Prostate specific antigen releases a kinin-like substance on proteolysis of seminal vesicle fluid that stimulates smooth muscle contraction.

PURPOSE: We investigated whether purified prostate specific antigen (PSA), a seminal plasma serine protease of the kallikrein enzyme family, is capable of releasing kinin-like peptides from natural substrate glycoproteins in human seminal vesicle fluid. MATERIALS AND METHODS: An in vivo rat bladder model was used to monitor for release of substances capable of inducing smooth muscle contractions. Purified PSA, seminal vesicle fluid (SVF) from radical prostatectomy specimens, bradykinin, saline and a bradykinin antagonist were injected intravesically into urethane-anesthetized rats, and the resulting bladder contractions were measured. RESULTS: Injection of either PSA or SVF alone did not induce bladder contractions. Injection of a mixture of SVF and PSA preincubated 15 minutes, however, induced strong bladder contractions (23 +/- 7 cm. H2O) that decreased with time (4 +/- 2 cm. H2O, after 90 minutes). Similar contractions were observed after injection of bradykinin (10(-4) M. = 39 +/- 14, 10(-6) M. = 27 +/- 9, 10(-8) M. = 7 +/- 4 cm. H2O). Addition of a bradykinin antagonist to the PSA-SVF mixture prior to injection blocked the observed bladder contractions (23 +/- 7 cm. H2O before, versus 0.3 +/- 1.2 cm. H2O after adding antagonist). CONCLUSIONS: We conclude that PSA produces a kinin-like substance by enzymatic cleavage of glycoproteins in human seminal fluid. This substance induces smooth muscle contractions which can be specifically blocked by addition of a bradykinin antagonist.

Molecular mass and carbohydrate structure of prostate specific antigen: studies for establishment of an international PSA standard.

Despite the widely accepted use of prostate specific antigen (PSA) as a marker of prostate cancer, this molecule has not yet been completely characterized. Past studies have well established, however, using both amino acid and cDNA sequencing techniques, that PSA contains 237 amino acids, with a molecular mass of 26,079 Da for the peptide moiety of the molecule. The present study reports analysis of this protein by ion spray mass spectrometry (ISMS) and analysis of its carbohydrate moiety by NMR spectroscopy. The predominant PSA molecular species detected by ISMS was at relative molecular mass (M(r)) of 28,430, indicating that PSA contains a carbohydrate residue of M(r) 2,351, for a total percentage of carbohydrate of 8.3%. Analysis of PSA by SDS-PAGE, however, showed a M(r) of 32,000 to 33,000, suggesting an overestimation of the molecular weight by the latter technique. The complete primary structure of the PSA carbohydrate chain was determined by NMR spectroscopy in combination with carbohydrate composition analysis. The experimentally determined carbohydrate content of PSA confirms that only one N-glycosylation site is occupied in the protein. The proposed carbohydrate structure is a diantennary N-linked oligosaccharide of the N-acetyllactosamine type, with a sialic acid group at the end of each of the two branches. In addition, our data indicate that approximately 70% of the PSA molecules contain a fucose group in the core chitobiose moiety. The calculated molecular weight of this carbohydrate structure (M(r) 2,351.8) is in excellent agreement with the predicted molecular weight of the carbohydrate group, based on the M(r) 28,430 for PSA measured by ion spray mass spectrometry and M(r) 26,079 calculated from the consensus sequence for the peptide portion of the molecule. ISMS of PSA is thus proposed as a convenient and reliable method of quality control, an indispensible step towards international standardization of this very important tumor marker for detection and monitoring of prostatic diseases, especially prostate cancer.

Identity of PSA purified from seminal fluid by different methods: comparison by amino acid analysis and assigned extinction coefficients.

