Monosaccharides react non-enzymically with the N-terminal amino group and lysyl side-chain amino groups of proteins, under physiological conditions, to produce glycated proteins'. Examples of proteins known to undergo this reaction are haemoglobin, serum albumin, immunoglobulins, collagen, and lens crystallins". The acyclic aldehyde or ketone form of the monosaccharide reacts with the amino group to form a Schiff s base or imine that undergoes an Amadori rearrangement to form a ketoamine7. With glucose, the ketoamine is an NC-(l-deoxy-D-fructos-1-yl)-amino acid residue, a fructosamine8.
The concentration of fructosamines in samples of blood plasma from diabetic patients is used as a measure of dietary or pharmacological control of blood-glucose concentration'. This concentration is assayed routinely by a simple, calorimetric assay based on the reducing activity of glycated serum proteins in alkaline solution. At pH 10.35, the fructosamine reduces Nitro Blue Tetrazolium (NBT) to formazan; the corresponding rate of change in absorbance at 520 nm is directly proportional to the concentration of fructosamine".
Fructosamines are also oxidised slowly under physiological conditions". Oxidative reactions and rearrangements lead to the irreversible formation of advanced glycation end-products (AGES). AGES alter the physicochemical and biochemical characteristics of proteins, usually associated with a deterioration in the biological function of the proteins'*. Several AGES have been identified and include 3-deoxygluco-sone13, 5-hydroxymethyl-1 -alkylpyrrole-2-carbaldehyde'4, iV,-carboxymethyl-lysine'5, N,-(3-lactato)lysinei6 , and 2-furoyl-4( 5)-(2-furanyl)-1 H-imidazole (FFI)"; although the formation of FFI is now thought to be artifactual'2~'9~20. The mechanism by which AGES are formed has not yet been fully explained'.
The synthesis and characteristics of fructosamines derived from amino acids and peptides are currently under investigation as prospective standards in the fructosamine test" and to study the mechanism of fructosamine degradation'3"0. The fructosamine * To whom correspondence should be addressed.