determined soy proteins and egg albumin were studied also for comparison. In a typical experi-... 

Many pharmaceutical compounds are weak acids or bases that can be analyzed by an aqueous or nonaqueous acid-base titration examples include salicylic acid, phenobarbital, caffeine, and sulfanilamide. Amino acids and proteins can be analyzed in glacial acetic acid, using HCIO4 as the titrant. For example, a procedure for determining the amount of nutritionally available protein has been developed that is based on an acid-base titration of lysine residues. ... 

Lysine is an essential amino acid with an e-amino group on the side chain that can react with various food components. As known, reaction of the e-amine can render lysine nutritionally unavailable reducing the nutritional value of food. While the determination of total lysine is straightforward (it is stable to acid hydrolysis), the determination of available lysine is difficult as lysine adducts are labile to the standard acid hydrolysis. A solution to this problem consists of derivatizing the e-amino group with a chromophore such as l-fluoro-2,4-dinitrobenzene (FDNB) to form a derivate which is stable to optimized hydrolysis conditions [222]. 

While the determination of total lysine is rather straightforward, the determination of free or available lysine is more problematic. In this situation, the term free is meant to imply that the e-amine of the lysine side chain has not reacted with various components of the sample matrix (most commonly carbohydrates via Maillard browning). This is important because reaction of the e-amine can render lysine nutritionally unavailable and the nutritive value for that protein is then diminished if lysine is the limiting amino acid (which is often the situation with soy proteins). While enzymatic digestion in the human gut may not release the modified lysine in a nutritionally available state, often these lysine adducts are labile to the standard acid hydrolysis in 6N HC1 at 110°C. This results in total lysine values that overestimate the amount of nutritionally available lysine. 

As shown in Table IX, the lysine availability (%) showed changes for the three samples. However, the unavailable lysine (total lysine minus available lysine) contents in bread (whole), bread crust and crumb were only 0.04, 0.05, 0.03%, respectively. Table 7 shows that the unavailable lysine contents for all pizza crusts, baked and unbaked, varied only from 0.02 to 0.03%. These data indicate the reduction of lysine caused by baking is mainly shown by the total lysine analysis. It appears then that there is no need to run available lysine determinations for such bakery foods. This finding also suggests that the nutritive loss of bread and pizza crusts was primarily due to the destruction of lysine in those products to a lesser extent baking caused it to become unavailable. 

Of the eight amino acids essential for man and other animals, lysine is the most easily damaged by processing and/or storage of food. This damage or modification results in a reduction in nutritional availability of lysine. Available lysine can be... 

The nutritional availability of lysine in foods of plant and animal origin may be significantly decreased by the ready involvement of the C-NH2 group of lysine in intra- and intermolecular... 

Lysine locked up as e-glycosylamine appears to be nutritionally available.7... 

Because of the partial recovery of lysine, the interpretation of the results of the analysis is complicated. It is important to bear in mind that lysine locked up in the Amadori compound, although partially recoverable by amino acid analysis, is no longer nutritionally available. 

The determination of nutritionally available lysine was dealt in Chapter 2. 

Nutritional Availability of Acylated Lysine and Proteins. The successful use of chemically derivatized proteins as food ingredients requires that they be digestible and nontoxic, and that the modified amino acid residues should be available nutritionally. Nutritional studies using modified food proteins are limited. The nutritional availability of several acylated lysines were studied by Bjarnason and Carpenter (105) and Mauron (106) and the results are summarized in Table III. The bioavailability of the acylated lysine varied significantly with the type of the acyl groups (see Table III). In addition acylated proteins (acetylated and succinylated) gave lower responses to the growth activity for the rats than equivalent supplements of unmodified proteins... 

Table III. Nutritional Availability of Acylated Lysine Percent Utilization as a Source Derivatives ...

Dehydrated milk-based foods is the major source of D-lactulose-amino acids in human nutrition. The dairy industry widely uses milk dehydration to manufacture powdered milk as a base for numerous dairy products, such as infant formulas, confectionaries, reconstituted milk etc. During the process of heating, drying and storage, lactose in milk can readily interact with amino compounds that are naturally present, primarily lysine residues in milk proteins. Estimated contents of D-lactulose-amino acids in selected products are compiled in Table 2. Commercial dairy products may contain, therefore, up to 40% of protein lysine in form of lactulose-lysine. Dietary availability of D-lactulose-amino acids is similar to that of the fructosamine derivatives. An ample evidence exists that the Amadori-type lysine glycoconjugates are not available to mammals as a nutrient and that lactulose-lysine is partially absorbed into the bloodstream and excreted unchanged. ... 

The difference between total and available lysine for each pizza crust was small, indicating that the nutritive loss in pizza crust should be attributed largely to the destruction of a portion of lysine in such crust. 

Lysine locked up in e-Amadori compounds becomes nutritionally not available this poses an analytical problem. 

These reactions create cross-links between the protein chains, which are responsible for the loss in the nutritive value of the severely heated proteins. This loss can be attributed either to the reduction of the total nitrogen digestibility or to the reduction of the availability of the lysine residues engaged in such isopeptides. 

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. 

Limiting essential amino acids covalently attached to proteins by using activated amino acid derivatives can improve the nutritional quality and change the functional properties of proteins. The best chemical methods for incorporating amino acids into water-soluble proteins involve using car-bodiimides, N-hydroxysuccinimide esters of acylated amino acids, or N-carboxy-a-amino acid anhydrides. The last two methods can give up to 75% incorporation of the amount of amino acid derivative used. With the anhydride method, as many as 50 residues of methionine have been linked to the 12 lysine residues of casein. The newly formed peptide and isopeptide bonds are hydrolyzed readily by intestinal aminopeptidase, making the added amino acids and the lysine from the protein available nutritionally. 

Tt is well recognized that the nutritional value of dietary proteins de-pends primarily on the content of their constituent amino acids, especially of their essential amino acids. Because of deficiencies of lysine and methionine, and to a lesser extent of a few other amino acids, proteins from plants and other alternative sources have low biological quality. Moreover, incomplete digestion of the protein also may result in a lack of complete availability (I) of the essential amino acids and may further reduce its value. In many raw plant foodstuffs such as soybeans, common beans, or unprocessed protein foods, undenatured... 

Such chemical changes may lead to compounds that are not hydrolyzable by intestinal enzymes or to modifications of the peptide side chains that render certain amino acids unavailable. Mild heat treatments in the presence of water can significantly improve the protein s nutritional value in some cases. Sulfur-containing amino acids may become more available and certain antinutritional factors such as the trypsin inhibitors of soybeans may be deactivated. Excessive heat in the absence of water can be detrimental to protein quality for example, in fish proteins, tryptophan, arginine, methionine, and lysine may be damaged. A number of chemical reactions may take place during heat treatment including decomposition, dehydration of serine and threonine, loss of sulfur from cysteine, oxidation of cysteine and methio-... 

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