To determine the true extinction coefficient of prostate specific antigen (PSA) and to measure any differences in PSA when isolated from seminal fluid by four different published methods, we studied 10 different lots of PSA by quantitative amino acid analysis. Despite an expected PSA concentration of 1 mg/ml based on gravimetric analysis at an average optical density of 1.45 at 280 nm, we recovered only 0.79 mg/ml by quantitative amino acid analysis (range 0.752 to 0.820 mg/ml with a coefficient of variation [C.V.] of 3.3% among the 10 lots). The concentration of 0.79 mg/ml was based on a molecular weight of 28,430 daltons for glycosylated PSA determined by ion spray mass spectroscopy [Bélanger et al: Prostate, 27:187-197, 1995]. From these 10 amino acid analyses, we calculated the extinction coefficient of PSA at 280 nm as 1.84 +/- 0.04 ml x mg-1 x cm-1 (range 1.78 to 1.90 with a C.V. of 2.2%). Similar concentrations of PSA were obtained by amino acid analysis regardless of the method of purification. These observations support the presence of a single form of PSA in seminal fluid and are consistent with the molecular evidence that PSA is transcribed from a single gene locus on the long arm of chromosome 19 [Riegman et al.: Genomics 14:6-11, 1992]. They do not support the recent contention by the Roswell Park group that the PSA they isolated in 1979 [Wang et al.: Invest Urol 17:159-163, 1979; Wang et al.: Prostate 24:107-108, 1994] is different from p30 reported a year earlier [Sensabaugh: J Forensic Sci 23:106-115, 1978].

Biochemical analysis of prostate specific antigen-proteolyzed insulin-like growth factor binding protein-3.

Prostate specific antigen (PSA) has been shown to proteolyze IGFBP-3. However, the cleavage sites and mechanism of proteolysis are unknown. In this study, we proteolyzed recombinant human IGFBP-3 with PSA bound to a solid phase support. The reaction mixture was separated by centrifugation, with PSA remaining in the solid phase and the proteolyzed IGFBP-3 in the aqueous phase. The IGFBP-3 fragments were functionally analyzed by affinity labeling and Western ligand blotting (WLB). Further biochemical analyses were provided by silver staining of total protein and Western immunoblotting (WIB) of immunoreactive fragments with an IGFBP-3 specific antiserum (alpha-BP-3 g1). N-terminal sequence analysis was performed on filter-immobilized IGFBP-3 fragments, following size separation by SDS-polyacrylamide electrophoresis. PSA proteolyzed IGFBP-3 into at least 7 fragments (M(r) of 26 kDa to 13 kDa) as identified by silver staining and WIB. At least 3 fragments were visible by affinity labeling with radiolabeled IGF-I or IGF-II and 4 were weakly visible by WLB. These data indicate that some IGFBP-3 fragments retain their ability to bind IGF. N-terminal sequence analysis revealed at least 5 different proteolytic recognition sites for PSA in IGFBP-3. Three of the 5 sites were consistent with a 'kallikrein-like' enzymatic activity and 2 sites were consistent with a 'chymotryptic-like' enzymatic activity. The chymotryptic activity of PSA was further confirmed by the ability of alpha-1-antichymotrypsin and chymostatin to block PSA cleavage of radiolabeled IGFBP-3.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological effects of prostate specific antigen as an insulin-like growth factor binding protein-3 protease.

Prostate specific antigen (PSA) is an insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) protease found in seminal plasma and produced by prostatic epithelial cells (PC-E) in vivo. We examined the effects of PSA-proteolysis of IGFBP-3 on the affinity of IGFBP-3 fragments for IGFs and on the mitogenic action of IGFs on PC-E. Recombinant human IGFBP-3 was cleaved by PSA, then incubated with 125I-IGF-I or -II in the presence of varying concentrations of unlabelled peptides, and then cross-linking electrophoresis and densitometric analysis were performed. While the affinity of IGF-II for the PSA-generated IGFBP-3 fragments fell slightly compared to intact IGFBP-3, the affinity of the PSA-generated IGFBP-3 fragments for IGF-I fell by ten fold. The addition of IGF-I or -II to PC-E in serum-free culture conditions resulted in a two-fold stimulation of cell number compared to control. The presence of IGFBP-3 in the media blocked the IGF-induced stimulation, but had no independent effect in the absence of IGFs. When PSA was added to PC-E cultures to which both IGF-I or -II and IGFBP-3 were added, the inhibitory effects of IGFBP-3 on IGF mitogenesis were reversed. We conclude that PSA decreases the affinity of IGFBP-3 for IGF and can potentiate IGF action in the presence of inhibitory IGFBP-3. This phenomenon may contribute to normal and malignant prostate growth.

