Prostate specific antigen--a new constituent of breast cyst fluid.

We demonstrate for the first time that prostate specific antigen (PSA) is a component of breast cyst fluid. At the cutoff level of 0.01 or 0.03 micrograms/L of PSA, 64% or 43% of cyst fluids are positive for PSA, respectively. PSA in cyst fluid, as characterized by high performance liquid chromatography, exists in almost equal concentrations as free PSA, with a molecular weight of 33 KDa, and as PSA bound to alpha 1-antichymotrypsin, with a molecular weight of 100 KDa. PSA presence was also characterized in cyst fluid by Western blot analysis. These data suggest that PSA is a frequent component of breast cyst fluid. More studies are needed to establish the role of this serine protease in normal breast, gross breast cystic disease, and breast cancer.