Caseins phosphorus

Casein phosphorus. Milk contains about 900mg phosphorusl"1, which occurs in five types of phosphate-containing compounds, as will be discussed in Chapter 5 ... 

In all the species that have been investigated, casein phosphorus is in the form of phosphomonoesters of seryl and, to a much lesser... 

As in the case of ovalbumin, the dephosphorylation of a-casein is accompanied by a change in the electrophoretic behavior. With the aid of Fig. 5, it is illustrated that the liberation of phosphorus is accompanied by the appearance of several new components with lower mobilities. The mobility decrements of these components at pH 6.8 are 0.5 X 10 or a multiple thereof. If compared with the base binding capacity of a-casein (31) this value corresponds to a change in the net charge of —2. This supports the initial assumption that some of the a-casein phosphorus is present in form of monoesters. 

The observation that prostate phosphatase liberates only 40% of the a-casein phosphorus and does not act on /3-casein foreshadowed the existence of phosphodiesters and pyrophosphate bonds in these proteins. Thus the action on these materials of enzymes specific for such bonds should establish not only the nature of the linkages but also the proportions in which they occur. The results of such stepwise enzymatic analysis can be best followed with the aid of Table V. 

If a-casein is treated with either the crystalline pyrophosphatase of yeast at pH 7.0 (35) or with the snake venom diesterase at pH 8.2 (87), no inorganic phosphorus is released. However, if the diesterase reaction is carried out in weakly buffered solutions a small drop of pH takes place (71), indicating the exposure of acidic groups. Subsequent incubation of the diesterase-treated a-casein with prostate phosphatase at pH 6.0 hberates no more phosphorus than in the absence of the diesterase. If, how ever, prostate and intestinal phosphatase are added, 78 % of the a-casein phosphorus is set free. Since the intestinal enzyme at pH 6.0 acts on low... 

In a recent communication Sundararajan and Sarma report that a phosphoprotein phosphatase from rat spleen dephosphorylates a-, /3-, and unfractionated casein (90). Since these authors state that their enzyme differs in its action from that of a phosphomonoesterase, their results are in accord with the occurrence of a variety of phosphorus bonds in proteins. In this connection it should be noted that intestinal phosphatase used in our work at pH 9.0 also liberates all of the a-casein phosphorus (72). As discussed earlier, although this enzyme at pH 6.0 hydrolyzes —N—P—... 

PHOSPHOPROTEINS. These proteins have phosphate groups esterified to the hydroxyls of serine, threonine, or tyrosine residues. Casein, the major protein of milk, contains many phosphates and serves to bring essential phosphorus to the growing infant. Many key steps in metabolism are regulated between states of activity or inactivity, depending on the presence or absence of phosphate groups on proteins, as we shall see in Chapter 15. Glycogen phospho-rylase a is one well-studied example. 

According to previous analyses of these paintings, the colour layers contain a high concentration of phosphorus (in the order of units of per cent) [40] this led restorers to the assumption that they contain casein which was ruled out by our detailed analysis. Thus, a high concentration of phosphorus coming from the proteinaceous binder is excluded because of the low concentration of phosphorus in the most phosphorylated binder (casein, max. 5% phosphoms) and the amount in the colour layer (up to 10%).The source of phosphorus was discovered by powder X-ray microdifffaction it comes from aluminium phosphate that was probably used as a substrate for the precipitation of red organic lake [40],... 

All of the studies were conducted with weanling, male albino rats of the Sprague-Dawley strain (Holtzman company). The basal diet used for these studies consisted of casein, starch, vegetable oil, vitamin and mineral mixtures, and cellulose. The Wesson Modification of the Osborne-Mendel mineral mixture was used in all studies. This mineral mixture contained no zinc, but it was adequate in the other minerals required by the rat. Most of the non-zinc-supplemented diets used in the various experiments contained approximately 7 ppm zinc. The level of mineral mixture used in the basal diets was 4%, and based on the chemical composition of the mixture, the basal diets contained approximately 0.57% calcium and 0.41% phosphorus ... 

A safety match requires for its ignition a rubbing on a "striking strip" which is covered with mixture such as consisting of red phosphorus 53, Sb sulfide 42 charcoal 5% in a binder such as NC lacquer, animal glue, dextrin, casein, plus hardener. Sb sulfide and charcoal serve as extenders to the phosphorus. 

Total phosphorus includes colloidal inorganic phosphate, casein (organic) phosphate, soluble inorganic phosphate, ester phosphate and phospholipids. 

The phosphorus of milk occurs in five classes of compounds phospholipids, lipid, casein, small soluble organic esters, soluble and colloidal inorganic salts (Figure 5.7). 

