Synthesis of milk proteins

Synthesis of the major milk proteins occurs in the mammary gland the principal exceptions are serum albumin and some of the immunoglobulins, which are transferred from the blood. Polymerization of the amino acids occurs on ribosomes fixed on the rough endoplasmic reticulum of the secretory cells, apparently by a method common to all cells. 

The primary blueprint for the amino add sequence of proteins is contained in deoxyribonucleic acid (DNA) within the cell nucleus. The requisite information is transcribed in the nucleus to ribonucleic acid (RNA) of which there are three types messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). These are transferred to the cytoplasm where each plays a specific role in protein synthesis. 

Protein synthesis actually takes place in the ribosomes of the rough endoplasmic reticulum (RER) which contain rRNA. There is a spedfic tRNA for each amino acid, with which it forms an acyl complex  

Ammo acid -I- tRNA -I- ATP — amino acyl-tRNA -H AMP -I- PPi amino acyl-tRNA synthetase 

There is a specific amino acyl-tRNA synthetase for each amino acid these enzymes have two specific binding sites, one for the amino acid and the 

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In addition to its effects on synthesis of milk proteins themselves, prolactin also induces the production in the mammary gland of other proteins/enzymes involved in production of milk components. Lactose synthetase and enzymes of lipid metabolism, including acetyl-CoA carboxylase and lipoprotein lipase, are among these inducible enzymes [75]. 

A role for cyclic AMP as a mediator for the actions of prolactin has been proposed, but the bulk of the evidence available suggests that it is not directly involved. It is possible that levels of cyclic AMP-dependent protein kinase are limiting, rather than cyclic AMP itself, and that these can be increased by prolactin, presumably by induction of the appropriate genes [67]. Cyclic AMP derivatives do not mimic the actions of prolactin in in vitro systems. Indeed, they may inhibit some actions of the hormone, including stimulation of synthesis of milk proteins, fatty acids, DNA and RNA in mammary gland explants [79-81]. 

Prolactin, produced in response to suckling of an infant, stimulates the synthesis of milk proteins during lactation. 

A study of the mRNAs that participate in the synthesis of milk proteins in guinea pigs identified pre-a-lactalbumin as a translation product in a heterologous cell-free system. ... 

Animal Nutrition. Sulfur in the diets of mminant animals is beneficial to the animals growth (see Eeedsand FEED ADDITIVES). Sulfur increases feed intake, cellulose and dry matter digestion, and the synthesis of microbial protein. This results in increased meat, milk, and wool production (43). The special uses for sulfur in agriculture demonstrate a significant and continuing need for increased use of sulfur (44). 

The concentration of milk protein is controlled by the rate of milk protein synthesis (milk protein yield) in relation to milk yield (milk volume). The... 

Heald, C. W. and Saacke, R. G. 1972. Cytological comparison of milk protein synthesis of rat mammary tissue in vivo and in vitro. J. Dairy Sci. 55, 621-628. 

Although there are many gaps in our knowledge of the synthesis and secretion of milk proteins by the lactating mammary gland, the essential features at the cellular level bear a close resemblance to pathways identified in many other eukaryotes (Mercier and Gaye, 1983 Kelly, 1985 Keenan and Dylewski, 1985). 

Hemorrhagic disease of the newborn can develop readily because of (1) poor placental transfer of vitamin K, (2) hepatic immaturity leading to inadequate synthesis of coagulation proteins, and (3) the low vitamin K content of early breast milk. Prothrombin levels during this period are only about 25% of the adult levels. Severe diarrhea and antibiotics used to suppress diarrhea readily exacerbate the situation, so prothrombin levels can drop below 5% of the adult level and bleeding can occur. This condition is routinely prevented by the prophylactic administration of 0.5 to 1.0 mg of phylloquinone intramuscularly, or 2.0 mg given orally immediately after birth. 

The synthesis and secretion of milk proteins have been studied in considerable detail reviews include Mercier and Gaye (1983), Mepham (1987) and Mepham et al. (1992). 

Thus, the synthesis and secretion of milk proteins involves eight steps transcription, translation, segregation, modification, concentration, packaging, storage and exocytosis, as summarized schematically in Figure 4.35. 

The production of milk and eggs rarely occurs alone and is usually accompanied by either gains or losses of fat or protein from the body of the lactating mammal or laying bird. This means that estimates of the partial efficiency of ME utUisation for milk or egg synthesis are usuaUy made by mathematical partition of the ME utUised. As the synthesis of milk or eggs requires more complex diets, it is not possible to provide efficiency coefficients for single nutrients as was done for maintenance and growth in Tables 11.5 11.7. 

Between 1.5% (cow 1) and 3.6% (cow 2) of the Tyr secreted into milk had been synthesized from intracellular Phe within the MG. Total mammary Tyr synthesis from Phe, however, also includes that directed towards synthesis of constitutive protein plus that exported to the mammary vein. Assuming that milk protein secretion represent 75% of MG protein synthesis (Raggio et al, 2006), and that return to the mammary vein can be estimated from the ratios of the IE Tyr/IE Phe in the artery, vein and milk protein, then total conversion of Phe to Tyr within the MG would be the equivalent of 5.2% (cow 1) to 9.4% (cow 2) of Tyr secretion in milk. This is despite the suggestion that hydroxylation of Phe to Tyr is markedly increased at high substrate concentration (Verbeke et al., 1972). 

In this work, two complementary approaches were applied. First, a meta-analysis approach was used to determine how the uptakes of major nutrients by the mammary gland were modified to support increased secretion of milk components in response to increased supplies of protein and/or GN. This approach was chosen to discern the common and the differing metabolic pathways used by the mammary gland to increase milk protein and lactose yields. Second, the contributions of nutrients to the synthesis of milk components per se and the ATP needed for these syntheses by the mammary were both estimated. For this purpose, the biochemical model proposed by Van Milgen (2002) adapted to the mammary gland of ruminants (Lemosquet et al., 2010a) was used in one example. 

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