7 Non-enzymatic glycosylation and browning of proteins in diabetes

Abstract

Non-enzymatic glycosylation is the first of a series of steps described by food chemists as the 'Maillard' or non-enzymatic browning reaction which occurs in baked or stored foodstuffs. In this reaction, reducing sugars such as glucose react with amino groups of peptides and proteins to form labile Schiff base adducts, which can then undergo an Amadori rearrangement to form the more stable adduct which has the structure 1-amino-l-deoxy-2ketose (Amadori product) (Figure 1, top). When non-enzymatically glycosylated proteins are heated or stored over a prolonged time, dehydration and rearrangement of the Amadori product occurs. Several highly reactive carbonyl group compounds are formed which can condense with amino groups to form melanoidins (Reynolds, 1965) (Figure 1, lower part). These yellow-brown products are often fluorescent, highly cross-linked and poorly soluble. The process of non-enzymatic browning may be of particular importance in the ageing of long-lived proteins such as collagen or lens crystallins since the off-rate of the brown products is probably non-existent. The interest in glycosylated proteins in vivo began with the observation by Rahbar (1968) that diabetic patients had elevated amounts of a 'fastmoving' haemoglobin as analysed by electrophoresis. Subsequent work in several laboratories led to the identification of haemoglobin Aic as glycosylated haemoglobin (Bookchin and Gallop, 1968; Fltickiger and Winterhalter, 1976; Koenig, Blobstein and Cerami, 1977) with the Amadori product of glucose attached to the amino terminal valine of the/J-chain. The demonstration by Koenig and colleagues (1975) that the amount of haemoglobin A~c reflected the integrated glycaemia over the previous three to four weeks spurred the use of this measurement as a clinical tool for diabetes control (Figure 2). In addition, the observation that the complications of diabetes occur preferentially in tissues that do not require insulin for glucose transport led to the hypothesis that increased non-enzymatic protein glycosylation might be involved in the complications of diabetes. Non-enzymatic glycosylation of proteins can theoretically alter their function in several ways. The presence of a bulky sugar molecule might lead to decreased enzymatic activity, changes in conformation, modification of