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Department of Agrifood Molecular Sciences, Section of Chemistry, University of Milan, Via Celoria 2, Milano, I-20133, Italy.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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The lysinoalanine content of 18 different kinds of formulas for enteral nutrition was determined by HPLC after derivatization. The liquid formulas have an average value of 528 µg/g protein LAL, ranging from 160 to 800 µg/g protein (average content of formulas for pediatric use 747 µg/g protein). These values are rather high considering that the average value detected in UHT-treated drinkable milk is 117 µg/g protein. In principle, the preparation of caseinates and the thermal stabilization of the end products are the two steps more favorable for the formation of LAL. The fact that the five samples stabilized by an UHT-treatment have an average value of 512 µg/g protein suggests that the LAL content depends more on the quality of the starting ingredients than on the sterilization process. A better selection of the starting ingredients should improve the quality of formulas for enteral nutrition, which is very desirable when formulating foods for consumers with very high nutritional demands.
Key Words: lysinoalanine • thermal damage • cross-linked amino acids • enteral nutrition
Abbreviation key: LAL = lysinoalanine, FMOC = 9-fluorenylmethylformate, SPE = solid phase extraction
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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In particular, protein-bound amino acids may undergo modifications in their side chains, that may require the participation of sugars, as in the Maillard reaction, or not, as in the formation of cross-linked amino acids. The most investigated of these unusual amino acids is lysinoalanine [LAL, N
-(R,S-2-amino-2-carboxyethyl)-S-lysine], which has been used as a marker of thermal damage to foods because it has the analytical advantage of being stable in the acidic conditions of protein hydrolysis (Maga, 1984; Friedman, 1999).
The mechanism of the formation of LAL is relatively simple: the first step is the elimination of a leaving group from O-phosphorylserine, O-glycosylserine, or cystine to generate a dehydroalanine residue, which may undergo Michael addition by the nucleophilic side chain of another amino acid, such as the -amino group of lysine. This process is favored by aqueous alkali treatments, such as those applied in preparing some milk derivatives, in particular sodium caseinate, and soybean protein concentrates (Friedman, 1999).
Many studies have investigated the effects of the formation of LAL on protein functionality and nutritional value. Generally the concentration decrease of essential amino acids is not the most relevant consequence, but LAL, being like a bridge or a cross-linker between two different parts of the protein chain (Pellegrino et al., 1998), impairs the approach of enzymes with the consequence of decreasing protein digestibility. For this reason, many groups have studied the digestibility of proteins treated with alkali (Savoie et al., 1991; Anantharaman and Finot, 1993; Guo et al., 1999) and other nutritional and toxicological consequences of LAL formation in foods (Karajiannis et al., 1980; Maga, 1984; Friedman, 1999). Adverse effects on growth, protein digestibility, protein quality, and mineral bioavailability and utilization have been described (Sarwar et al., 1999).
There are also indications about toxicity, as LAL was shown to provoke lesions in rat kidney cells causing nephrocitomegaly (Friedman et al., 1984; Friedman and Pearce, 1989). Although the effects seem very species-specific, these observations have promoted investigation on humans, in particular on preterm infants. The higher level of Maillard reaction products and LAL in infant formulas compared to breast milk had no influence on creatinine clearance or electrolyte excretion and there was no evidence of tubular damage as determined by the urinary excretion of four kidney-derived enzymes, however feeding with formulas resulted in a general increase in urinary microprotein levels (Langhendries et al., 1992).
The thermal damage of the formulas for parenteral nutrition has been investigated by several groups, especially regarding the formation of color and the degradation of essential amino acids (Fry and Stegink, 1982; Labuza and Massaro, 1990; Stegink et al., 1981). We verified, instead, that the thermal damage of formulas for enteral nutrition had never been studied before, although caseinates are particularly susceptible to the formation of LAL. This has prompted us to collect samples of commercial formulas and to quantify this cross-linked amino acid as a marker of their nutritional quality.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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LAL Determination
The procedure for LAL determination included sample acid hydrolysis, FMOC derivatization of the amino compounds, selective solid phase extraction (SPE) to purify LAL derivatives, and subsequent HPLC analysis with fluorescence detection (Pellegrino et al., 1996). The HPLC elution solvents were prepared from stock solution I (0.5% tetrahydrofuran and 0.1% ethyl acetate in 30 mM potassium acetate; vol/vol/vol) and stock solution II (80% acetonitrile in 100 mM sodium acetate; vol/vol). The stock solutions were mixed (vol/vol) for preparing the elution solvent A (I:II=70:30) and the elution solvent B (I:II=25:75). The elution solvent C was 100% of stock solution II. An aliquot of 20 µL of each sample purified by SPE was injected into the HPLC column protected by a guard column cartridge; the separation was carried out by a linear elution gradient. Fluorescence detection was performed with excitation at 266 nm and emission at 310 nm. Under the conditions described, the isomers SR- and SS-LAL were eluted as two different peaks with tR 25 and 26 min respectively.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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The analysis of LAL can be performed by different methods, for example by ion-exchange chromatography (Friedman et al., 1984; Sanderson et al., 1978) or by GC/MS after derivatization of both amino and carboxy groups (Büser and Erbersdobler, 1988; Hasegawa et al., 1987; Liardon et al., 1991). In recent years, however, HPLC has become more and more important. Warthesen and Wood-Rethwill (1984) have reported a method in reversed phase after derivatization of the three amino groups with dansylchloride, that has been improved in more recent times (Faist et al., 2000; Moret et al., 1994). Another HPLC method, based on derivatization with FMOC-Cl, solid phase extraction, reverse phase chromatography and fluorescence detection, has been proposed by Pellegrino et al. (1996) for the quantification of LAL at very low level in cheese.
