Enzymatic identification of the load-bearing boundary lubricant in the joint.

Bovine articular cartilage and synovial fluid (SF) were co-incubated with one of three enzymes selected to destroy each of the three major contenders for the active ingredient imparting such remarkable load-bearing lubrication to the normal joint. Destroying hyaluronic acid (HA), alias hyaluronan, with hyaluronidase, both frictional and wear tests displayed no significant change in accordance with most previous studies of SF alone. Destroying surface-active phospholipid (SAPL) with phospholipase A2, there was a highly significant dose-dependent compromise of lubrication as recorded on both tests. Trypsin produced a somewhat surprising result in that lubrication of the cartilage actually improved. This result can be interpreted as indicating that lubricin is not the lubricant per se, but, as a water-soluble, macromolecular, proteinaceous carrier for phospholipid, its destruction caused more SAPL to be deposited as the true load-bearing lubricant. These results are discussed in the context that SAPL, lubricin and HA each have specific roles in a comprehensive lubrication system.