The Production of Threose as a Degradation Product from L-Ascorbic Acid

Abstract

In order to examine the nature of in. vivo Maillard reactions that involve L-ascorbate and proteins, rates of formation of threose from L-ascorbic acid (1), dehydro-L-ascorbic acid (2), and 2,3-diketo-L-gulonic acid (3) were measured in the presence and absence of oxygen at pH 7.0 (phosphate buffer) and at 37 °C. Threose is produced in measurable quantities from L-ascorbate only in the presence of oxygen. Compounds 2 and 3 both give rise to threose in both the presence and absence of oxygen. Compound 3 gives threose at a faster rate and in higher amounts than 1 or 2, suggesting that it is the primary source of threose in this reaction. Incubations of dehydro-L-ascorbate with N^a -acetyl-L-lysine in the presence of cyanoborohydride gave N^a -acetyl-N^e -(1-deoxy-L-threitol-l-yl)-L-lysine which was chemically synthesized (from threose and N'-acetyl-L-lysine) and unequivocally characterized. The data suggest that the Maillard reaction observed when L-ascorbic acid is incubated with protein may well arise as a result of interactions of L-threose with amino groups, and that the function of oxygen in the reaction is to convert 1 into the more reactive 2. and 3. Oxygen does not appear to be necessary for the further degradation of 2 or 3.