On Glucose Transport and Non-enzymic Glycation of Proteins In Vivo

Non-enzymic glycation is a chance event which may occur whenever a protein is in solution with a reducing sugar. The product of this reaction is a covalently linked glycated protein. Plasma proteins are commonly targets of glycation, particularly in the hyperglycaemic circulation, and yet little is known of the changes to protein function caused by glycation or the catabolic fate of these glycated proteins in vivo. The following article examines the effect of glycation on the catabolism of albumin, the principal glycated protein in plasma, by comparing the catabolisms of radio-labelled samples of rabbit naturally glycated and unglycated albumins in normal and diabetic rabbits. We reason that the relatively small changes (5-10%) to the half-life (T1/2) of albumin and to its distribution to body compartments caused by glycation in vivo may reflect an evolutionary adaptation of albumin (and of all plasma proteins) to resist glycation and, possibly, an accelerated catabolism. Presumably, any increase in the rate of catabolism of a glycated protein followed by its de novo replacement with native (unglycated) protein would create an extra energy demand of the host. In contrast to the vertebrates, the use of non-reducing disaccharides as transport fuels in the circulation systems of insects and higher plants may be an adaptation to allow transport of relatively high concentrations of sugar without the problems caused by glycation.