A general affinity method to purify peroxidase-tagged antibodies

Antibodies tagged with enzymes, e.g. horseradish peroxidase (HRPO) are used extensively in a broad range of immunoassay, immunohistochemical, and prodrug-based immunotherapeutic applications. These antibodies may be polyclonal, monoclonal, bispecific or genetically engineered in origin. Often, purification of the antibody is the single greatest obstacle to obtaining immunoprobes with high specific activity [Milstein and Cuello, Nature 305 (1983) 537]. We have circumvented this problem by utilising benzhydroxamic acid-agarose to purify the antibodies tagged with HRPO as a preformed immune complex. Benzhydroxamic acid has been shown to have affinity for the active site of HRPO [de Ropp et al., Biochemistry 38 (1999) 1077]. A preliminary ammonium sulfate precipitation of 250 ml of bispecific antibody supernatant was performed and the pellet resuspended and dialysed against phosphate buffer (pH 7.0). This fraction was incubated with HRPO, then loaded on the affinity column which was washed, and the labelled bispecfic monoclonal antibodies were eluted under mild conditions (borate buffer pH 9.0). The effective yield of this bispecific antibody-HRPO complex was 30 assay plates or 3000 wells. We have also successfully co-purified covalent polyclonal-HRPO conjugates and HRPO-labelled streptavidin using a similar strategy to obtain enzyme-labelled probes with high specific activities for a multitude of applications.

ELISA for a complexed antigen with a monoclonal antibody blocking reaction with the free antigen-assay-specific for complexed prostate-specific antigen

We have developed an enzyme-linked immunoassay (ELISA) for serum complexed-antigen (prostate-specific antigen; PSA)(c-PSA) with simultaneous blocking of free-antigen (PSA)(f-PSA). The assay utilizes three different monoclonal antibodies (MAbs) recognising three distinct PSA epitopes. The detection limit was established as 0.19 microg/l (n=20, mean of zero standard+2S.D.) and the average recovery of f-PSA was 98-100%. The within-run and between-day coefficients of variation (CV) ranged from 2.1% to 3.2% and 2.8% to 6.3%, respectively. There was a good correlation between serum c-PSA measured by the present ELISA and PSA-alpha(1)-antichymotrypsin complex (PSA-ACT) concentrations (r=0.991). This method should provide a better tool for discriminating between benign and malignant prostatic disease.

A highly sensitive enzyme-linked immunoassay for serum free prostate specific antigen (f-PSA)

Monoclonal antibodies (MAbs) were generated against human prostate specific antigen (PSA) to allow development of a sensitive free-PSA (f-PSA) assay. Of a total of 211, 12 could detect only f-PSA, the other 199 MAbs binding to both f-PSA and complex-PSA. In the present study, one MAb (no. 5C6) specific for f-PSA and another (no. 79) reacting with both forms were used to develop an enzyme-linked immunoassay (ELISA) for serum f-PSA. The detection limit was established as 0.008 microg/l (n = 20, mean of zero standard + 3 S.D.) and the average recovery of f-PSA was 93-102%. The within-run and between-day coefficient of variation (CV) varied from 5.4-7.4% and 4.8-6.5%, respectively. The cross-reactivity of the assay to PSA-alpha1-antichymotrypsin complex was determined to be < 0.4%.

Using proportions of free to total prostate-specific antigen, age, and total prostate-specific antigen to predict the probability of prostate cancer

OBJECTIVES

This study was undertaken to define the probability of prostate cancer as a function of the proportion of free to total prostate-specific antigen (FTPSA), total PSA, and age for those patients with total PSA levels between 2.5 and 20.0 ng/mL.

METHODS

Prebiopsy serums were obtained from 428 untreated patients (165 malignant, 263 benign) who had undergone sextant six-core biopsy. Each patient had no prior history of prostate cancer and a prebiopsy total PSA value between 2.5 and 20.0 ng/mL. Total PSA levels were determined using the PA immunoassay performed on the TOSOH AIA-1200 automated immunoassay instrument. Free PSA levels were determined using a monoclonal-polyclonal antibody sandwich radioimmunoassay.

RESULTS

In men with total PSA values between 2.5 and 20.0 ng/mL, the FTPSA significantly differentiated between patients with benign and malignant histologic states. Log linear modeling indicated distinct differences in the risk for cancer as a function of FTPSA, total PSA, and age. The highest probability for cancer was observed in men greater than 70 years of age who had a FTPSA less than 7% and total PSA more than 10.0 ng/mL. Conversely, the lowest probability for cancer was observed in patients less than 60 years of age who had a FTPSA more than 25% and a total PSA less than 4 ng/mL.

CONCLUSIONS

The probability that prostate cancer will be found on biopsy has a marked gradient that is associated with age, total PSA, and FTPSA. The extreme ends of FTPSA of less than 7% and more than 25% are diagnostic for prostate cancer and benign prostatic disease, respectively.

