Amadori compounds are well known as important intermediates in the browning reaction of amino acids and reducing sugars during food processing'**. The same reaction of reducing sugars and proteins under physiological conditions produces glycated proteins that may contribute to the development of pathologies associated with diabetes and aging in vivo %'. Glycation gives Amadori derivatives, and thus may cause the denaturation, polymerization, cross linking, and insolubilization of tissue proteins in vivo. Oxidative changes of glycated proteins leading to the formation of cross-linked products?', the degradation of their sugar moietie?", and oxygen radical production by their autoxidation'2-'3 have been observed in vitro.
We previously reported that ascorbic acid causes the fragmentation of proteins and the oxidative degradation of their histidine residues in the presence of copper ion 14-16. This oxidative damage was due to oxygen radicals formed by a one electron reduction through the ascorbic acid-copper ion-oxygen complex. More recently, we showed that the browning solution prepared from amino acids and giucose caused the oxidative fragmentation of bovine serum albumin in the presence of copper ion17, and found that the oxidative damage was dependent on the Amadori product. This fragmentation also seemed to be caused by the action of oxygen radicals, presumably generated in the autoxidation of Amadori compounds.
In the present work, we have investigated in detail the oxidative degradation of Amadori compounds through complexes with copper ion. These degradations may also proceed autoxidatively, much as with ascorbic acid and a metal ion. To clarify the oxidative processes four different Amadori compounds, namely fructose-&alanine* (FA), fructosephenylalanine (FP), ZV*-Boc-1V6-fiuctoselysine (FL), and fructose-p-toluidine (FT), were used. The first experiment was to determine whether the decomposition of Amadori compounds is catalyzed by cupric ion at 40". As shown in Fig. 1, FA was not degraded at all in aqueous solution in the absence of cupric ion, but some degradation was observed in phosphate buffer at pH 7.2. However, the addition of cupric ion to FA * A more systematic name for FA would be N-( 1 -deoxy-D-fructos-1 -yl)-p-alanine. Similar names could be. constructed for FP, FL, and FT.