Enzyme casein

Enzymic casein denaturation and coagulation are not covered in this chapter. Coagulation of the casein complex in milk initiated by enzymic cleavage of K-casein has been recently reviewed (McMahon and Brown 1984B) and is covered in Chapter 12. 

Biocatalysts which are reversibly soluble as a function of Ca + concentration were produced by covalent conpling of phosphoglyceromutase, enolase, peroxidase, and pyruvate kinase to ofsi-casein. The enzyme casein conjugates are soluble at a Ca + concentration below 20 mM bnt precipitate completely at a Ca + concentration above 50 mM. The precipitate redissolves when EDTA, a strong Ca +-binding agent, is added (128). 

Important molecules of this class are dentin phosphoryns or sialoproteins containing high aspartic acid (Asp) and extremely high serine (Ser) amoimts, which can be phosphorylated by the enzymes casein kinases I and II. Examples include rat dentin phosphoprotein with 43% Ser and 31% Asp [155], rat phosphoryn with 56% Ser and 32% Asp [ 156], hiunan dentin phosphophoryn with 58% Ser and 26% Asp [157] or a moUuscan shell glycoprotein with 32% Ser and 20% Asp [158]. It seems that phosphoserin (P-Ser) is of especial importance for polymer-crystal interactions, very Ukely in blocked sequences. In addition, the so-called Asprich proteins were recently identified in nacre and can contain up to 53 mol % Asp and 11% Glu [ 159,160]. 

Casein hydrolyzates are produced from dried casein. With appropriate heat treatment and the addition of alkaHes and enzymes, digestion proceeds. FoUowing pasteurization, evaporation (qv), and spray drying, a dried product of 2—4% is obtained. Many so-called nondairy products such as coffee cream, topping, and icings utilize caseinates (see Dairy SUBSTITUTES). In addition to fulfilling a nutritional role, the caseinates impart creaminess, firmness, smoothness, and consistency of products. Imitation meats and soups use caseinates as an extender and to improve moistness and smoothness. 

Destruction of the casein micelles in the milk with subsequent precipitation of the casein can be accomplished in a number of ways. The action of heat or the action of alcohols, acids, salts and the enzyme rennet all bring about precipitation. In commercial practise the two techniques used employ either acid coagulation or rennet coagulation mechanisms. 

The following description is from U.S. Patent 2,701,227 To 50 liters of distilled water there was added 10.17 kg of enzyme hydrolyzed casein (N-Z-Amine). The temperature was raised to 100°C and held until the casein digest solution was clear. The container was then cooled rapidly to 15°C and the cooled solution filtered through a coarse grade of filter paper. A small amount of toluene was added as a preservative and the solution... 

Milk from cows contains 3.2% protein, about 80% of which is casein. Casein is isolated by a precipitation process from milk, involving heating, rinsing to remove whey, and drying to a powder. The yield is about 3 kg/ 100 kg skim milk. Rennet casein is obtained when the casein is precipitated by chymosin enzyme, also known as rennet, and acid casein is produced when precipitation is accomplished by acidification. Acid casein is usually found in the form of sodium caseinate or calcium caseinate, which are water-soluble salts. Caseinates are made by reacting NaOH or CaOH with a slurry of casein curd or powder and then spray drying (Southward, 2010). 

The reason that the caseins, which constitute nearly 80% of bovine milk, are unfolded in their native states appears to be to facilitate digestion, since the open rheomorphic structures allow rapid and extensive degradation to smaller peptides by proteolytic enzymes. The natively unfolded structures of many cereal proteins may serve an analogous purpose since they provide nutrition for seedlings. The physiological function of the synucleins in the brain is as yet unclear, but tau is known to promote and stabilize the assembly of microtubules. 

