Novel fluorescent compound (DDP) in calf, rabbit, and human articular cartilage and synovial fluid

Objective. To investigate the presence of and quantify 2,6-dimethyldifuro-8-pyrone (DDP), a novel fluorescent compound identified as in various calf, rabbit, and human tissue/fluid samples, to determine the DDP level in articular cartilage (AC) laminae, and to investigate the changes in cartilage DDP content with cartilage maturation. Methods. Samples were obtained from calf(< 2 years), rabbit (< 2 weeks to 2 years) or human AC and synovial fluid (SF) as well as other non-cartilaginous tissues. SF and tissue samples were hydrolyzed with 6 M HCl 124 hours at 110°C), lyophilized, and dissolved in HPLC mobile phase. DDP and collagen crosslink peaks were measured using a fluorescence detector at excitation and emission wavelengths of 295 and 395 nm, respectively. Results. DDP was detected from calf metacarpophalangeal dint AC (362 ± 48 pmol/mg dry weight), SF (4.5 ± 0.3 pmol/μl SF), and intervertebral disc (24 ± 4 pmol/mg). DDP was not detected in calf ligament, tendon, bone, ocular lens, cornea, or elastic cartilage. The DDP amount was greater in mid-deep cartilage lamina (448 ± 63 pmol/mg) than superficial-mid lamina (129 ± 52 pmol/mg) (p =0.008). DDP level decreased with maturation in rabbit knee joint AC from 185 ± 40 (< 2 weeks) to 27 ± 3 (2 years) pmol/mg dry weight. DDP was not detected in adult rabbit ligament, tendon, meniscus, or bone. DDP was detected in human knee joint AC and SF. The DDP level in osteoarthritic lesions was present in lower concentrations (range: 0 to 96 pmol/mg dried AC) compared to intact AC (range: 63 to 236 pmol/mg) of the same knee. Conclusion. DDP is a hyaline cartilage specific compound present in all articular cartilage samples from various articulating joints/animal species. DDP level increases with AC depth and decreases with cartilage maturation. DDP is a potential indicator of cartilage metabolism during normal growth, ageing, and cartilage disease.