Plasma proteins, glycosylated

At present, antioxidants are extensively studied as supplements for the treatment diabetic patients. Several clinical trials have been carried out with vitamin E. In 1991, Ceriello et al. [136] showed that supplementation of vitamin E to insulin-requiring diabetic patients reduced protein glycosylation without changing plasma glucose, probably due to the inhibition of the Maillard reaction. Then, Paolisso et al. [137] found that vitamin E decreased glucose level and improved insulin action in noninsulin-dependent diabetic patients. Recently, Jain et al. [138] showed that vitamin E supplementation increased glutathione level and diminished lipid peroxidation and HbAi level in erythrocytes of type 1 diabetic children. Similarly, Skyrme-Jones et al. [139] demonstrated that vitamin E supplementation improved endothelial vasodilator function in type 1 diabetic children supposedly due to the suppression of LDL oxidation. Devaraj et al. [140] used the urinary F2-isoprostane test for the estimate of LDL oxidation in type 2 diabetics. They also found that LDL oxidation decreased after vitamin E supplementation to patients. 

The most frequent protein in the plasma, at around 45 g is albumin. Due to its high concentration, it plays a crucial role in maintaining the blood s colloid osmotic pressure and represents an important amino acid reserve for the body. Albumin has binding sites for apolar substances and therefore functions as a transport protein for long-chain fatty acids, bilirubin, drugs, and some steroid hormones and vitamins. In addition, serum albumin binds Ca "" and Mg "" ions. It is the only important plasma protein that is not glycosylated. 

Posttranslational modification Examples of posttranslational modification POSTTRANSLATIONAL MODIFICATION OF POLYPEPTIDE CHAINS (p. 440) Many polypeptide chains are covalently modified after translation. Such modifications include trimming excess amino acids, phosphorylation which may activate or inactivate the protein, glycosylation which targets a protein to become part of a plasma membrane or lysosome or be secreted from the cell, or hydroxylation such as that seen in collagen. 

Gould, B. J., Hall, P. M., and Cook, J. G. (1984). A sensitive method for the measurement of glycosylated plasma proteins using affinity chromatography. Ann. Clin. Biochem. 21, 16-21. 

Acute phase proteins are plasma proteins produced mainly by hepatocytes. Most APPs are glycoproteins with one or more N-linked complex glycans. Stimulants that commonly induce the acute phase response include tissue injury, rheumatoid arthritis, bacterial infection, inflammation, and neoplasms. Cytokines, notably interleukin-6, induce striking alterations in the concentration and glycosylation pattern of APPs in response to these stimuli [160]. 

Factor X is a plasma protein involved in both the intrinsic and extrinsic pathways of blood coagulation. Factor X has a mass of 55kDa and the activated Xa of 40kDa. The normal concentration in plasma is 6-8 pg/ml. Post-translational modifications of the protein involve y-carboxylation of specific glutamic acid residues, 3-hydroxylation of one aspartic acid residue, and N- and 0-linked glycosylation. 

Other plasma proteins exhibit glycosylation, but there are currently few established reports of functional effects. Of interest is glycosylated transferrin with its short half-life of 8 days (K4), glycosylated transferrin assays can be used to follow short-term variations in glycemia (K3). 

If preparatory prerequisites are followed, samples for glycosylated hemoglobin and plasma protein assays may be stored, depending on refrigeration temperatures, for months and years without loss of activity. 

The TBA method has been used not only to assay glycosylated hemoglobin, but also almost exclusively for the following glycosylated proteins plasma albumin (D22, El, NIO, M12), plasma proteins and albumin (K6), capillary whole-blood protein collected on filter paper (L14), erythrocyte spectrin (M13), skin collagen (P2), lens basement membrane (M5), and crystallins (M28). In assays on hair and epidermal keratin, cyclohexanone extraction of the color developed with TBA and reading the absorbance of the cyclohexanone layer at 433 nm has provided increased sensitivity and accuracy (T4). A possible objection to the procedure is the use of oxalic and thiobarbituric acids, which are potentially toxic chemicals. 

Because the presence of free glucose in plasma or serum will continue the glycosylation process (K5), it is not feasible to store such samples unless the glucose is first removed by dialysis. Preferably, the total plasma protein must be precipitated and stored as such or with further purification alternatively, fractions may be prepared by suitable fractionation procedures and stored in solid form or reconstituted in glucose-free media. 

Ml. Ma, A., Naughton, M. A., and Cameron, D. P., Glycosylated plasma proteins A simple method for the elimination of interference by glucose in its estimation. Clin. Chim. Acta 115, 111-117 (1981). 

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