Lysine unavailable

Raw defatted cottonseed flours contain 1.2—2.0% gossypol [303-45-7] (7) (19). When cottonseed is treated with moist heat, the S-amino group of lysine and gossypol forms a derivative that is biologically unavailable thereby inactivating gossypol but further lowering the effective content of lysine. 

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]. 

It may be concluded that cross-linking due to isopeptide formation is probably responsible for decreased overall digestibility of the protein (, 22) thus, if the digestive enzymes are not able to release the smaller peptides, then the question of availability of the isopeptides per se is beside the point. Ford (13) points out that the rate of digestibility of isopeptide links may preclude the maximum availability of the lysine as a result of the lysine entering the system too late to be effectively utilized by the tissue. Thus, the lysine in these linkages would be at least partly unavailable. Other workers have shown that small quantities of isopeptides are found in the urine (14). 

Recently, Jeunink and Cheftel (47) have attempted to illuminate the mechanism of extrusion texturization. They attributed the low product solubility to new disulfide bonds and non-covalent interactions, but they could not rule out a contribution due to formation of isopeptide links. The small observed increase in unavailable lysine would be consistent with isopeptide formation. 

The authors recognized that exhaustive acylation may lower PER of the protein due to unavailability of acylated lysine, and suggested that such derivatized proteins serve a functional role and not constitute a significant source of nutritive protein. 

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. 

Of all amino acids involved in the browning reaction, lysine with its c-amino group is especially susceptible to side reaction and crosslinking and becomes unavailable. 

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. 

This leads to a decrease in the biological value of the protein. Milk is a good model to study the nutritional effects of the Maillard reaction because of its high levels of lysine and lactose. In processed milks, unavailable lysine is present only as e-deoxylactulosyllysine (9,24) its level which depends on the type of treatment can be measured by the furosine method (24,31). 

These effects were described by de Groot and Slump (99) followed by Provansal et al. (94). Later, it was confirmed that the lysine moiety of lysinoalanine was completely unavailable to the rat and only partly available to the chick, while a certain part of the cysteine moiety of lanthionine (32% to 52% according to the racemic mixture) was available to chicks (100) (see Table I). 

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-... 

A proportion of the vitamin Be in foods may be biologically unavailable after heating, as a result of the formation of (phospho)pyridoxyllysine by reduction of the alditnine (Schiff base) by which pyridoxal and the phosphate are bound to the e-amino groups of lysine residues in proteins. A proportion of this pyridoxyUysine may be useable, because it is a substrate for pyridoxamine phosphate oxidase to form pyridoxal and pyridoxal phosphate. However, it is also a vitamin Be antimetaboUte, and even at relatively low concentrations can accelerate the development of deficiency in experimental animals maintained on vitamin Be-deficient diets (Gregory, 1980a, 1980b). 

On the contrary, the nutritional value of oxidized lipid-protein interaction products is substantially lower than that of the original lipoproteins. The main reason is the lower digestibility most covalent bonds formed in the interactions are not attacked by proteases under the conditions of digestion. The 6-amino group of bound lysine is particularly sensitive to interactions with carbonylic oxidation products (Janitz et al., 1990), and the resulting imine bonds substantially reduce the lysine availability. Lysine losses correlate with the increase in fluorescence. Other amino acids, such as tyrosine, tryptophan, and methionine, are also partially converted into unavailable products. Interaction products may be allergenic even when allergenic proteins have reacted (Doke et al., 1989). 

Kinsey and Grant (384) have noted that casein after treatment with mustard at pH 9.3 and subsequently refluxed with 1 1 HCl no longer supported growth of rats. They were able to show that the histidine lysine, methionine and threonine were unavailable. When the treatment with mustard was performed at pH 7.4 the first three of the above amino acids wero still unavailable but the threonine w>.e free. Mustardization of horse serum globulin and albumin at pH 7.5 affected the methionine and lysine of these proteins. 

When foods are heated, pyridoxal and pyridoxal phosphate, can react with the e-amino groups of lysine to form a Schiff base (aldimine). This renders both the vitamin Bg and the lysine biologically unavailable more importantly, the pyridoxyl-lysine released during digestion is absorbed and has anti-vitamin B antimetabolite activity. 

Reaction of a malondialdehyde (MDA) carbonyl group with amino groups leads to the formation of imines, but formation of these structures is of no nutritional concern, because they are hydrolysed at the acidic pH of the stomach. AT-Prop-2-enals, which are absorbed from the gut, are also formed in neutral or acidic aqueous media, but most of the absorbed material is not metaboHsed. A third type of reaction products are unavailable 4-substituted l,4-dihydropyridine-3,5-dicarbaldehydes, which arise in reactions of malondialdehyde with amino compounds, such as lysine, in the presence of alkanals. Examples of malondialdehyde reaction products with lysine are lV -(prop-2-enal)lysine, so-called MDA-lysine, JV -(prop-2-enal)lysine, ArAf -di(prop-2-enal)lysine (3-150) and conjugated cross-hnk in proteins termed lysine-MDA-lysine. An example of the reaction product of lysine with malondialdehyde and acetaldehyde is Ar,Ar-di(4-methyl-l,4-dihydropyridine-3,5-dicarbaldehyde)lysine, which, for example, arises in the reaction of bovine serum albumin with malondialdehyde and acetaldehyde (3-151). 

Amino acids— These nutrients—which are provided in the form of pure, white powders—are usually added to flours or meals prior to their incorporation into various products. In certain cases, increased amounts should be added because heat processing by baking and/or toasting renders some of the lysine and other amino acids unavailable. 

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