Issues on standardization of immunoassays for prostate-specific antigen: a review.

The proposed use of serum prostate-specific antigen (PSA) for annual screening of men over age 50 will require careful standardization of the various commercial immunoassays to allow year-to-year comparisons for individual patients. Some current PSA assays give significantly different results on testing the same sample. The standardization process will require several steps. First, a primary antigen standard should be purified from seminal plasma (a convenient source), using a reproducible technique. The modified Sensabaugh-Blake purification of PSA yields a suitable pure antigen. Next, PSA values need to be assigned to PSA-containing serum samples. These secondary serum-based reference materials can be used by manufacturers and regulatory agencies to develop and monitor the performance of PSA assays. In serum, 2 forms of PSA are detected immunologically: a free form (M(r) = 30 kD) and a form complexed with alpha-1-antichymotrypsin (M(r) = 100 kD). Different immunoassays for PSA detect these 2 forms in different molar ratios. The most promising approach to this problem is to select a reference immunoassay that detects both forms of PSA in equal molar ratios. A series of samples containing various and known levels of naturally occurring serum PSA can then serve as secondary serum-based reference materials for calibration of other commercial PSA immunoassays. Equimolar standardization is a useful method for any set of assays that detect free and bound forms of a ligand in differing molar ratios. The technical simplicity and power of this approach should allow early agreement on standardization of PSA immunoassays and will greatly assist PSA screening programs for prostate cancer now in progress.

An analysis of urinary prostate specific antigen before and after radical prostatectomy: evidence for secretion of prostate specific antigen by the periurethral glands.

We investigated whether urinary prostate specific antigen (PSA) might be a useful marker to detect locally recurrent tumor after radical prostatectomy. We also investigated whether PSA in the first 1 to 5 cc of voided urine is a more useful indicator of urinary PSA levels than the midstream urine PSA, since the first portion of the voided urine contains the highest concentration of prostatic and urethral secretions. To determine the response of urinary PSA to radical prostatectomy, we obtained first voided and midstream urine PSA levels in 18 patients with adenocarcinoma of the prostate before and after surgery. Mean first voided urine PSA levels decreased from 915.1 ng./ml. (range 21 to 2,853) preoperatively to 21.4 ng./ml. (range 0.9 to 88) postoperatively, while mean midstream urine PSA levels decreased from 245.9 ng./ml. (range 5 to 1,312) preoperatively to 1.8 ng./ml. (range 0 to 20.4) postoperatively. We also obtained postoperative first voided and midstream urine PSA levels in 9 prostate cancer patients who had undergone radical prostatectomy, and were considered to be cured by rigid clinical and histological criteria. To distinguish bladder versus urethral sources of urinary PSA in patients without a prostate, we additionally studied 11 patients without prostate cancer who had undergone cystoprostatectomy with orthotopic bladder substitution and who had undetectable serum PSA levels by the ultrasensitive assay. In the cured prostatectomy patients the mean first voided urine PSA level was 40.2 ng./ml. (range 2.8 to 100) and the mean midstream urine PSA level was 3.4 ng./ml. (range 0.1 to 15.2), while in the cystoprostatectomy patients these levels were 15.5 ng./ml. (range 0.8 to 49.9) and 1.2 ng./ml. (range 0 to 6.4), respectively. We conclude that the first voided urine sample better reflects local PSA production by the prostate than the midstream sample, first voided urine PSA decreases significantly in response to radical prostatectomy but is still present in measurable amounts even in surgically cured prostate cancer patients and urethral secretion of low levels of PSA persists after radical prostatectomy. This finding diminishes the chance that the first voided urine PSA level will be a useful marker to detect locally recurrent tumor after radical prostatectomy. Further studies are needed to determine if there is a critical level of first voided urine PSA after radical prostatectomy above which there is an increased likelihood of locally recurrent tumor but overall urinary PSA is highly unlikely to be clinically useful.

