Se-Containing Amino Acids

All selenium-containing proteins and enzymes in animals, microorganisms and plants incorporate selenocysteine non-spedfically or as part of Se-dependent antioxidant enzymes such as glutathione peroxidase, (EC 1.11.1.9) which has a Se-cysteine residue in its active site. An active form of Se, selenophosphate, is produced by selenophosphate synthetase in several bacteria. This active form is required for the production of Secys-tRNA, a precursor for Se-cysteine. In a similar vein, a Se-containing modified-tRNA nucleoside, 5-methylaminomethyl-2-selenouridine, encodes a selenouridine synthase which replaces sulfur in tRNA with selenium. 

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Metalloids, such as As and Se, are known to be metabolized by living organisms in a way that leads to the formation of a covalent bond between the heteroatom and the C atom incorporated in a larger structure (e.g., arsenosugars, Se-containing amino acids, peptides, and proteins). The complexation of metals by components of food leads to a number of relatively poorly characterized metal... 

Selenium is an essential trace element and an integral component of heme oxidase. It appears to augment the antioxidant action of vitamin E to protect membrane lipids from oxidation. The exact mechanism of this interaction is not known however, selenium compounds are found in the selenium analogs of the sulfur-containing amino acids, such as cysteine and methionine. Se-cysteine is found in the active sites of the enzyme glutathione peroxidase, which acts to use glutathione to reduce organic hydroperoxides. 

Figs. 22 top) and 23 bottom). Gas-Kquid chromatograms of the trimethylsilyl derivatives of sulfur-containing amino acids. Column 05% w/w SE-30 on acid-washed, dichlorodimethylsilane-treated Chromosorb G (80-100 mesh) 1 meter X 35 mm i.d. glass initial temperature, 75°, programmed at 4.6° per minute Nj flow 40 ml per minute. Injected mixture contained ca, 5 /tg of each amino acid. Eeproduced from Shahrokhi and Gehrke (S4) with permission. 

Streptokinase. Streptokinase is a single-chain protein containing 415 amino acids, mol wt of 45,000 to 50,000 (261,284—286). It is produced by P-hemolytic streptococci and is not an enzyme per se. Only after streptokinase combines with plasminogen on a 1 1 basis to form a... 

In their initial studies on the effects of vasoactive peptides on mast-cell secretion, Johnson and Erdos [31] concluded that the ability of peptides to elicit a secretory response from mast cells depended upon the number of basic groups a peptide contained, and not on the structure of the N-terminal or C-terminal amino acids per se. They found, for example, that Polistes kinin, with... 

The evolutionary classification has a rational basis, since, to date, the catalytic mechanisms for most peptidases have been established, and the elucidation of their amino acid sequences is progressing rapidly. This classification has the major advantage of fitting well with the catalytic types, but allows no prediction about the types of reaction being catalyzed. For example, some families contain endo- and exopeptidases, e.g., SB-S8, SC-S9 and CA-Cl. Other families exhibit a single type of specificity, e.g., all families in clan MB are endopeptidases, family MC-M14 is almost exclusively composed of carboxypeptidases, and family MF-M17 is composed of aminopeptidases. Furthermore, the same enzyme specificity can sometimes be found in more than one family, e.g., D-Ala-D-Ala carboxypeptidases are found in four different families (SE-S11, SE-S12, SE-S13, and MD-M15). 

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