The role of hyaluronic acid in protecting surface-active phospholipids from lysis by exogenous phospholipase A(2)

Background. This in vitro study aimed to elucidate the extent and kind of i nvolvement of hyaluronic acid (HA) in the currently accepted view of synovi al joint lubrication, in which surface-active phospholipids (SAPL) constitu te the main boundary lubricant. The integrity of SAPL is apparently threate ned by the lysing activity of phospholipase A(2) (PLA(2)). Methods. The effects of increasing concentrations of HA degraded by free ra dicals and non-degraded HA on the lysing activity of PLA2 were examined in vitro. Liposomes (lipid model membrane) containing phosphatidylcholine (PC) were used as the substrate, on the assumption that they are appropriate re presentatives of SAPL. Results. HA adhered to the phospholipid membrane (liposomes), inhibiting th eir lysis by PLA(2). However, in its degraded form, IIA not only failed to inhibit PLA(2)-lysing activity, but accelerated it. Conclusions. It is reasonable to assume that IIA plays an important indirec t role in the steady state of the boundary lubrication process of joints by protecting SAPL from being lysed by PLA(2). However, as excessive loading generates free radicals within the joint (among other effects), the HA that is degraded in this way is incapable of protecting SAPL front lysis by PLA (2). When the rate of degradation exceeds that of synthesis, there will be insufficient replacement of HA and/or SAPL, resulting in denudation of the articular surfaces. These are then exposed to increasing friction, and henc e increased danger of degenerative joint changes.