Lysine incorporation into tissue proteins

For the untreated (3H-lysine) casein, the radioactive urinary and fecal excretions were low, about 10% and 4%, respectively, after 74 h. The unexcreted radiactivity was retained in the organism and incorporated into the proteins. The radioactivity measured in the liver, muscle, and kidneys (expressed as percentage of the ingested dose per gram of tissue) can be considered as a function of the quantity of lysine incorporated into the protein of these tissues and proportional to the biological availability of 3H-lysine in the casein samples. The value of radioactivity measured in the liver, muscle, and kidneys can be considered as being given by a protein whose lysine is 100% available (see Table III, Column 5). 

Carnitine biosynthesis utilizes the essential amino acid lysine, with terminal methyl groups donated by S-adenosylmethionine. Only lysine incorporated into proteins is a substrate for the methylation reaction. In humans, the final reaction in the biosynthetic pathway, catalyzed by a cytosolic hydroxylase, occurs in liver and kidney but not in cardiac or skeletal muscle. The carnitine requirement of these tissues is met by carnitine transported to them via the plasma... 

In addition to the 20 common amino acids, proteins may contain residues created by modification of common residues already incorporated into a polypeptide (Fig. 3-8a). Among these uncommon amino acids are 4-hydroxyproline, a derivative of proline, and 5-hydroxylysine, derived from lysine. The former is found in plant cell wall proteins, and both are found in collagen, a fibrous protein of connective tissues. 6-N-Methyllysine is a constituent of myosin, a contractile protein of muscle. Another important uncommon amino acid is y-carboxyglutamate, found in the bloodclotting protein prothrombin and in certain other proteins that bind Ca2+ as part of their biological function. More complex is desmosine, a derivative of four Lys residues, which is found in the fibrous protein elastin. 

Metabolic Transit. Free Amadori Compounds. It is well known that the synthetic Amadori compounds of the free amino acids are absorbed by the intestine and excreted unchanged in the urine (9,28,30). The transport is not active as observed with deoxyfructosyltryptophan (30) and c-deoxyfructosyllysine (40), and the level of absorption depends on the nature of the amino acid and on the conditions of ingestion. Nutritional assays and metabolic transit studies performed with radioactive Amadori compounds of tryptophan (12,30), leucine (12), and lysine (9,28,41) given orally or intravenously on normal or anti-biotics-treated animals have shown that the intestinal microflora can regenerate part of the amino acid. This can be absorbed subsequently at a very low level by the caecum or the large intestine and incorporated into the tissue proteins or utilized by the intestinal microflora. Barbiroli (13) showed also that some intestinal enzymes were able to liberate some amino acids from their Amadori compounds but to a very small... 

The conclusions from the findings cited above may be summarized as follows. The mechanism of amino acid incorporation into proteins appears to be the same in many animal tissues and for many amino acids but one special and different mechanism, that incorporating lysine in guinea pig liver homogenate, has already been found. 

Collagen constitutes 20-25% of the total protein in mammals. Table 12.6 shows data on its amino acid composition. The high contents of glycine and proline and the occurrence of 4-hydro-xyproline and 5-hydroxy lysine are characteristic. Since the occurrence of hydroxyproline is confined to connective tissue, its determination may provide quantitative data on the extent of connective tissue incorporation into a meat product. 

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