Isolation and characterization of free form prostate specific antigen (f-PSA) in sera of men with prostate cancer.

PURPOSE: The free uncomplexed form of prostate specific antigen (f-PSA) from prostate cancer sera was partially isolated and characterized because the molecular form of f-PSA in the serum is unknown.
MATERIALS AND METHODS: 230 ml. of sera from 59 men with bone metastasis and individual PSA values of >2000 ng./mL were combined and centrifuged for 60 minutes at 30,000 RPM (4C). The sera were fractionated by gel filtration column chromatography (Sephacryl S-200, 2.5 cm. x 92 cm.). Free and complexed PSA in the eluted fractions were isolated by measuring immunoreactivity of PSA (Tosoh AIA-600 assay); f-PSA from 23 separate runs were combined, concentrated and re-chromatographed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to immobilize the isolated proteins onto a nitrocellulose membrane and a polyvinylidene difluoride (PVDF) membrane. Monoclonal antibody (F5) was used to probe PSA on nitrocellulose membrane and the PSA band was detected by Emission Chemoluminescence (ECL) kit. Amino terminal sequence analysis of the isolated f-PSA was performed with a gas-phase sequentor (Applied Biosyntens 4760 A) using the program designed by the manufacturer.
RESULTS: 0.5 cc of f-PSA (27,000 ng./mL) was obtained from serums after rechromatography. SDS-PAGE showed one double band around 30 kDa; with ECL technique, one major band at 30-kDa was identified as PSA. The amino terminal sequence analysis of this band showed residue 1 through 9 and 146 through 152.
CONCLUSIONS: In our preliminary experiment, the free form of serum PSA is partially isolated directly from human sera. Amino terminal sequence analysis has shown that serum f-PSA is not a pre-mature or zymogen form of PSA because serum f-PSA has a N-terminus identical to that of seminal fluid PSA. A nicked form of f-PSA is also found in these patient sera.