The Effect of Phospholipase Digestion Upon the Boundary Lubricating Ability of Synovial Fluid

Objective: To identify the boundary lubricant in synovial fluid (SF). Is synovial lubrication mediated by surface active phospholipid as opposed to mucinous glycoprotein?

Methods: A sonicated preparation of phosphatidylcholine and bovine SF were tested in vitro in a bearing of latex oscillating against polished glass under a load of 0.35 x 10(6) N/m2. The friction apparatus isolates conditions of boundary lubrication and has been validated against a cartilaginous bearing. Coefficient of friction (mu) was measured and compared against mu from physiologic saline, which served as a control. Separate digestions were carried out upon the SF with trypsin, phospholipase C, and phospholipase A2 in the presence and absence of proteolytic inhibitors.

Results: Digestions of bovine SF by phospholipase C in the presence of protease 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 purified phospholipase C contained trypsin-like activity when activity was tested with N alpha-benzoyl-L-arginine ethyl ester as substrate. Similar results were observed for phospholipase A2, which possesses a lower amount of trypsin activity.

Conclusion: The results indicate that phospholipid does not play a prominent role in synovial fluid's ability to lubricate an artificial bearing. Rather, the boundary lubricating ability of SF is attributable to lubricin, a mucinous glycoprotein.