Iron Metabolism in Kidney Allograft Recipients: Still a Mystery?

Citrate and albumin facilitate transferrin iron loading in the presence of phosphate

Labile plasma iron (LPI) is redox active, exchangeable iron that catalyzes the formation of reactive oxygen species. Serum transferrin binds iron in a non-exchangeable form and delivers iron to cells. In several inflammatory diseases serum LPI increases but the reason LPI forms is unknown. This work evaluates possible pathways leading to LPI and examines potential mediators of apo transferrin iron loading to prevent LPI. Previously phosphate was shown to inhibit iron loading into apo transferrin by competitively binding free Fe³⁺. The reaction of Fe³⁺ with phosphate produced a soluble ferric phosphate complex. In this study we evaluate iron loading into transferrin under physiologically relevant phosphate conditions to evaluate the roles of citrate and albumin in mediating iron delivery into apo transferrin. We report that preformed Fe³⁺-citrate was loaded into apo transferrin and was not inhibited by phosphate. A competition study evaluated reactions when Fe³⁺ was added to a solution with citrate, phosphate and apo transferrin. The results showed citrate marginally improved the delivery of Fe³⁺ to apo transferrin. Studies adding Fe³⁺ to a solution with phosphate, albumin and apo transferrin showed that albumin improved Fe³⁺ loading into apo transferrin. The most efficient Fe³⁺ loading into apo transferrin in a phosphate solution occurred when both citrate and albumin were present at physiological concentrations. Citrate and albumin overcame phosphate inhibition and loaded apo transferrin equal to the control of Fe³⁺ added to apo transferrin. Our results suggest a physiologically important role for albumin and citrate for apo transferrin iron loading.

Iron metabolism in hemodialyzed patients - a story half told?

INTRODUCTION

All living organisms have evolved sophisticated mechanisms to maintain appropriate iron levels in their cells and within their body. Recently our understanding of iron metabolism has dramatically increased. Overt labile plasma iron (LPI) represents a component of non-transferrin bound iron (NTBI) that is both redox active and chelatable, capable of permeating into organs and inducing tissue iron overload. The LPI measures the iron-specific capacity of a given sample to produce reactive oxygen species. We studied for the first time NTBI correlations with markers of iron status and inflammation in prevalent hemodialyzed patients.

MATERIAL AND METHODS

Complete blood count, urea, serum lipids, fasting glucose, creatinine, ferritin, serum iron, total iron binding capacity (TIBC) were studied by standard laboratory method. The NTBI was assessed commercially available kits from Aferrix Ltd in Tel Aviv, Israel. A test result of 0.6 units of LPI or more indicates a potential for iron-mediated production of reactive oxygen species in the sample.

RESULTS

Patients with LPI units ≥ 0.6 had higher serum iron, erythropoiesis stimulating agents (ESA) dose, ferritin, high-sensitivity C-reactive protein (hsCRP), hepcidin and lower hemojuvelin. In hemodialyzed patients NTBI correlated with hsCRP (r = 0.37, p < 0.01), ferritin (r = 0.41, p < 0.001), IL-6 (r = 0.43, p < 0.001). In multivariate analysis predictors of NTBI were hemoglobin and alkaline phosphatase, explaining 58% of the variability.

CONCLUSIONS

Elevated NTBI in HD may be due to disturbed iron metabolism. Anemia and liver function might also contribute to the presence of NTBI in this population.