Modeling formalin fixation and antigen retrieval with bovine pancreatic ribonuclease A: I—Structural and functional alterations

Abstract

Understanding the chemistry of protein modification by formaldehyde is central to developing improved methods to recover proteins from formalin-fixed paraffin-embedded tissues for proteomic analysis and to improve protein immunoreactivity for immunohistochemical studies. We used biophysical techniques to investigate the effects of formaldehyde treatment on bovine pancreatic ribonuclease A (RNase A). Treatment of RNase A with formaldehyde was shown by gel electrophoresis to lead to the rapid formation of intra- and intermolecular protein cross-links. Thermal studies revealed that these protein cross-links significantly increased the thermal denaturation temperature of RNase A preparations. Analysis of formaldehyde-treated RNase A oligomers isolated by gel chromatography revealed that intramolecular protein cross-links are primarily responsible for the increase in protein thermostability. Formaldehyde treatment also lowered the isoelectric point of the enzyme from 9.45 to the 6.0–7.4 range. Optical spectroscopic studies demonstrated that the formaldehyde-induced modifications did not significantly alter the secondary or tertiary structure of RNase A. Heating formaldehyde-treated RNase A at 65°C resulted in a significant reversal of the protein intra- and intermolecular cross-links and led to a partial restoration of enzymatic activity.