Screening of Pancreatic Ribonuclease A Inhibitors from an Autodisplayed Fv-Antibody Library.

Pancreatic ribonuclease A (RNase A) inhibitors were screened from an autodisplayed Fv-antibody library, which was prepared by randomizing amino acid sequences of the third complementary-determining region (CDR3) within the heavy chain variable region (V H region) of immunoglobulin G (called "Fv-antibody" comprising three CDRs and four frame regions (FRs)) through site-directed mutagenesis. The library was autodisplayed on the outer membrane of Escherichia coli . Target Fv-variants (clones) with specific binding affinity for RNase A were screened using fluorescein-labeled RNase A and flow cytometry. Three Fv variants (clones) were screened, and CDR3 amino acid sequences were analyzed. The screened Fv-antibodies were expressed as soluble proteins, and CDR3 was synthesized into peptides (11 residues). The binding affinity constants ( K D ) of the expressed Fv-antibodies and synthesized peptides to RNase A were estimated using surface plasmon resonance. Fitting analysis based on the adsorption model showed that K D values of the three expressed Fv-antibodies were estimated to be 17.5 ± 4.1, 28.8 ± 9.7, and 33.9 ± 8.9 nM ( n = 3), and those of the three synthesized peptides were 1.3 ± 0.1, 1.3 ± 0.3, and 3.7 ± 1.3 μM ( n = 3). From the RNase activity assay with an RNA probe labeled with fluorophore and quencher, inhibition constants (IC 50 ) of the three expressed Fv-antibodies were estimated to be 90.2, 65.3, and 98.8 nM ( n = 3), and those of the three synthesized peptides were 8.1, 3.6, and 0.4 μM ( n = 3). The activity of RNase inhibitors constituting the expressed Fv-antibodies and synthesized peptides was demonstrated via an RNA cleavage test using the total RNA from HeLa cells.