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

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

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.

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.

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.