Early detection of residual prostate cancer after radical prostatectomy by an ultrasensitive assay for prostate specific antigen.

We evaluated the usefulness of an ultrasensitive immunoassay for prostate specific antigen (PSA), modified from the standard Yang Pros-Check PSA test and with a biological detection limit for PSA in serum of 0.07 ng./ml., to detect residual prostate cancer at an earlier date. We studied retrospectively serial frozen serum samples from 22 prostate cancer patients after radical prostatectomy who later had residual cancer with detectable PSA levels of 0.3 ng./ml. or more by the standard PSA test. As controls we studied 33 cystoprostatectomy patients (for bladder cancer) without histological evidence of prostate cancer and 23 patients after radical prostatectomy who had the highest probability of cure of the cancer. All control patients without cancer had PSA values (282 of 283 samples, 99.6%) of less than 0.1 ng./ml. This value was called the residual cancer detection limit. In the 22 patients with recurrent cancer the ultrasensitive assay detected cancer recurrence (PSA 0.1 ng./ml. or more) much earlier (median 202 and mean 310 days) than the standard assay (PSA 0.3 ng./ml. or more). On screening 187 current post-radical prostatectomy patients without evidence of cancer by the standard assay the ultrasensitive assay detected 21 (11.2%) with evidence of residual cancer, that is PSA level of 0.1 ng./ml. or more. We conclude that an ultrasensitive assay for PSA can detect residual cancer after radical prostatectomy much earlier than current immunoassays for PSA. Earlier detection of residual cancer may improve long-term survival by allowing for earlier institution of adjuvant therapy.

Prostate-specific antigen (PSA) is an insulin-like growth factor binding protein-3 protease found in seminal plasma.

Insulin-like growth factor binding protein-3 (IGFBP-3), the major serum carrier protein for the IGFs, is absent from Western ligand blots of seminal plasma, but is detectable by RIA. IGFBP-3 protease activity has recently been described in pregnancy serum. We investigated the possibility that seminal plasma contains an IGFBP-3 protease, by incubating seminal plasma with 125I-labeled human IGFBP-3. Seminal plasma was found to have potent IGFBP-3 protease activity with a cleavage pattern different from that of pregnancy serum. Prostate-specific antigen (PSA) is a serine protease found in semen. Autoradiographs measuring IGFBP-3 protease activity demonstrated that purified PSA cleaved IGFBP-3, yielding a cleavage pattern identical to that of seminal plasma. IGFBP-2 and -4 in seminal plasma were not degraded by PSA. Cleavage of IGFBP-3 by PSA resulted in a marked reduction in the binding affinity of the fragments to IGF-I, but not IGF-II. We speculate that PSA may serve to modulate IGF function within the reproductive system or in prostate cancer by altering IGF-IGFBP-3 interactions.

Ultrasensitive radioimmunoassay of prostate-specific antigen.

