The effect of phospholipase digestion upon the boundary lubricating ability of synovial fluid

Objective. To identify the boundary lubricant in synovial fluid (SF). Is sy novial lubrication mediated by surface active phospholipid as opposed to mu cinous glycoprotein? Methods. A sonicated preparation of phosphatidylcholine and bovine SF were tested in vitro in a bearing of latex oscillating against polished glass un der a load of 0.35 x 10(6) N/m(2). The friction apparatus isolates conditio ns of boundary lubrication and has been validated against a cartilaginous b earing. Coefficient of friction (mu) was measured and compared against mu f rom physiologic saline, which served as a control. Separate digestions were carried out upon the SF with trypsin, phospholipase C, and phospholipase A (2) in the presence and absence of proteolytic inhibitors. Results, Digestions of bovine SF by phospholipase C in the presence of prot ease inhibitors did not remove boundary lubricating ability compared to an undigested control (p = 0.89). Digestion of bovine SF with trypsin removed all lubricating ability and raised friction (p = 0.004). Commercial purifie d phospholipase C contained trypsin-like activity when activity was tested with N alpha-benzoyl-L-arginine ethyl ester as substrate. Similar results w ere observed for phospholipase A(2), which possesses a lower amount of tryp sin activity. Conclusion, The results indicate that phospholipid does not play a prominen t role in synovial fluid's ability to lubricate an artificial bearing. Rath er, the boundary lubricating ability of SF is attributable to lubricin, a m ucinous glycoprotein.