Lactose synthesis

Milk from cows with mastitic infections contains a low level of total solids, especially lactose, and high levels of sodium and chloride, the concentration of which are directly related (Figure 5.5). The sodium and chloride ions come from the blood to compensate osmotically for the depressed lactose synthesis or vice versa. 

As many as 60 indigenous enzymes have been reported in normal bovine milk. With the exception of a-lactalbumin, which is an enzyme modifier in lactose synthesis (Chapter 2) most, if not all, of the indigenous enzymes in milk have no obvious physiological role. They arise from three principal sources ... 

The general scheme for lactose synthesis (Jones 1978) is considered to be ... 

Powell, J. T. and Brew, K. 1975. On the interaction of a-lactalbumin and galactosyl-transferase during lactose synthesis. J. BioL Chem. 250, 6337-6343. 

Keenan, 1975 Neville et al., 1981 Watters, 1984 Virk et al., 1985), the presumption is that the formation of casein micelles is orchestrated with the transport of ions, the phosphorylation and glycosy-lation of the caseins, and lactose synthesis, such that the intravesicular ionic environment and casein concentration change continuously during the 20 min or so required for micelle assembly. Patton and Jensen (1975) observed, in electron micrographs, the same density of micellar particles in the alveolus as in mature vesicles, suggesting that, by this stage, the vesicular concentrations are virtually identical to those in the aqueous phase of milk. 

In summary, intact milk proteins have their specific functions such as casein micelle formation and regulation of lactose synthesis. Milk proteins exhibit various biological activities when they are partially digested as described before. They are finally a source of essential amino acids. This means milk proteins are highly functional substances. 

Mellenberger, R.W., Bauman, D.E., Nelson, D.R. 1973. Fatty acid and lactose synthesis in cow mammary tissue. Biochem. J. 136, 741-748. 

With the onset of lactation, a-lactalbumin is formed in the mammary gland and alters the substrate specificity of the transferase from N-acetylglucosamine to glucose, enabling lactose synthesis to be effected ... 

In 1958 Yasunobu and Wilcox drew attention to certain similarities between a-lactalbumin and lysozyme (see Gordon, 1971). A few years later Brew and Campbell (1967) also drew attention to their marked similarity in molecular weights, amino acid composition, and the amino-and carboxy-terminal amino acid residues. They stated, To the extent that the properties mentioned reflect similar primary structures, the a-lactalbumins may have evolved by gradual modification from lysozyme, which is found in the milk of many species (p. 263). This proposal prompted Brew etal. (1967, 1970) to determine the amino acid sequence of bovine a-lactalbumin, which proved to have a high level of sequence identity with domestic hen egg-white lysozyme. Thus, these studies were in accordance with the proposal that the two proteins had diverged from a common ancestor (see also Hill etal., 1969, 1974). They stated that although lysozyme does not participate in lactose synthesis and a-lactalbu-... 

According to Lonnerdahl and Glazier (1985), only 1% of the calcium content of human milk and 0.15% of the calcium content of cow milk are bound to a-lactalbumin. Hence, this protein is quantitadvely unimportant for calcium nutridon of the infant. They point out, rather, that the primary role of calcium may be to regulate lactose synthesis and possibly to aid in the secredon of a-lactalbumin. On the other hand, Rao and Brew (1989) have found that Ca(II) is essential for the formation of correct disulfide bonds and the development of nadve conformadon. They suggest that Ca(II) may funcdon to guide the folding of the nascent protein. 

FUNCTION Stimulates breast development and lactose synthesis in pregnancy, among other functions. 

Which of the following statements concerning lactose synthesis is TRUE ... 

A) can convert galactose to UDP-glucose for lactose synthesis during lactation... 

D. Prolactin causes synthesis of the milk protein a-lactalbumin, which stimulates lactose synthesis. 

This modified enzyme is lactose synthase. Synthesis of ot-lactalbumin is activated hormonally in mothers shortly after giving birth. The ot-lactalbumin protein combines with preexisting galactosyltransferase, changes its specificity, and activates the large amount of lactose synthesis needed for milk production. 

Lactose synthesis in milk Lactose synthase Galactosemia... 

Conversion to glucose-6-phosphate (phosphoglucomutase) Lactose synthesis in milk Lactose synthase Galactosemia... 

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