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The nonenzymatic glycosylation reaction that is accelerated in diabetes is the first step of the Maillard or nonenzymatic browning reaction that occurs in stored food. The glucose-protein adduct rearranges and dehydrates to form brown and fluorescent pigments, which can act as crosslinks, resulting in decreased protein solubility and altered mechanical properties. Evidence suggesting that this process occurs in vivo has been found in lens crystallins. The observation that nonenzymatic glycosylation and insolubility increases in collagen with age and diabetes led us to investigate the possible browning of human collagen. Several complications of diabetes mellitus occur in collagenrich tissues and resemble processes and diseases characteristic of aging. These include earlier onset and greater severity of atherosclerosis (1), stiffening of lungs (2) and large arteries (3, 4), thickening of capillary and glomerular basement membranes (5, 6), periarticular rigidity (7), and osteoarthritis (8). With age, collagen becomes less soluble (9, 10), more crosslinked (11), and more glycosylated (10,12), and it accumulates yellow and fluorescent pigments (13,14). In diabetes, several of these changes occur at an earlier age, suggesting an apparent acceleration of the aging process. Compared with age-matched nondiabetics, collagen from diabetics is less soluble (10), is more resistant to digestion by collagenase (15) and cyanogen bromide (16), has more pepsin-releasable high molecular weight peptides (10), and is more nonenzymatically glycosylated (10,15,17). Glycosylation and subsequent crosslinking occur in food proteins that are stored or heated in the presence of reducing sugars and are due to the Maillard or nonenzymatic browning reaction. Glucose, for example, first reacts nonenzymatically with free amino groups on proteins to form a stable amino 1-deoxyketosyl adduct, also called the Amadori product. Nonenzymatic glycosylation has been shown to occur with hemoglobin to form hemoglobin Al, and with a variety of other body proteins-e.g., lens crystallins, collagen, and nerve proteins (18 proteins (20). The recent demonstration that insolubility and resistance to enzymatic digestion of human collagen increase with age and diabetes in parallel with nonenzymatic glycosylation (10) led to the present investigation of the browning process in the same subject population. Insoluble dura mater collagen from nondiabetics and subjects with type I and type II diabetes was examined for the presence of yellow and fluorescent pigments similar to those that form in the nonenzymatic browning reaction with glucose. MATERIAL AND METHODSThe samples of dura mater used in this study were obtained at autopsy from the same subjects in whom age-and diabetes-related changes in skin collagen solubility and glycosylation were reported by Schnider and Kohn (10). These include 17 samples of dura mater from subjects without clinical or pathological evidence of connective tissue disease or diabetes mellitus and similar samples from 3 subjects w...

Nonenzymatically glycosylated proteins gradually form fluorescent cross-linked protein adducts--a process termed "browning." The rate of this reaction increases with the glucose concentration. Assaying for the presence of browning products in long-lived proteins should therefore provide information on long-term metabolic control. We measured collagen-linked fluorescence typical for nonenzymatic browning in skin-biopsy specimens from 41 subjects with longstanding Type I diabetes and from 25 controls. Fluorescence correlated with age and (weakly) with the duration of diabetes. Mean age-adjusted fluorescence values were twice as high in diabetic subjects as in control subjects (P less than 0.0001) and increased with the severity of retinopathy, nephropathy, and arterial and joint stiffness. The correlation was significant for retinopathy (r = 0.42; P less than 0.01), arterial stiffness (r = 0.41; P less than 0.01), joint stiffness (r = 0.34; P less than 0.05), and the sum of all complications (r = 0.47; P less than 0.01). Fluorescence also correlated with systolic (r = 0.42; P less than 0.01) and diastolic (r = 0.36; P less than 0.05) blood pressures. If one can assume that the fluorescence results from a browning product of glucose, our data suggest that there is an overall correlation between the severity of diabetic complications and cumulative glycemia over many years.