The ability of reactive oxygen species produced by triggered neutrophilic leukocytes, hypoxanthine/xanthine oxidase (HX/XAO), hydrogen peroxide, and hypochlorous acid/myeloperoxidase (HOCl/MPO) systems to degrade hyaluronate (HA) in human synovial fluid (SF) and purified umbilical cord HA was compared by measuring the molecular weight distribution of HA using high-performance liquid chromatography with a size-exclusion column. The exposure of noninflammatory SF to phorbol myristic acetate (PMA)-activated neutrophils or to hydrogen peroxide (H2O2) caused depolymerization of SF HA to the degree corresponding to that found in rheumatoid SFs. When HX/XAO was used as radical generator, the molecular weight of SF HA decreased from 3.42 x 10(6) to 1.40 x 10(4) daltons with concomitant decrease of SF viscosity to 36% from the original value. The HOCl/MPO system caused no depolymerization of SF HA, even at very high unphysiological HOCl concentrations that induced the precipitation of SF HA together with SF proteins. This effect was found to be comparable to conventional mucin clot formation in SF. However, purified human umbilical cord HA was easily depolymerized with HOCl/MPO or with H2O2, but these effects were sensitive to the hydroxyl radical scavenger mannitol and iron chelator desferrioxamine, indicating that the formation of reactive hydroxyl radical (OH.) is likely to participate in these reactions. Thus we conclude that in inflammatory SF HA is mainly depolymerized by OH. produced by decomposition of H2O2 catalyzed by iron, free or locally bound to HA itself. In contrast to what has been reported earlier, HOCl/MPO only depolymerizes purified umbilical cord HA (in a hydroxyl radical-dependent manner) but does not depolymerize HA in SF. As a matter of fact, HOCl/MPO has a scavenging action on SF HA by consuming H2O2 and thus preventing the formation of reactive hydroxyl radicals.