P.W. Robertson, L.R. Whybin, and J. Cox, Reduction in non-specific binding in enzyme immunoassays using casein hydrolysate in serum diluents. J. Immunol. Methods 76, 195—197 (1985). 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

The present study was undertaken to determine the influence of several levels of dietary casein upon the activities of trout hepatic enzyme systems which may be involved in the in vitro activation and detoxification of AFB. In addition, the effect of Technical Paper No. 4883, Oregon Agricultural Experiment Station. 

The decrease in cytochrome P-450 content correlated with a lowering of trout AE activity observed in hepatic microsomes recovered from fish fed high levels of casein, versus those from fish fed low casein diets. As shown in Table III, up to a 32% decrease in the production of the epoxide dieldrin was noted. Similar results have been observed in 10 month old rainbow trout with a nearly identical maximum decrease (unpublished data). Since AFB activation has been shown to involve a cytochrome P-450 dependent enzyme system (19, 20, 22) and trout... 

Unlike EH and GTr activities, the production of AFL from AFB by trout hepatic enzymes was observed to increase in fish fed the higher casein diets (Table III). AFL has been shown to be carcinogenic (30) and mutagenic (53) and may be oxidized back to AFB by trout hepatic enzymes (54). It has been suggested that AFL represents a reserve pool of toxin in vivo (35, 55) and that its production is indicative of a sensitive animal species (56, 57). If AFL does indeed represent a reserve pool of AFB in trout, then its increased production by fish fed high levels of casein may predispose these particular animals to the induction of cancer. 

The observed mutagenic responses to AFB reflect the overall effects of activation and detoxification systems on the in vitro metabolism of AFB. It appeared that the effect of high casein levels fed to trout was that a greater amount of activated AFB was produced and/or that less could be detoxified by these fish than by those fed lower casein diets. If lowered cytochrome P-450 content and AE activities in fish fed the high casein diets represented a decrease in the activation of AFB, then these effects were overcome by the observed decreases in GTr activity and/or increases in AFB conversion to AFL relative to those of trout fed lower casein diets. Alternately, the results could be explained by dietary effects upon some unknown OAFB metabolizing enzyme system in trout, upon free GSH levels in hepatic tissue, or that the levels of the cytochrome P-450 involved in AFB activation were not reflected by the observed total cytochrome P-450 levels. 

The effect of sphingosine on other enzymes may also contribute to its apoptotic effect. These include the inhibition of calcium/calmodulin-requiring enzymes and DNA primase and the stimulation of casein kinase II and several unidentified kinases (Alessenko, 2000). In addition, sphingosine can increase the cellular concentration of cyclic AMP, which is inhibitory for proliferation in many cell types (Pyne and Pyne, 1996). 

The Daily Industiy. The first step in cheese manufacture is the coagulation of milk. Coagulation can be divided into two distinct phases, enzymatic and the non-enzymatic. In the primary enzymatic phase a proteol ic enzyme such as chymosin (rennet), or less effectively, pepsin, carries out an extremely specific and limited proteolysis, cleaving a phenylalanine-methionine bond of /c-casein, making the casein micelle metastabie. In the second, non-enzymatic phase, the... 

This enzyme [EC 3.4.21.53], also known as endopepti-dase La, ATP-dependent serine proteinase, and ATP-dependent protease La, catalyzes the hydrolysis of peptide bonds in large proteins (for example, globin, casein, and denaturated serum albumin) in the presence of ATP (which is hydrolyzed to ADP and orthophosphate). Vanadate ion inhibits both reactions. A similar enzyme occurs in animal mitochondria. Protease La belongs to the peptidase family S16. 

Proteolytic breakdown of the casein Bacterial or native plasmin enzymes that are resistant to heat treatment may lead to the formation of a gel. 

In vitro, the enzyme is able to catalyze crosslinking of whey proteins, soy proteins, wheat proteins, beef myosin, casein, and crude actomyosin (which is refined from mechanically deboned meat), improving functional properties such as the texture of food products [49-53], Bonds formed by transglutaminase exhibit a high resistance to proteolytic degradation [54],... 

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