Does hydrogen peroxide exist “free” in biological systems?

Abstract

Hydrogen peroxide (H 2O 2) can diffuse far from the site of production to intracellular locations where biological effects may be greater. The diffusion range is extended by H 2O 2 carriers formed spontaneously by hydrogen bonding with monomeric and polymeric compounds, including amino and dicarboxylic acids, peptides, proteins, nucleic acid bases, and nucleosides. Hydrogen peroxide adducts (HPAs) are readily synthesized, e.g., crystalline histidine (His)-H 2O 2 adducts. An equilibrium exists between an adduct-forming compound and H 2O 2. The detection and relative stabilities of HPAs are measured by the degree of decomposition of H 2O 2 as influenced by test compounds in buffered solution competing with glucose or fructose for H 2O 2. The HPAs delay decomposition of H 2O 2 up to several hundredfold. The overall charge on an HPA, i.e., its ability to penetrate cell membranes, influences the cytotoxic and clastogenic effects of H 2O 2. Growth inhibition of Salmonella typhimurium LT 2 by H 2O 2 is enhanced by neutral HPAs but decreased by anionic HPAs. Addition of catalase 1, 10, or 30 min after inoculation of S. typhimurium LT 2 reduces or nearly eliminates partial growth inhibition by H 2O 2, but a neutral HPA, expecially his-H 2O 2, transported H 2O 2 into the cells within 1 min, and in about 10 min completely inhibited growth. The stability of HPAs decreases with increasing pH or increasing temperature, while added Fe(II) in the presence and absence of EDTA accelerates H 2O 2 and HPA decomposition. Calculations indicate H 2O 2 hydrogen bonds with nucleic acid-base pairs with no apparent bond strain and energy stabilization comparable to normal hydrogen bonding.