Retinol-binding protein electrophoresis

Transthyretin amyloidosis (also called familial amyloid polyneuropathy) is an autosomal dominant syndrome characterized by peripheral neuropathy. This disease results from one of five mutations identified thus far in the gene for transthyretin. Transthyretin is also called prealbumin (although it has no structural relationship to albumin) because it migrates ahead of albumin in standard electrophoresis at pH 8.6. Transthyretin is synthesized in the liver and is a normal plasma protein with a concentration of 20-40 mg/dL. It transports thyroxine and retinol binding protein (Chapter 38). The concentration of transthyretin is significantly decreased in malnutrition and plasma levels are diagnostic of disorders of malnutrition (Chapter 17). 

Figure 7. Enlarged zoom on retinol binding protein in renal insufficiency after two-dimensional polyacrylamide gel electrophoresis. First dimension immobiUzed pH 3 to 10 gradient, second dimension 9 to 16% gradient polyacrylamide gel. Retinol binding protein appeared as two large spots. This particular pattern modification is related to the accumulation, in renal failure, of a truncated variant of retinol binding protein (-266 Da) (arrow on the left) relative to the normal form (arrow on the right) (Kieman et al., 2002).

Vitamin A is found in the blood in the form of retinol bound to a specific protein called the retinol binding protein (RBP). RBP has been purified from human plasma it has a molecular weight of 21,000 and binds one molecule of retinol per molecule of protein and moves as an a 1 protein on electrophoresis. 

Siegenthaler, G. (1990) Gel electrophoresis of cellular retinoic acid-binding protein, cellular retinol-binding protein, and serum retinol-binding protein. Meth. Enzymol 189, 299-307... 

To take advantage of the fact that retinol is much more stable when com-plexed with plasma retinol-binding protein, its transport protein, Oliver et al. (43) allowed 200- J,L serum aliquots to dry onto ethanol-washed, dry filter paper. The retinol was redissolved for HPLC analysis by suspending the paper in phosphate buffer, followed by shaking with ethanol, and then extracted with hexane. Shi et al. (44) similarly shipped and stored whole blood or serum samples dried on filter paper, with subsequent analysis by capillary electrophoresis. This procedure allows simple, inexpensive shipment of samples (e.g., population screening surveys) over great distances. 

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