Autoxidation of ascorbate mediates lysine N-pyrrolation

Abstract

Protein N-pyrrolation, which converts lysine residues to N^ε-pyrrole-l-lysine (pyrK), is a naturally occurring covalent modification. The pyrrolated proteins have a unique property of binding to DNA-staining agents, such as SYBR Green I (SG), and anti-DNA antibodies, suggesting a physiologically relevant modification that gives rise to DNA mimic protein. These properties of pyrrolated protein are suggested to be associated with innate and autoimmune responses. Short-chain aldehydes derived from lipid peroxidation are thought to be involved in the formation of pyrK. We now report that similar lysine N-pyrrolation also occurs during the metal-catalyzed oxidation of proteins with ascorbate. When human serum albumin (HSA) was incubated with Fe²⁺/ascorbate in the presence and absence of docosahexaenoic acid, the protein was converted to SG-binding proteins even without the polyunsaturated fatty acid. The formation of SG-binding proteins by Fe²⁺/ascorbate was accompanied by the formation of pyrK, which was also detected in ascorbate-treated hemoglobin. Moreover, the metal-catalyzed oxidation of ascorbate produced the pyrrolation factors, glycolaldehyde and glyoxal. These results and the observations that sera from autoimmune-prone MRL-lpr mice recognized modified proteins with Fe²⁺/ascorbate and with glycolaldehyde/glyoxal suggest that the autoxidation of ascorbate, as well as lipid peroxidation, can be a source of autoantigenic N-pyrrolated proteins. Our findings revealed a possible function of ascorbate as an endogenous source of pyrrolated proteins and suggested that the pyrK residues generated in proteins may play a role in the innate and autoimmune responses associated with the oxidative metabolism of ascorbate.

Keywords:

• Lysine modification
• pyrrolation
• ascorbate
• autoxidation
• glycolaldehyde

Acknowledgements

We thank Ms Yuki Hondoh for her excellent editorial support.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conceptualization: K.U.; draft preparation: J.Y., T.S., and K.U.; performed experiments: J.Y.; administrative, technical support: M.C.; and study supervision: K.U.

Disclosure statement

The authors declare no competing financial interest.

Additional information

Funding

This work was supported in part by Grant-in-Aid for Scientific Research (S). 17H06170 (to K. U.), Grant-in-Aid for Scientific Research (C). 21K11694 (to J. Y.), and Grant-in-Aid for Scientific Research on Innovative Areas “Oxygen Biology: a new criterion for integrated understanding of life” 26111011 (to K. U.) of the Ministry of Education, Sciences, Sports, Technology, Japan. This research was also supported by AMED-CREST from AMED (to K. U.).