Protein determination from amino acid

Hopp TP, Woods KR. (1981) Prediction of protein antigenic determinants from amino acid sequences. Proc Natl Acad Sci U S A78, 3824-8. 

Urea is synthesized from the ammonia derived from protein, polypeptide, and amino acid catabolism. Consequently, the amount of urea in the blood is partly determined by protein and amino acid metabolism. Therefore, in considering urea production, the source of the ammonia and the biosynthesis of urea must be discussed. The ammonia is derived mainly from amino acid breakdown. In liver, the amino acid pool is derived from the breakdown products of ingested and endogenous proteins and from amino acids synthesized de novo. 

The importance of Heinrich Ritthausen s fundamental studies, 1862 to 1899, on analytical procedures for the determination of amino acids in proteins has been emphasized in the biographical sketches which have been presented by Osborne (210), Vickery (289), and Chibnall (47). It is of particular interest to note here the prediction made by Ritt-hausen about 1870 that the amino acid composition would prove to be the most adequate basis for the characterization of proteins. Ritthausen and Kreusler (230) were the first, in 1871, to determine amino acids derived from proteins, and some of the values which they found for aspartic and glutamic acids are given in Table III (cited by Chibnall, 47, and Vickery, 286). 

The isotope dilution principle, first employed by Hevesy and Hobbie (133) in 1932 for the determination of lead in ores, was applied by Schoenheimer et al. (241) to the determination of amino acids. [Shemin and Foster (248) have reviewed this topic.] An N15-amino acid derivative was added to a protein hydrolyzate, a sample of the amino acid to be determined was isolated and purified, the excess N15 in this product was estimated with the mass spectrograph, and the grams of amino acid originally present were calculated from Equation 2. 

With methods for the quantitative analysis of amino acids to hand, the way was now open for the determination of amino acid sequences. Purified bovine insulin was relatively freely available. On the basis of ultracentrifugal analysis (Gutfreund and Ogston), a molecular weight of 12,000 was assumed—as it later emerged, a factor of 2 too high. One of the advantages from the choice of insulin as the protein to sequence was that tryptophan is absent. A 100% recovery of the amino acids could therefore be obtained easily by simple hydrolysis with HC1. In 1948 Tristram reported the complete amino acid composition of the protein. 

There are two basic considerations when attempting SDM. One is to determine the amino acids that should be mutated and the other is to decide what to replace them with. The first question is, of course, dependant upon information gathered from previous experimentation in order to target residues that are appropriate. Such information may be derived from biochemical techniques. For instance, in our binding site studies, we have specifically mutated amino acids that had previously shown to be covalently labeled by photoactive ligands. Additionally, we have used comparisons between the sequences of different receptor subunits that correlate with receptor function to identify domains of interest. Chimeragenesis, the technique described in the first half of this chapter, can provide important information in this regard. Obviously, those proteins for which a detailed structural model is available will lend themselves to more rational substitutions. 

Hydroperoxides may be determined by measuring at 290 nm (e = 44100 M cm ) or 360 nm (e = 28000 cm ) the concentration of 13 formed in the presence of a large excess of ions. The reaction may be too slow for practical purposes, unless a catalyst is present. For example, an assay for lipid hydroperoxides conducted without a catalyst may require several measurements every 6 min until the absorbance reaches a maximum. Exclusion of air from the sample cuvette is important. The method is about 1000-fold more sensitive than thiosulfate titration The iodometric method with UVD at 360 was adopted for detecting the presence of hydroperoxides derived from protein, peptide or amino acid substrates subjected to y-radiation, after destroying the generated H2O2 with catalase. ... 

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