Bacterial endotoxin (lipopolysaccharide) stimulates the rate of iron oxidation

Abstract

Bacterial endotoxin (lipopolysaccharide) has affinity for a number of cations, including iron. Previous investigations have demonstrated that lipopolysaccharide can affect the oxidation rate of iron; heme-bound ferrous iron in hemoglobin is oxidized to ferric iron when hemoglobin binds lipopolysaccharide. In the present study, we directly examined the interaction between lipopolysaccharide and iron. Lipopolysaccharide caused a concentration-dependent increase in the rate of iron oxidation, with up to a 23-fold increase in oxidation in the presence of 200 µg/ml Escherichia coli lipopolysaccharide. This effect was seen both with several carbohydrate-rich smooth lipopolysaccharides and also with carbohydrate-poor rough lipopolysaccharide. Extensively deacylated rough lipopolysaccharide had no effect, suggesting a role of the fatty acid components of lipopolysaccharide in this process. Purified lipid A produced inconsistent results: some preparations stimulated iron oxidation and others did not. A series of sugars, starches and a preparation of purified O-chain polysaccharide (the carbohydrate portion of the lipopolysaccharide macromolecule) had no effect on the rate of iron oxidation, whereas phospholipid-enriched brain tissue extracts (similar to the lipid A component of lipopolysaccharide) stimulated oxidation. We conclude that the lipid moiety of bacterial lipopolysaccharide is responsible for the stimulation of iron oxidation. This process may contribute to the ability of lipopolysaccharide to cause oxidation of heme-bound iron in hemoglobin.

Life at the Energetic Edge: Kinetics of Circumneutral Iron Oxidation by Lithotrophic Iron-Oxidizing Bacteria Isolated from the Wetland-Plant Rhizosphere

“…Theis and Singer (33) proposed that organic matter could affect chemical Fe(II) oxidation rates by binding to free Fe(II). Depending on the stability constant of the Fe(II)-organic matter complex (33), the concentrations of organic matter, O 2 , and Fe(II) (19), and the nature of the organic matter (24,30,33), organic matter can either inhibit, accelerate, or have no effect on abiotic Fe(II) oxidation. For example, the inhibition of chemical oxidation rates by bacterial exopolysaccharides was also proposed as a mechanism to explain higher rates of Fe plaque (oxide) formation on axenic rice (Oryza sativa) roots than on nonaxenic rice roots (18).…”

Redox-active metals in commercial preparations of lipopolysaccharide: implications for studies of cellular responses to bacterial products

“…Therefore, association of LPS with metals, although an experimental nuisance, is likely to be the biological norm, as some liberation of metals is a likely consequence of tissue necrosis at the site of infection. Some authors have suggested that LPS may serve as a metal‐binding substance for Gram‐negative bacteria, in the microbe versus host struggle for biologically important metal ions (Roth et al ., 2000). Although studies of the strength of LPS–metal binding interactions suggest that endotoxin would be a poor competitor for metals with most host metal‐sequestering proteins (Šourek and Tichý, 1975), it may nevertheless represent an effective scavenger of ‘free’ metal ions.…”

“…Although studies of the strength of LPS–metal binding interactions suggest that endotoxin would be a poor competitor for metals with most host metal‐sequestering proteins (Šourek and Tichý, 1975), it may nevertheless represent an effective scavenger of ‘free’ metal ions. Furthermore, there is preliminary evidence to show that interactions between LPS and metals may have pathological relevance, as LPS is known to speed the oxidation of ferrous iron, and may thus aggravate damage due to oxidative stress (Roth et al ., 2000). Other studies have shown synergistic interactions between LPS and metals in induction of cytokine production by mammalian immune cells (Scuderi, 1990).…”