Development of a sandwich ELISA with potential for selective quantification of human lactoferrin protein nitrated through disease or environmental exposure

Development of an Indirect ELISA for the Detection of Lactoferrin in Type 2 Diabetes Plasma: A Novel Approach

Background:

In biological systems, lactoferrin (LF) is a crucial protein for protecting the body against diseases and pathogens that can affect both humans and animals. LF is a multifunction protein that binds to different surface receptors to stimulate the innate immune system. In diabetes, lactoferrin has a direct association with inflammation. The effects of inflammation interaction are unknown but reasonably could include changes in LF, a body protein whose changed concentration correlates with type 2 diabetes (T2D). The LF content in plasma has been used as a disease biomarker, and there is a need for convenient and reliable assays.

Method:

An innovative indirect enzyme-linked immunosorbent assay (ELISA) was developed and applied to measure circulating lactoferrin levels as an inflammation marker in human samples, including healthy and type 2 diabetes.

Results:

Under optimized conditions, the proposed indirect ELISA was evaluated and linearly responded to LF standards in a 0.05–0.5 µgmL−1 range. The limit of detection (LOD) was 0.05 µgmL−1, and a reliable limit of quantification (LOQ) was 0.240 µgmL−1 .

Conclusion:

The developed assay showed both specificity and reproducibility, indicating the utility of this indirect ELISA in LF monitoring. This study provides a definitive indirect ELISA protocol to detect various lactoferrin antigens with accurate, reliable, and reproducible data, and it could be applied for diagnosing lactoferrin-related diseases, such as type 2 diabetes. Our innovative approach provides a relatively cost-effective, sensitive, and precise way to assess LF in various human plasmas.

L-ergothioneine reduces nitration of lactoferrin and loss of antibacterial activity associated with nitrosative stress

Lactoferrin (LF) is a multifunctional antimicrobial, anti-inflammatory, and antioxidant protein that occurs naturally in mammals, most notably in exocrine gland tissues and fluids, such as in the eye. Nitrosative stress can promote changes to tyrosine and other amino acid residues of the protein, which also reduces the activity of LF. l-ergothioneine (ET) is a potent anti-inflammatory antioxidant present in the eye and other tissues through nutrition or supplementation and that may play a role in the prevention or treatment of a variety of diseases. Here we investigated the ability of ET to reduce 3-nitrotyrosine (NTyr) formation using two separate substrates, with the goal of determining whether ET can protect the antibacterial function of LF and other proteins when exposed separately to peroxynitrite and tetranitromethane as nitrating reagents. Native human LF was used as a simple protein substrate, and lamb corneal lysate was chosen as one example of mammalian tissue with a more complex mixture of proteins and other biomolecules. Nitration was monitored by absorbance and fluorescence spectroscopy as well as sandwich (nitrated LF) and direct NTyr (corneal lysate) enzyme-linked immunosorbent assays (ELISAs). We found that pretreatment with ET reduced chemical modification of both native LF and corneal lysate samples and loss of antibacterial LF function due to exposure to the nitrating reagents. These initial results suggest that ET, raised to sufficiently elevated levels, could be tailored as a therapeutic agent to reduce effects of nitrosative stress on LF and in turn sustain the protein activity.

Modification of lactoferrin by peroxynitrite reduces its antibacterial activity and changes protein structure

Lactoferrin (LF) is a multifunctional protein that plays important physiological roles as one of the most concentrated proteins in many human and other mammalian fluids and tissues. In particular, LF provides antibacterial properties to human milk, saliva, and tear fluid. LF also protects against stress-induced lipid peroxidation at inflammation sites through its iron-binding ability. Previous studies have shown that LF can be efficiently nitrated via biologically relevant mediators such as peroxynitrite (ONOO^- ), which are also present at high intracellular concentrations during inflammation and nitrosative stress. Here, we examine changes in antibacterial properties and structure of LF following ONOO^- treatment. The reaction induces nitration of tyrosine and tryptophan residues, which are commonly used as biomarker molecules for several diseases. Treatment with ONOO^- at a 10/1 M ratio of ONOO^- to tyrosine inhibited all antibacterial activity exhibited by native LF. Secondary structural changes in LF were assessed using circular dichroism spectroscopy. Nitration products with and without the addition of Fe³⁺ show significant reduction in alpha-helical properties, suggesting partial protein unfolding. Iron-binding capacity of LF was also reduced after treatment with ONOO^- , suggesting a decreased ability of LF to protect against cellular damage. LC-MS/MS spectrometry was used to identify LF peptide fragments nitrated by ONOO^- , including tyrosine residue Y92 located in the iron-binding region. These results suggest that posttranslational modification of LF by ONOO^- could be an important pathway to exacerbate infection, for example, in inflamed tissues and to reduce the ability of LF to act as an immune responder and decrease oxidative damage.