Signal peptide albumin

Albumin (69 kDa) is the major protein of human plasma (3.4-4.7 g/dL) and makes up approximately 60% of the total plasma protein. About 40% of albumin is present in the plasma, and the other 60% is present in the extracellular space. The liver produces about 12 g of albumin per day, representing about 25% of total hepatic protein synthesis and half its secreted protein. Albumin is initially synthesized as a preproprotein. Its signal peptide is removed as it passes into the cisternae of the rough endoplasmic reticulum, and a hexapeptide at the resulting amino terminal is subsequently cleaved off farther along the secretory pathway. The synthesis of albumin is depressed in a variety of diseases, particularly those of the liver. The plasma of patients with liver disease often shows a decrease in the ratio of albumin to globulins (decreased albumin-globuhn ratio). The synthesis of albumin decreases rela-... 

The reaction of Cr(VI) with bovine serum albumin (BSA) at pH 7.4 results in the formation of Cr(V) species (detected by EPR spectroscopy, gi o = 1.9790) (411). The reported giso value is characteristic of hve-coordinate Cr -oxo complexes with 1,2-diolato coordination (I in Chart 1 Section II.A.2) (12, 18, 27). This complex, however, may be an artifact due to the presence of trace carbohydrates in BSA, as no signihcant EPR signals were observed during the reactions of high purity BSA with Cr(VI) (412). Photocatalyzed reactions of Cr(VI) with amino acids (Gly or Ala) or with small peptides (Gly or Ala , n = 3-5) in methanol solutions led to the formation of relatively stable Cr -amino acid(peptide) complexes (such as II in Chart 1), characterized by EPR spectroscopy and electrospray mass spectrometry (22, 23, 34). An EPR silent Cr -Ala dimer (III in Chart 1) has been isolated and characterized by XAFS spectroscopy this is the first structurally characterized Cr -amino acid complex... 

In this work, the solutions of human serum albumin (HSA) (>96%, Sigma) and of bovine serum albumin (BSA) (>98%, MP Biomedicals) in a phosphate buffer (0.01 M, pH 7.4) have been used. The proteins concentrations were lO- (absorption spectra measurement) and 10- M (fluorescence measurement at the nanosecond laser fluorimeter). All of the experiments were performed at a temperature of 25 1 °C. The structure and biological functions of HSA and BSA can be found in (Peters, 1996). Tryptophan, tyrosine, and phenylalanine (with relative contents of 1 18 31 in HSA and 2 20 27 in BSA) are the absorption groups in these proteins (as in many other natural proteins). The tyrosine fluorescence in HSA and BSA (as in many other natural proteins) is quenched due to the effect of adjacent peptide bonds, polar groups (such as CO, NH2), and other factors, and phenylalanine has a low fluorescence quantum yield (0.03) (Permyakov, 1992). Therefore, the fluorescence signal in these proteins is determined mainly by tryptophan groups. In that case the fluorescence, registered in nonlinear and kinetic laser fluorimetry measurements, correspond to tryptophan residues (this fact will be used in Section 6.1). 

Calibration of peak position for accurate mass determination can be performed internally or externally to minimize systematic errors. Internal calibration can be conducted when compounds with known molecular weight (called calibration compounds or calibrants) are mixed with the sample prior to the introduction into the ion source. This calibration can be performed, for example, by adding the calibrant to the liquid-phase sample while diluting it prior to analysis. The best result is achieved when multiple calibration signals are used to interpolate the m/z of ions within the range of interest. In proteomics, a tryptic digest of albumin from horse heart is typically used as the calibrant because it covers a wide m/z range (e.g., m/z 800-3000) that is ideal for mass calibration of low- to medium-sized peptides. In external calibration, the calibrants are analyzed before the analysis of real samples. The peaks of the calibrants are used to create and set the calibration equation in the data acquisition software. This method provides less mass accuracy because the instrument condition may still vary between the calibration and analyses of real samples. However, external calibrations save time and calibration compounds, and such methods also make analyses of analytes free from interferences caused by calibrants. 

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