Single-step purification of prostatic acid phosphatase: immunoaffinity chromatography with a monoclonal antibody

BACKGROUND

Prostatic acid phosphatase (PAP) is an important protein which should be studied further as a tumor marker or as a biologically functional molecule. The purpose of the study was to establish a simple and reliable method to obtain highly pure PAP.

METHODS

Spleen cells from mice immunized with prostatic epithelial cells prepared from benign prostatic hyperplasia tissue were fused with myeloma cells X63Ag8-653. Hybrid cells of interest were selected using the indirect immunofluorescence method with unfixed frozen tissue sections. One clone of the hybrid cell lines was established which secreted the monoclonal antibody specifically reactive to prostatic acid phosphatase. Using this monoclonal antibody, we purified the antigen from human prostatic tissue by means of single-step immunoaffinity chromatography.

RESULTS

SDS-PAGE profiling under reducing conditions indicated that the protein recognized by this antibody consisted of several components of molecular weight 41,000-45,000. Partial amino acid sequence analysis of this protein indicated that these components involved a heterogeneously modified single polypeptide, and that this antigen is identical to human prostatic acid phosphatase.

CONCLUSIONS

This single-step method saves the time needed to purify prostatic acid phosphatase and requires only half a day for the whole procedure. Moreover, the purity of the isolated protein was extremely high. This method seems to be useful not only for purifying prostatic acid phosphatase but also for purifying other proteins from the prostate gland and for analysis of antigenic macromolecules.

Measurement of the proportion of free to total prostate-specific antigen improves diagnostic performance of prostate-specific antigen in the diagnostic gray zone of total prostate-specific antigen

OBJECTIVES

This study examined the clinical significance of non-complexed (free) prostate-specific antigen (PSA) in the differential diagnosis of prostate cancer with an emphasis on patients with total PSA values between 4.0 and 10.0 ng/mL (the diagnostic gray zone).

METHODS

Serum samples were obtained from three specimen banks. Patient samples consisted of 55 untreated histologically confirmed primary cancer, 62 men with untreated benign prostatic disease histologically confirmed by 6 negative sextant biopsies, and 64 asymptomatic healthy male controls with normal digital rectal examinations and PSA values less than 4.0 ng/mL. All patients were between the ages of 50 and 75 years. Total PSA levels were determined using the PA immunoassay performed on the TOSOH AIA-1200 automated immunoassay instrument. Free PSA levels were determined using a monoclonal-polyclonal antibody sandwich radioimmunoassay. The proportion of free to total PSA was calculated by dividing the patient's free PSA value by the total PSA value.

RESULTS

When all subjects were included, both total PSA and the proportion of free to total PSA significantly differentiated between patients with prostate cancer and patients with benign histologic conditions (P < 0.0001). However, in men with total PSA values between 4.0 and 10.0 ng/mL, the proportion of free to total PSA significantly differentiated between patients with benign and malignant histologic conditions (P = 0.0004), whereas the total PSA did not (P = 0.13). Among this subgroup of patients, the analysis of sensitivity and specificity showed that the proportion of free to total PSA had a clearly higher specificity compared with that of the total PSA at the same level of sensitivity.

CONCLUSIONS

Measurement of the free PSA level in a patient's serum and calculation of the proportion of free to total PSA enhances the ability to distinguish benign histologic conditions from cancer while retaining high sensitivity for detecting cancer in men who present with total PSA levels between 4.0 and 10.0 ng/mL. A large-scale population-based study is currently in progress to confirm this preliminary finding.

Purification of 5'-nucleotidase from human seminal plasma

5'-Nucleotidase from human seminal plasma was purified to electrophoretic homogeneity and some of its kinetic and molecular properties compared with those of 5'-nucleotidase from bull seminal plasma. The purification of the enzyme was achieved by using the same affinity chromatography media (Con A-Sepharose and AMP-Agarose or ADP-Agarose) previously used for the purification of bull seminal plasma 5'-nucleotidase (Fini, C., Ipata, P.L., Palmerini, C.A. and Floridi, A. (1983) Biochim. Biophys. Acta 748, 405-412). However, in the present purification procedure no detergent was used as it had been necessary for the purification of the bovine enzyme. The experimental data reveal some main differences between these two enzymes; first, the human enzyme seems to be constituted of a single polypeptide chain of about 71 kDa, while the 5'-nucleotidase of bull seminal plasma, in non denaturing detergent solutions, is a homodimer of about 160 kDa. Another most remarkable difference is that the human enzyme does not seem to contain a phosphatidylinositol anchoring system like the one present in the bovine enzyme and in 5'-nucleotidase of different sources (Low, M.G. (1987) Biochem. J. 244, 1-13). Finally, the AMPase activity of 5'-nucleotidase from human seminal plasma is not affected by dithiothreitol which, on the contrary, is a powerful inhibitor of the bovine enzyme causing the dissociation of its subunits which are held together by disulphide bridges (Fini, C., Minelli, A., Camici, M. and Floridi, A. (1985) Biochem. Biophys. Acta 827, 403-409).