Age-related accumulation of Maillard reaction products in human articular cartilage collagen

Non-enzymic modification of tissue proteins by reducing sugars, the so-call ed Maillard reaction, is a prominent feature of aging. In articular cartila ge, relatively high levels of the advanced glycation end product (AGE) pent osidine accumulate with age. Higher pentosidine levels have been associated with a stiffer collagen network in cartilage. However, even in cartilage, pentosidine levels themselves represent < 1 cross-link per 20 collagen mole cules, and as such cannot be expected to contribute substantially to the in crease in collagen network stiffness. In the present study, we investigated a broad range of Maillard reaction products in cartilage collagen in order to determine whether pentosidine serves as an adequate marker for AGE leve ls. Not only did the well-characterized AGEs pentosidine, N-epsilon-(carbox ymethyl)lysine, and N-epsilon-(carboxyethyl)lysine increase with age in car tilage collagen (all P < 0.0001), but also general measures of AGE cross-li nking, such as browning and fluorescence (both P < 0.0001), increased. The levels of these AGEs are all higher in cartilage collagen than in skin coll agen. As a functional measure of glycation the digestibility of articular c ollagen by bacterial collagenase was investigated; digestibility decreased linearly with age, proportional to the extent of glycation. Furthermore, th e arginine content and the sum of the hydroxylysine and lysine content of c artilage collagen decrease significantly with age (P < 0.0001 and P < 0.01 respectively), possibly due to modification by the Maillard reaction. The o bserved relationship between glycation and amino acid modification has not been reported previously in vivo. Our present results indicate that extensi ve accumulation of a variety of Maillard reaction products occurs in cartil age collagen with age. Altogether our results support the hypothesis that g lycation contributes to stiffer and more brittle cartilage with advancing a ge.