Casein tryptophane reaction

Tryptophane Reactions. To 2 ml of protein solution add 2 ml glacial acetic acid, and mix. Add this mixture very carefully to 5 ml of concentrated sulfuric acid, pouring it slowly down the side of the tube so that the two liquids do not mix. If the proteins contain tryptophane, a violet ring will form at the interface of the two liquids. In the same manner test solutions of gelatin, casein, and egg albumen. 

To study the effect of the Maillard reaction on nutritive value of protein, Patton, et a l. (57) heated purified casein and soybean globulin in 57. glucose solution for 24 hrs at 96.5°C, and found significant losses of lysine, arginine, tryptophan, and histidine (52). 

Tryptophan was first isolated only at the beginning of this century (411). A number of color reactions of proteins were extensively studied in the latter half of last century and numerous attempts were made to isolate the chromogen responsible. The name tryptophan was given to this chromogen in 1890 by Neumeister (645). The chromogen was soon associated with the substance giving rise to indole on bacterial putrefaction of proteins. The failure of many early attempts to isolate tryptophan was probably due to the fact that it is destroyed on acid hydrolysis. The successful isolation by Hopkins and Cole (411) used enzymic hydrolysis of casein, but the chief reasons for their success were their discovery of mercury salts as... 

Traditionally fermented dairy products have been used as beverages, meal components, and ingredients for many new products [60], The formation of flavor in fermented dairy products is a result of reactions of milk components lactose, fat, and casein. Particularly, the enzymatic degradation of proteins leads to the formation of key-flavor components that contribute to the sensory perception of the products [55], Methyl ketones are responsible for the fruity, musty, and blue cheese flavors of cheese and other dairy products. Aromatic amino acids, branched-chain amino acids, and methionine are the most relevant substrates for cheese flavor development [55]. Volatile sulfur compounds derived from methionine, such as methanethiol, dimethylsulflde, and dimethyltrisul-fide, are regarded as essential components in many cheese varieties [61], Conversion of tryptophan or phenylalanine can also lead to benzaldehyde formation. This compound, which is found in various hard- and soft-type cheeses, contributes positively to the overall flavor [57,62]. The conversion of caseins is undoubtedly the most important biochemical pathway for flavor formation in several cheese types [62,63]. A good balance between proteolysis and peptidolysis prevents the formation of bitterness in cheese [64,65],... 

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. 

Protein-quinone reactions seem to have little influence on protein digestibility or on the bioavailability of tryptophan. Methionine does not appear to react covalently but may be extensively oxidized to its sulphoxide with some reduction in bioavailability. The reactions of cysteine were not investigated in our study because of the low level of cyst(e)ine in casein. Pierpoint (1969ab) has reported that cysteine reacts like lysine via a substitution into the quinone ring and it is also possible that cyst(e)ine residues may be oxidized (Finley and Lundin, 1980). 

---