Aspartic acid racemization reveals a high turnover state in knee compared with hip osteoarthritic cartilage.
OBJECTIVE
We investigated tissue turnover in healthy and osteoarthritic cartilage. We challenge long held views that osteoarthritis (OA) is dominated by a similar turnover process in all joints and present evidence that hip and knee cartilage respond very differently to OA.


METHODS
d- and l-Aspartate (Asp) were quantified for whole cartilage, collagen and non-collagenous components of cartilage obtained at the time of joint replacement. We computed the Asp racemization ratio (Asp-RR = d/d + l Asp), reflecting the proportion of old to total protein, for each component.


RESULTS
Compared with hip OA, knee OA collagen fibrils (P < 0.0001), collagen (P = 0.007), and non-collagenous proteins (P = 0.0003) had significantly lower age-adjusted mean Asp-RRs consistent with elevated protein synthesis in knee OA. Knee OA collagen had a mean hydroxyproline/proline (H/P) ratio of 1.2 consistent with the presence of type III collagen whereas hip OA collagen had a mean H/P ratio of 0.99 consistent with type II collagen. Based on Asp-RR, the relative age was significantly different in knee and hip OA (P < 0.0005); on average OA knees were estimated to be 30 yrs 'younger', and OA hips 10 yrs 'older' than non-OA.


CONCLUSIONS
The metabolic response to OA was strikingly different by joint site. Knee OA cartilage evinced an anabolic response that appeared to be absent in hip OA cartilage. These results challenge the long held view that OA cartilage is capable of only minimal repair and that collagen loss is irreversible.
['Catterall|J B|JB|', 'Zura|R D|RD|', 'Bolognesi|M P|MP|', 'Kraus|V B|VB|']
D000293:Adolescent D000328:Adult D000367:Age Factors D000368:Aged D000369:Aged, 80 and over D001224:Aspartic Acid D002358:Cartilage, Articular D016022:Case-Control Studies D003094:Collagen D024043:Collagen Type II D024061:Collagen Type III D026603:D-Aspartic Acid D005260:Female D006801:Humans D006909:Hydroxyproline D008297:Male D008875:Middle Aged D015207:Osteoarthritis, Hip D020370:Osteoarthritis, Knee D011392:Proline D055815:Young Adult
Racemization Osteoarthritis Collagen Cartilage turnover