We describe a modification of the Yang Pros-check radioimmunoassay for prostate-specific antigen (PSA) that increases the analytical sensitivity of the assay approximately threefold (from a working range of 0.3-50 to 0.1-1.2 micrograms/L). It can detect PSA added to zero-concentration diluent (bovine serum albumin solution) at 0.10 microgram/L or added to zero-concentration control female serum at 0.20 microgram/L (P less than 0.05). In 26 patients tested after cystoprostatectomy for bladder cancer (who had normal prostates without cancer on histologic examination), PSA values by this ultrasensitive assay were all less than 0.10 microgram/L. Therefore, we propose this value as the upper limit of the 95% reference interval. In a retrospective study of two patients who developed recurrent prostate cancer, serum PSA values increased above the 0.1 microgram/L detection limit 175 and 581 days before increasing above the 0.3 microgram/L detection limit of the standard Yang assay. This ultrasensitive radioimmunoassay of PSA should prove more useful than current methods for detecting early recurrence of prostate cancer.

Synthetic substrates and inhibitors for serine proteases from lymphocytes, mast cells, semen, and blood.

The substrate specificity of several serine proteases from cytotoxic T lymphocytes, natural killer cells, mast cells, seminal fluid, and blood plasma has been determined with synthetic peptide thiobenzyl ester and p-nitroanilide substrates. Several new enzymatic activities have been discovered. A variety of inhibitors such as isocoumarins, trifluoromethyl ketones, and peptide chloromethyl ketones were used to study these enzymes and were found to be potent inhibitors.

Comparison of a polyclonal and monoclonal immunoassay for PSA: need for an international antigen standard.

Two leading commercial immunoassays for prostate specific antigen (the Yang Laboratories polyclonal radioimmunoassay and the Hybritech two-site monoclonal radioimmunometric assay) were compared using independently purified antigen as the calibrator. The Yang polyclonal assay yielded values from 1.4 to 1.9 times higher than the Hybritech monoclonal assay on the same sample, similar to results of previous investigators. When an independent prostate specific antigen calibrator was substituted for the respective kit calibrators, both assays yielded essentially identical results, thereby confirming that differences in assay values arise from differences in the assigned values of the kit calibrators. Otherwise, both assays had similar responses in the high and low range. Recovery studies of prostate specific antigen in various diluents demonstrated that antigen recovery was lowest in diluents with the highest protein content. We conclude that the kit manufacturers have assigned different values to their antigen calibrators. Sample values between the assays can be compared by use of an appropriate conversion table. Differences in their recommended ranges of normal for prostate specific antigen in male serum (zero to 2.5 ng./ml. for the Yang assay and zero to 4.0 ng./ml. for the Hybritech assay) reflect two factors: a difference in assigned calibrator values and differences in the selection of the "normal" population of men used to define the normal range of values. We urge adoption of an international standard calibrator for prostate specific antigen as a means to correct these differences.

Identity of prostate specific antigen and the semen protein P30 purified by a rapid chromatography technique.

Previous investigators have reported the identity of prostate specific antigen (PSA) and the semen protein p30, but data to support these claims have not been published. We report here the rapid and continuous purification of PSA using a simple two column chromatography technique originally developed for purification of p30. Using commercial antisera to PSA and original antisera to p30 for detection, we show that the two glycoproteins have identical purification profiles by this technique which uses cation exchange chromatography and sizing chromatography. The antigens also have identical molecular weights by gel electrophoresis and gel filtration. Furthermore, both antigens correspond to the same prominent protein band in seminal plasma by sodium dodecylsulfate polyacrylamide gel electrophoresis. On commercial immunoassay for PSA, purified p30 gives a calibration curve identical to the commercial PSA kit calibrator (Pros-checkTM PSA Radioimmunoassay, Yang Laboratories). We conclude that PSA and the semen protein p30 are identical and can be easily purified by a rapid continuous technique.

The effect of surface charge on non-specific binding of rabbit immunoglobulin G in solid-phase immunoassays.

