Lysine FDNB reactive

Table 13.6 Mean amounts of total lysine, FDNB-reactive lysine and truly digestible lysine in heat-treated meat and bone meals...

Fluorodinitrobenzene ( FDNB )-reactive lysine The direct FDNB method of Carpenter ( 8 ) was used with some further ( % J 0 ) modifications. 

Table I. Simple linear regression equations correlation between FDNB-reactive lysine and lysine determined by the other methods...

The gross protein value is probably the most commonly used biological method for evaluating proteins. Gross protein values measure the ability of proteins to supplement diets consisting largely of cereals and they correlate well with FDNB-reactive lysine figures. 

The chemical and enzymatic browning reactions of plant polyphenols and their effects on amino acids and proteins are reviewed. A model system of casein and oxidizing caffeic acid has been studied in more detail. The effects of pH, time, caffeic acid level and the presence or not of tyrosinase on the decrease of FDNB-reactive lysine are described. The chemical loss of lysine, methionine and tryptophan and the change in the bioavailability of these amino acids to rats has been evaluated in two systems pH 7.0 with tyrosinase and pH 10.0 without tyrosinase. At pH 10.0, reactive lysine was more reduced. At pH 7.0 plus tyrosinase methionine was more extensively oxidized to its sulphoxide. Tryptophan was not chemically reduced under either condition. At pH 10.0 there was a decrease in the protein digestibility which was responsible for a corresponding reduction in tryptophan availability and partly responsible for lower methionine availability. Metabolic transit of casein labelled with tritiated lysine treated under the same conditions indicated that the lower lysine availability in rats was due to a lower digestibility of the lysine-caffeoquinone complexes. 

The influence of pH, time and temperature of treatment was investigated only in relation to the loss of fluorodinitroben-zene FDNB-reactive lysine. Fig. 2 shows the influence of pH in solutions containing 5 % casein and 0.2 % caffeic acid stirred for 3 h at room temperature. In the presence of tyrosinase, there was an extremely narrow region for maximum reactive lysine loss at pH 7. At pH 6.8 and pH 7.5, the loss was already half that at pH 7. In the absence of tyrosinase, there was little loss of reactive lysine up until pH 8.8 however, at pH 10.0 only 73 % of the original reactive lysine remained. The loss only occurred in systems which were stirred and oxygenated. 

COMPARISON OF DYE-BINDING LYSINE (DB-L) AND FDNB-REACTIVE LYSINE (F-DNB-L). (AFTER REFERENCE 15)... 

In Figure 1 it is clear that all five procedures showed at least some measure of sensitivity to lysine damage in all heated samples and that generally with increased severity of heat treatment a corresponding decrease in reactive lysine was detected. The SA and DAN methods and to a smaller degree the DB method were more sensitive than the FDNB method, particularly for the most severely damaged samples. The total lysine method was least sensitive. 

Although other newer chemical methods are starting to take over as a method of choice for reactive lysine ( JJ9 ), in the present work the classical direct FDNB method was selected as standard reference since many times it has been shown to give results that correspond closely with those from both biological evaluation and in vitro enzymic digestion ( 19, 20 ). 

Comparison with in vivo procedures Although the FDNB procedure proved to be a suitable reference method, there is no doubt that all methods should be ultimately compared to in vivo procedures. For this reason selected samples were also analyzed by plasma amino acid and digestibility methods. Preliminary results ( Table II ) show that plasma lysine results correlated very well with results for lysine digestibility and FDNB lysine ( r =0.95 ), reasonably well with those for dansyl chloride lysine, succinic anhydride reactive lysine and dye binding lysine, but poorly with total lysine. Although the absolute values were in many cases very different, it is apparent that all methods except total lysine can be used to at least indicate the extent of lysine damage. 

If the four reactive-lysine methods studied are compared on the basis of simplicity and rapidity, the FDNB method is not preferred since it is fairly complicated, takes ca. 20 h per assay, and reqires special precautions due to the vesicant effects of the... 

We heated an albumin mix with 1 % formaldehyde for 2 h at 80 °C (Table 4) and found that lysine digestibility and protein digestibility for chicks were reduced to 75 % and 72 % of their original values and that chemically reactive lysine as measured by fluorodinitrobenzene (FDNB) was 76% of its original value. Biologically available lysine... 

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