**2. AGEs formation**

It was not until 1980 that the pathophysiological significance of AGEs emerged in medical science, particularly in relation to diabetic complications [10]. AGEs are a heterogeneous group of molecules that form from the non-enzymatic addition of sugar moieties onto arginine and lysine residues of proteins, free amino groups on lipids, or guanine nucleic acids [11]. Glycation has to be distinguished from glycosylation, which is an enzymatic reaction. First described by Louis Camille Maillard in the 1900s, non-enzymatic glycation involves condensation reaction of the carbonyl group of sugar aldehydes with the N-terminus or free-amino groups of proteins via a nucleophilic addition, resulting first in the rapid formation of a Schiff base. The physiological consequences of the Maillard reaction in the etiology of a range of important diabetic complications have already been indicated [12]. The Schiff base then goes through rearrangements to form the more stable Amadori products. Among most cellular and plasma proteins, Amadori products can change with glucose. That is to say, the levels of Amadori products will rise and fall depending on the levels of glucose. The most well-known example of an Amadori product is hemoglobin A1c (HbA1c), a naturally occurring modification to the N-terminal valine amino group of the β chain of hemoglobin [13]. Schiff bases and Amadori products are reversible reaction products. However, they can react irreversibly with amino acid residues of peptides or proteins to form protein adducts or protein crosslinks [14] (**Figure 1**).

risk both micro- and macro-vascular complications: the former result from damage to retinal, renal, and neural tissues, which is the cause of blindness, end-stage renal failure, and non-traumatic lower limb amputation, respectively [1]. Here, we will focus on diabetic wound. Impaired wound healing is associated with increased morbidity and mortality in diabetes mellitus. The majority of non-healing wounds often lead to amputation, increasing

According to a national survey, the prevalence of chronic cutaneous wounds among hospitalized patients was 1.7% in China. The leading causes were diabetes (31.3% men, 35.3% women) and trauma (26.4% men, 19.2% women). Therefore, diabetes has recently become the leading cause of chronic cutaneous wounds in China [3]. In Shuliang Lu's study, it was indicated that new diabetic foot ulcers were already in poor condition when patients first visited the diabetic foot clinic. Concomitantly, patients had worse health-related quality of life compared with the

Several mechanisms have played a role in this condition, such as neuropathy, peripheral arterial disease, biomechanical factors, infection, and wound healing. Brownlee identifies the production of reactive oxygen species (ROS) as the unifying mechanism behind the main pathological pathways triggered by hyperglycemia, one of which leads to the formation of heterogeneous moieties called advanced glycation end products (AGEs) via non-enzymatic glycation and glycoxidation processes [5]. AGEs affect the wound healing process either directly by their interference with various components involved or indirectly through their association with diabetic neuropathy or angiopathy [6, 7]. In addition, RAGE was discovered as a receptor for AGEs, such as carboxymethyl lysine (CML) [8]. RAGE has been postulated to contribute to the development of diabetic complications [9]. The mechanism of RAGE has

In this chapter, we will present data regarding the formation and the metabolism of AGEs, the role of RAGE involved in diabetic conditions, evidence emerging from in vitro and in vivo studies as well as studies using anti-AGEs and other related agents to support a pathogenic

It was not until 1980 that the pathophysiological significance of AGEs emerged in medical science, particularly in relation to diabetic complications [10]. AGEs are a heterogeneous group of molecules that form from the non-enzymatic addition of sugar moieties onto arginine and lysine residues of proteins, free amino groups on lipids, or guanine nucleic acids [11]. Glycation has to be distinguished from glycosylation, which is an enzymatic reaction. First described by Louis Camille Maillard in the 1900s, non-enzymatic glycation involves condensation reaction of the carbonyl group of sugar aldehydes with the N-terminus or free-amino groups of proteins via a nucleophilic addition, resulting first in the rapid formation of a Schiff base. The physiological consequences of the Maillard reaction in the etiology of a range of important diabetic complications have already been indicated [12]. The Schiff base then goes through rearrange-

role for AGEs in the impaired process of diabetic wound healing.

the direct costs of their care, rehabilitation, and lost productivity [2].

general population [4].

224 Wound Healing - New insights into Ancient Challenges

also been widely discussed.

**2. AGEs formation**

**Figure 1.** Schematic presentation of the Maillard reaction. Reactive carbonyl groups of a reducing sugar react with neutrophilic free amino groups of proteins to form a reversible Schiff base. Through rearrangement, a more stable Amadori product is formed. Depending on the nature of these early glycation end products, protein adducts or protein crosslinks are formed. (Illustrated from Ref. [54]).

In the context of intracellular glycation, it is important to note that glucose has the slowest rate in the glycation reaction of any sugar [15]. Because of the slow formation, it is believed that AGEs accumulate only on long-lived extracellular proteins. However, later a rapid extracellular AGEs formation on short-lived proteins and intracellular AGEs formation by reactive dicarbonyl compounds have attracted attention [16]. Thus, glycolytic intermediates such as dihydroxyacetone-phosphate, glyceraldehyde-3-phosphate and the dicarbonyl compounds glyoxal, methylglyoxal, and 3-deoxyglucosone are important for the intracellular Maillard reaction [17]. Among these compounds, methylglyoxal is regarded as the most potent glycating agent [18]. The transformation of glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate formed methylglyoxal [19]. It could be detoxified by the conversion to S-Dlactoylglutathione and D-lactate, catalyzed in the cytosol of all cells by glyoxalase I and II. It has been reported that overexpression of glyoxalase I in endothelial cells completely prevented AGEs formation, thus indicating the importance of methylglyoxal to form AGEs [20]. Moreover, several studies on different animal models have established that dietary AGEs could play an important role in the pathogenesis of various pathologic conditions and their complications, such as type 1 diabetes mellitus in non-obese diabetic mice [21], atherosclerosis in apoEdeficient mice [22], type 2 diabetes, and impaired wound healing in db/db (+/+) mice [23]. It should be emphasized that a large portion of AGEs in the human body is derived from exogenous sources, e.g. from regular food, smoking, etc. [24]. Much attention has been paid to the so-called exogenous AGEs, harmful products of "browning" (or the Maillard reaction) in various foods. Together with endogenous AGEs, these compounds form the majority of glycation-free adducts. Among the various food processing methods, heating, sterilizing, and microwaves contribute to the generation of exogenous AGEs, all of which tend to accelerate the non-enzymatic addition of non-reducing sugars to free NH2 groups of proteins and lipids [25].
