Racemization essential amino acids

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. 

The percentage of D-enanticmers relative to the total amount of the amino acid residue can be calculated by the relation (D/DfL) x 100. D-Aspartic acid accounts for 30% of that residue (which is thus 60% racemized) in treated casein, Pranine-D, and wheat gluten. In these three proteins, 22-30% of the phenylalanine (an essential amino acid) is the D-enanticmer, and in wheat gluten, 26% of glutamic acid has been converted to the D-form. 

Amino acids are racemized by concentrated acid (50, 51, 52) and base (52, 53, 54) at elevated temperatures, and some preliminary experiments have shown that at 116° aspartic acid is racemized slowly at neutral pH values (38, 55). Also, the amino acids in fossil shells are partially racemized, with the amounts of racemization increasing with the age of the shell (56, 57, 58) racemization is essentially complete in shells of Miocene age. Since the kinetics of racemization of amino acids have not been investigated in detail at any pH, I have recently carried out a detailed study of the kinetics of racemization of aspartic acid between pH 0 and 13 and also the kinetics of racemization of phenylalanine, alanine, and isoleucine at pH 7.6. The results of these investigations are reported herein. 

The nutritive quality of any protein depends on three factors amino acid composition, digestibility, and utilization of the released amino acids. Racemization brought about by processing can impair the nutritive value of proteins by (1) generating non-metabo-lizable forms of amino acids (some D-enantiomers), (2) creating peptide bonds inaccessible to proteolytic enzymes, possibly restricting release of essential amino acids (3) toxic action (or interaction) of specific D-enantiomers. Little is known concerning the nutritional consequences of human consumption of racemized proteins. 

With respect to racemization, a major consideration is whether humans can utilize the D-enantiomers of essential amino acids (Stegink, 1977 Masters and Friedman, 1980). Berg (1959) reviewed human and animal utilization of free D-amino acids. Uptake of L-amino acids is invariably faster than that of the D-enantiomers in the intestine (Gibson and Wiseman, 1951 Finch and Hird, 1960) and kidney (Rosenhagen and Segal, 1974). Once absorbed, D-amino acids can be utilized by two pathways (1) racemases (or epimerases) may convert D-isomers to DL-forms or (2) D-amino acid oxidases may catalyze oxidative deamination to a-keto acids, which can be specifically reaminated to the natural L-forms (Meister, 1965). Only the latter activity has been demonstrated in mammals. However, when mixtures of D-amino acids are fed to rats, the oxidase system can be overloaded so that the D-enantiomers of essential amino acids cannot be transaminated in sufficient quantity to support growth (Wretlind, 1952). 

W. C. Rose and coworkers (67) were able to maintain N-balance in young men with high caloric diets constructed from racemate containing mixtures of nine crystalline amino acids, purified fats, carbohydrates and vitamin mixtures. Since the nine essential amino acids of those diets had to be converted in part to the ten unessential amino acids, the requirement of each of-the nine essential amino acids appears to be about twice that shown in Table V. 

Synthetic pathways towards the dimethylene isosteres comprise several disconnections, indicated by the essential bond forming reactions (Scheme 2). Several methods yield racemic dipeptide analogues. These are usually incorporated into the peptide sequence and the resulting epimeric peptides are separated. When either R1 or R2 = H, asymmetric syntheses towards the required enantiomer are available. When both R1 and R2 H, only the reduction of the i )[CH=CH] precursor yields homochiral compounds. As many co-amino acids (R1 = R2 = H) are commercially available, their synthesis needs not be discussed here. 

Taking advantage of the ready availability of racemic ibuprofen, the resolution approach for production of (S)-(+)-ibuprofen becomes an attractive alternative. Merck s resolution process involves the formation of a diastereomeric salt of ibuprofen with (S)-lysine, an inexpensive and readily available natural amino acid.45 The racemic ibuprofen is mixed with 1.0 equivalent of (5)-lysine in aqueous ethanol. The slurry is agitated to allow full dissolution. The supernatant, which is a supersaturated solution of ibuprofen-lysine salt, is separated from the solid and seeded with (.S )-ibuprofcn-(.S )-lysine to induce crystallization. The precipitated solid is collected by filtration, and the mother liquor is recycled to the slurry of racemic ibuprofen and (S)-lysine. This process is continued until essentially all (S)-ibuprofen in the original slurry is recovered, resulting in the... 

Vitamins, cofactors, and metals have the potential to broaden the scope of antibody catalysis considerably. In addition to hydrolytic and redox reactions, they facilitate many complex functional group interconversions in natural enzymes.131 Pyridoxal, for example, plays a central role in amino acid metabolism. Among the reactions it makes possible are transaminations, decarboxylations, racemizations, and (3,y-eliminations. It is also essential for ethylene biosynthesis. Not surprisingly, then, several groups have sought to incorporate pyridoxal derivatives into antibody combining sites. 

The ease of racemization of chiral a-amino aldehydes under MBH conditions is undoubtedly a major difficulty in studying diastereoselective reactions [53]. Epi-merization can be essentially avoided by conducting the reaction at low temperature [54, 67], or it can be minimized at room temperature when a conformation-ally restricted amino aldehyde, such or N-trityl-azetidine 2-(S)-carboxyaldehyde is used [54]. The use of ultrasound also increases the rate of the MBH reaction, avoiding racemization almost completely, even at room temperature [55]. When adding various a-amino acid-derived aldehydes to methyl acrylate using DABCO... 

Enantiomeric Mannich bases may be obtained either by using optically active starting materials or by optical resolution of racemic derivatives. In the former case, the reactants are mostly provided by natural products, such as components of essential oils (e.g., camphor ), hormones, nucleic acids, employed as substrates, or a-amino acids - mainly used as amine reagents, etc. A list of optically active reactants reported in the literature is summarized in Table 12. 

Amino acid racemization analyses on shell are carried out in two ways sampling all the amino acids extracted from the shell (called total) and sampling only the protein-bound amino acids (referred to as protein). The first method is essentially identical to the processing... 

The effects of pH on racemization rates of free amino acids in aqueous solution are complex (21). However, when bone fragments were heated at 100°C in solutions of pH 2-9, the rate constant for aspartic acid racemization was essentially independent of pH (37). It was proposed that the phosphate from the bone s inorganic phase acts as a buffer. This same buffering action would take place in a bone under natural conditions of burial as well. 

The azide procedure for peptide synthesis and particularly for fragment condensations is considered to be a mainly racemization free method. This low racemization tendency of azides was explained by several theories, which have been reviewed.t l The most plausible cause of racemization is the formation of oxazoles (Scheme 3) and the related enolization. In presence of bases the a-carbon proton is readily abstracted to form an anionic oxazol-5(4//)-one resonance system.For the formation of the oxazol-5(4//)-one the influence of the substituent Y on the a-carbonyl is essential. Since the a-carbonyl group of amino acid azides are less activated and thus relatively insensitive to oxygen containing nucleophiles such as water and alcohols, oxazol-5(4//)-one formation is largely prevented. It was proposed that the soft electron shell of the azide shields the a-carbonyl atom, so that only strong nucleophiles can attack it.t 1 The reactivity towards amines can be explained in a manner analogous to the aminolysis of anchimerically assisted active esters.h 1... 

---