The binding of IgG to a series of surface-immobilized proteins and synthetic polypeptides with differing isoelectric points was investigated by ELISA. The binding of rabbit IgG to a protein-coated test well surface was found to be highly charge dependent and relatively linear over a wide range of isoelectric points of the surface coating species. Binding was high to positively charged surfaces and low to negatively charged ones. When an intermediately charged protein, the semen antigen p30 (also known as prostate-specific antigen), was co-coated on the test surface with polypeptides of varying charge, the specificity of surface binding between anti-p30 IgG and non-immune IgG was altered. Co-coating p30 with the anionic glycoprotein AGP significantly enhanced binding specificity as reflected in an increased signal-to-noise ratio, whereas co-coating p30 with less negatively charged proteins led to a progressive decrease in the signal-to-noise ratio. The sialic acid moiety of AGP was primarily responsible for its ability to inhibit non-specific IgG binding. The results indicate that non-specific binding of IgG to polypeptide-coated ELISA test surfaces is highly charge dependent. In solid-phase immunoassays which measure antigen-specific antibody, non-specific binding of immunoglobulins may be diminished by co-coating surface antigens with a hydrophilic anionic surface matrix which resembles in some respects the cell surface glycocalyx.

Noise control in solid-phase immunoassays by use of a matrix coat.

This paper describes the matrix coat noise control technique as applied in an ELISA to detect IgG antibody to the semen protein p30 (also known as prostate-specific antigen). Background noise due to non-specific binding of IgG in solid-phase immunoassays has recently been shown to be highly charge dependent. In this technique, the surface antigen (here, p30) is co-coated on the test surface with an anionic macromolecule, the noise reduction matrix component, to reduce non-specific IgG binding. A second matrix component, the noise balancing component, is added if necessary to balance non-specific IgG binding between a detecting well which contains matrix plus antigen and a control well which contains matrix alone. Sample noise can then be measured in the control well and subtracted to yield a noise-corrected signal. This approach both minimizes background noise and then precisely quantitates residual noise. In this study, human alpha 1-acid glycoprotein (AGP) served as the noise reduction component and bovine serum albumin (BSA) as the noise balancing component. Optimal coating concentrations of AGP and BSA for the matrix were determined by a new technique, the tetrad method of signal and noise analysis. A signal probe (immune serum), a noise probe (non-immune serum) and a set of four test wells are employed to analyze the noise control properties of a given combination of matrix components. For each such tetrad of wells, three ratios are calculated: the sensitivity, signal-to-noise and noise balance ratios, along with a composite index, the matrix index. These can be used to select the combination and concentrations of matrix components which optimize assay noise control and minimize losses of assay sensitivity. When an ELISA using matrix coat noise control (MCNC-ELISA) was compared with ELISAs using standard blocking techniques, the MCNC-ELISA was superior in its ability to control for false positive results due to non-specific IgG binding and binding of the polycation poly-L-lysine. Two subjects (14%) out of a clinic population of sexually active women were found to have significant serum levels of anti-p30 IgG (P less than or equal to 0.001) by MCNC-ELISA.

Postcoital detection of a male-specific semen protein. Application to the investigation of rape.

Identification of semen in vaginal fluid may provide documentation of sexual contact in alleged victims of rape. We describe an enzyme-linked immunosorbent assay for a semen glycoprotein of prostatic origin, designated p30. This test detects as little as 3 ng of the p30 antigen per milliliter in various body fluids. Semen from normal and vasectomized men contains high levels of p30 (mean, 1.55 mg per milliliter of seminal plasma), and urine from men contains low levels (mean, 260 ng per milliliter). However, the antigen cannot be detected in body fluids from women, including vaginal fluid and urine, suggesting that p30 may be a male-specific antigen. The p30 antigen was detectable in vaginal fluid for a mean period of 27 hours after coitus, as compared with 14 hours for prostatic acid phosphatase. Of 27 vaginal fluid samples from women who were allegedly raped in which the acid phosphatase test was negative, 7 (26 per cent) were unequivocally positive for p30 by our assay. We conclude that the assay for p30 offers a more sensitive and specific method of semen detection in rape investigation than the enzyme assay for prostatic acid phosphatase.