Mucin synovial fluid

The mucin precipitated from synovial fluids by weak acids appears to be a dissociable complex of hyaluronic acid and proteins. Electrophoretic studies of synovial fluids (at pH 8.6) have usually shown the presence of one fast-moving component considered to be free hyaluronic acid, but some fluids show a second fast component, possibly a hyaluronic acid -- protein complex (69). On the basis of studies in the ultracentrifuge, the mucin possibly may exist in synovial fluid partially as a mucoprotein with a molecular weight of one to ten million (70). Electrophoretic and ultracentrifugal measurements at varying pH values provide two of the best ways of demonstrating interaction in such systems. 

In 1943 E. A. Balazs and L. Filler published a paper in which they described a study of role of hyaluronan in dog knee joints. They found that the intercellular substance of connective tissue of the synovium contains sufficient viscous mucin that can replace the mucin removed from the knee [5]. These observations literally opened the door to further studies on the role of hyaluronan in normal and traumatic joints. In 1949, C. Ragan and K. Mayer published a very important paper in which they described the observation of hyaluronan in rheumatoid synovial fluid. This was the first study in which normal and pathological synovial fluids were compared by determination of the concentration and viscosity of hyaluronan [6]. 

Isolation from Synovial Fluid. The material obtained from the tarso-tibial joint of cattle or horses is diluted with 1 to 3 volumes of water and the hyaluronate and proteins present are precipitated by adding acetic acid to a concentration of 0.2%. Hyaluronic acid is isolated from the mucin precipitate either by solvent purification (10,88,133) or by enzymic digestion of the proteins with trypsin (161,163,168) or pancreatin (99). Digestion is considered complete when an aliquot does not give a mucin clot with acetic acid (168). The digest is purified by McClean s alcohol precipitation (107), by dialysis, or by adsorption on Lloyd s reagent (99, 161,168). Hyaluronate from synovial fluid frequently contains appreciable amounts of phosphorus. 

A variation of the M. C. P. method by Gunter (56) is based on the stringiness of the mucin clot and measures the length to which a filament of the substrate solution can be drawn at a standard velocity. Hyaluronidase destroys the stringiness. Dialyzed synovial fluid is used as the substrate. The method requires, a special apparatus for the determinations. The assay is independent of hydrogen ion concentration between pH values of 6.1 and 7.9. The optimum sodium chloride concentration lies between 0.1 and 0.2 M. A unit is defined as the amount of enzyme which will reduce the stringiness to 50% of the initial value in 20 minutes under the conditions of the experiment. The method appears to be considerably more sensitive than the M. C. P. test. 

The site of origin of hyaluronic acid in the body is unknown. The presence of hyaluronate in ocular fluids and its absence from serum have been cited as evidence that it represents a secretion rather than a dialyzate (270). Meyer (264) has pointed out that the concept of synovial fluid as a dialyzate to which the mucin is added during passage through the connective tissue does not seem probable since, if such a mechanism exists, pleura and peritoneal fluid and lymph should likewise contain the mucin, which has not been shown to be the case. The fact that a viscous fluid appears in cultures of synovial tissue (426) was cited by him as evidence that hyaluronate is a secretory product of some cells of the synovial lining. This seems to be borne out by the fact that hyaluronic acid has been isolated from a synovioma, not only at the site of the tumor but in metastases in the liver (264). 

---