The last method was selected for our study, because it is very sensitive, with a threshold limit around 1 µg/g protein. As already indicated by Pellegrino et al. (1996), LAL appears in the chromatograms as a double peak due to the presence of the S,S- and S,R-diastereoisomers. The application of this methodology to products for enteral nutrition was very simple and gave good reproducibility (the standard deviation was 2–3%). In total 18 different kinds of commercial formulas were analyzed (for each of them at least two samples from different lots were obtained, the differences of LAL concentration in different lots may reach 9–10%). The samples were labeled with letters, the therapeutic indication is reported in Materials and Methods, whereas details of the composition, obtained from the labels, are reported in Table 1
. Each sample was analyzed in triplicate: the results are shown in Figure 1.
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Very variable contents were detected in the 5 samples (A, B, C, E, F) that had been sterilized by an UHT treatment, as indicated in the label.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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So far infant formulas are certainly the foods, which have been investigated in most detail from the point of view of LAL formation. Most of these studies were performed in the 80’s (Bellomonte et al., 1987; De Koning and Van Rooijen, 1982; Friedman, 1999; Fritsch and Klostermeyer, 1981; Resmini et al., 1985), when the level of LAL could reach 150 to 920 µg/g protein in powdered formulas and 160 to 2100 µg/g protein in liquid formulas. These results induced manufacturers to look for milder technologies for manufacturing the ingredients and for stabilizing the end-products, which resulted in a dramatic decrease of the LAL content in the following years. A recent investigation have shown that current commercial spray dried formulas, produced with very mild technologies, contain only traces of LAL, whereas commercial liquid formulas 131 µg/g protein (Arnoldi, 2000; D’Agostina et al., 2002), comparable with UHT-treated drinkable milk (Faist et al., 2000). The consistent improvement of the quality of these products in respect to the past indicates that a careful choice of the manufacturing procedures permits to reduce considerably the thermal damage to proteins.
The liquid formulas for enteral nutrition investigated in this study have an average value of 528 µg/g protein LAL, ranging from 160 to 820 µg/g protein (only 3 samples out of 17 have less than 350 µg/g protein). These values are rather high, for example much higher than in UHT-treated drinkable milk, average 117 µg/g protein (Faist et al., 2000), and the values detected in the two samples for pediatric use (C and T, which contained 677 and 817 µg/g protein of LAL, respectively) are particularly worrying.
The two steps more favorable for the formation of LAL are the preparation of caseinates and the thermal stabilization of the end-products. The indication of the stabilization method on these formulas is optional, however, the 5 samples stabilized by an UHT-treatment, as indicated in the label, have an average value of 512 µg/g protein, suggesting that the LAL contents depends more on the quality of the starting ingredients than on the sterilization process of the end-products. Rennet casein has an average content of 114 µg/g protein (range 21–178 µg/g), whereas sodium and calcium caseinates an average content of 977 µg/g protein (Pellegrino et al., 1996), showing that the alkali process for producing caseinates is particularly favorable for the formation of LAL. However, the LAL content may vary from 128 to 2992 µg/g protein (Pellegrino et al., 1996) depending on the manufacturing conditions, a fact that clearly shows the possibility of producing caseinates with satisfactory nutritional features. A better selection of the raw ingredients should improve the quality of formulas for enteral nutrition, a very desirable achievement while formulating products for consumers with very high nutritional demands, at least in the case of products for pediatric use.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Corresponding author:
A. Arnoldi; e-mail:
anna.arnoldi{at}unimi.it.
Received for publication March 14, 2002. Accepted for publication February 6, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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-DL-(2-amino-2-carboxyethyl)-U-14C-L-lysine (LAL) in Sprague-Dawley rats. J. Nutr. 110:2065–2077.
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