Minerals found in milk which are insoluble remain in water in the curd and are more concentrated in the cheese than in milk. About two-thirds of the calcium and one-half of the phosphorus of milk remains in cheese. A major portion of the milk calcium is retained in the curd of cheese made with coagulating enzymes. Acid coagulation alone results in the loss of portions of both calcium and phosphorus salts in the acid whey, since these minerals are more soluble in the acidic medium. Most milk fat and fat-soluble vitamins are retained in the curd, but a considerable amount of water-soluble vitamins is lost during cheese manufacture. Retention of part of some B-complex vitamins in curd is due to their extended association with casein in the original milk. 

Originally, the caseins were defined as those phosphoproteins which precipitate from raw skim milk upon acidification to pH 4.6 at 20°C, and the individual families were identified by alkaline urea gel electrophoresis (Whitney et al 1976). With the resolution of their primary structure, it became possible to classify them according to their chemical structure, rather than on the basis of an operational definition. When one does this, it is apparent that not all of the caseins contain phosphorus (Table 3.1) some are also found in the acid whey after removal of the precipitated caseins. 

Differences in pH and buffering capacity among individual lots of fresh milk reflect compositional variations arising from the functions of the mammary gland. In general the pH is lower in colostrum (down to pH 6.0 McIntyre et al. 1952) and higher in cases of mastitis (up to pH 7.5 Prouty 1940) than in normal milk of mid-lactation. As discussed in Chapter 1, colostrum and mastitis milks are known to differ radically in their proportions of the proteins and certain salts. Milks of lower phosphorus, casein, and Ca2+ tend to be low in titratable acidity, while excessive acidity is related to hyperketonemia, inadequate calcium and excessive concentrates in the ration (Bonomi 1978). 

Fractionation of milk and titration of the fractions have been of considerable value. Rice and Markley (1924) made an attempt to assign contributions of the various milk components to titratable acidity. One scheme utilizes oxalate to precipitate calcium and rennet to remove the calcium caseinate phosphate micelles (Horst 1947 Ling 1936 Pyne and Ryan 1950). As formulated by Ling, the scheme involves titrations of milk, oxalated milk, rennet whey, and oxalated rennet whey to the phenolphthalein endpoint. From such titrations, Ling calculated that the caseinate contributed about 0.8 mEq of the total titer of 2.2 mEq/100 ml (0.19% lactic acid) in certain milks that he analyzed. These data are consistent with calculations based on the concentrations of phosphate and proteins present (Walstra and Jenness 1984). The casein, serum proteins, colloidal inorganic phosphorus, and dissolved inorganic phosphorus were accounted for by van der Have et al (1979) in their equation relating the titratable acidity of individual cow s milks to the composition. The casein and phosphates account for the major part of the titratable acidity of fresh milk. 

Casein is not coagulated by heat. It is precipitated by acids and by rennin. a proteolytic enzyme obtained from the stomach or calves. Casein is a conjugated protein belonging lo the group uf phosphoproteins. The enzyme trypsin can hydrolyze off a phosphorus-containing peptone. 

Enzymatic phosphorylation by phosphorylases and phosphatases produces phosphoesters such as phosphoserine and phosphothreonine. Chemical phosphorylation of proteins changes their functional properties, improving them sometimes (Yoshikawa et al., 1981 Hirotsuka et al., 1984 Huang and Kinsella, 1986 Chobert etal, 1989 Matheis, 1991). However, the properties of the phosphorylated proteins depend entirely on the degree of denaturation and substitution defined by the reaction conditions and the protein (Medina etal, 1992 Sitohy etal, 1994). Casein was phosphorylated by the commonly used methods, characterized by use of excessive amounts of phosphorus oxychloride and with important additions of concentrated inorganic bases (Matheis et al, 1983 Medina et al, 1992). Thus, obtained phosphorylated caseins were highly cross-linked and partially insoluble and difficult to characterize. Hence, there arose a need to produce monomeric over-phosphorylated caseins more suitable for use and for study of their... 

Matheis, G., Penner, H.M., Feeney, R.E., and Whitaker, J.R. 1983. Phosphorylation of casein and lysozyme by phosphorus oxychloride. J. Agric. Food Chem. 31, 379-387. 

The equilibria described here are represented schematically in Figure 5-2, and the levels of total and soluble calcium and phosphorus are listed in Table 5-3. The mineral equilibria in milk have been extensively studied because the ratio of ionic and total calcium exerts a profound effect on the stability of the caseinate particles in milk. Pro-... 

G. Hevesy was the first to label proteins with radionuclides as early as 1938. The Nobel Prize was awarded to him in 1943 for his discoveries in this field Hevesy fed the radionuclide of phosphorus to sheep per os and isolated P-casein from their milk. 

G5. Glomset, J. A., The further purification and properties of a phosphatase from spleen able to hydrolyze completely the phosphorus of L. Casein. Biochim. Biophys. Acta 32, 349-357 (1959). 

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