Protein. . 53 glutamic acid released from

FIG. 4 Protein turnover releases stored selenomethionine (SeMet) to the free amino acid pool (Gly = glycine, Glu = glutamic acid, Ala = alanine). Adapted from Jacques, 2001. 

The body contains many amino acids, some of which can be synthesized (non-essential amino acids), while others must be absorbed from the diet (the essential amino acids). Amino acids are the building blocks of protein, being attached together in groups to form the proteins that the body needs as part of its cellular structure or as enzymes, which are biological catalysts. Glutamate can be synthesized in the body, and it is also absorbed from food, in which it occurs as the free amino acid, or it can be released from proteins by digestion. 

Waldschmidt-Leitz and Reicheneder (1961) identified an NH2-terminal glutamic acid residue in a wheat protein bound by way of its 7-carboxyl group. They inferred this from the evolution of carbon dioxide during reaction with ninhydrin. In view of the fact that ninhydrin is able to split many peptide bonds (Dowmont and Fruton, 1952 Yanari, 1956) its use in this way is open to some question however, since glutamic acid could be identified as the NH2-terminal residue by the Sanger method and yet was not released by aminopeptidase, it seems probable that a 7-glutamyl link is present. 

Additional evidence for a bacterial contribution to HMW DOM proteins comes from molecular-level analyses of dissolved amino acids. Hydrolysis of HMW DON releases 11-29% of the nitrogen as amino acids (McCarthy et al., 1996). Specific amino acids include common protein amino acids, as well as /3-alanine and y-aminobutyric acid which are nonprotein amino acid degradation products. The distribution of amino acids is similar to that of fresh plankton cells, suspended particulate matter, and total dissolved amino acids. However, stereochemical analyses show HMW DOM amino acids to be elevated in the D-enantiomer, with d/l ratios for alanine, aspartic acid, glutamic acids, and serine ranging from 0.1 to 0.5 (McCarthy et al., 1998). Racemization of phytoplankton-derived L-amino acids is too slow at ocean temperatures to yield such high D/L ratios, but bacteria can synthesize D-amino acids, and it is likely that the D-amino acids in HMW DOM result from bacterial bioploymers rich in these particular amino acids. The high dA ratios of some amino acids and the abundance of amide nitrogen in HMW DOM N-NMR spectra led McCarthy et al. (1998) to... 

Although the precision of analysis with our present technology is often 1 to 3 %, the quantitative release of many amino acids and amino acid derivatives from proteins is often difficult and lowers the overall precision. For example, if constant-boiling HCl (about 5.7 N) is used to hydrolyze a protein in vacuo at 110°C for 24 hr (these conditions are those most commonly used), the amounts of aspartic acid, asparagine, serine, threonine, glutamic acid, glutamine, valine, isoleucine, methionine, tyrosine, tryptophan, cysteine and cystine present in the... 

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