Cu(II) reductase

This post addresses the existence of a putative Cu(II) reductase that sits adjacent to the Ctr1 Cu+ channel. Sukru Gulec and James Collins wrote an excellent review on absorption of iron (Fe) and copper (Cu). This is a composite of three images going from the entry of Cu in the duodenum, transfer to the hepatocyte, and the red blood cell, erythroid cell.

Let us start our journey with the

CYBDR1 , aka DCytB is a small membrane protein for which UniProt lists the following catalytic function:

Fe³⁺(out) + L-ascorbate(in) = Fe²⁺(out) + H⁺ + monodehydro-L-ascorbate radical(in)

Note that Cybdr1 has a lot in common with cytochrome P450s in the liver, hence the “Cy” as part of the name. The “D” must have come from “duodenum,” a location in the proximal small intestine responsible for absorbing Fe and Cu. The 31 kDa Cybdr1 gene was fused to the gene for Green Fluorescence protein and expressed in a kidney cell line. The kinetics of reducing Fe³⁺ and Cu²⁺ was measured. [2] The Fe(II)-ferrozine complex formation was detected by the absorbance at 562 nm. Capture of Cu(I) by BCS was monitored by the change in absorbance at 482 nm. [2]

This particular kidney cell line expresses the divalent metal ion tansporter, DMT1. The authors demonstrated that the Kcyb1 transfected cells had absorbed 5x as much ⁵⁹Fe originally added in the +3 ferric oxidation state as non transfected cells. [2] The authors cited literature showing that DCYTB knockout mice seemed normal and that no mutations in the human DCYTB gene have been associated with human disease at the time of the manuscript published in 2008. [2]

STEAP: Six Transmembrane Epithelial Antigen of Prostate

In 2006 a group from Millennium Pharmaceuticals and Children’s Hospital of Harvard characterized the tissue expression and reductase activity of Steap isoforms. [3] Tissue expression of STEAP1-4 was determined by use of radioactive RNA hybridization probes and whole body sections of mice. STEAP1-4 were expressed in HEK293 cells. [3] Fe³⁺ reduction to Fe²⁺ was measured with ferrozine, that absorbs 562 nm. The reduction of Cu²⁺ to Cu⁺ was measured with BCA that absorbs at 482 nm . [3]

Note that Cu²⁺ uptake was already pretty high with just the empty vector used to insert the Steap gene into HEK293 cells. Cu²⁺ does seem to compete with Fe³⁺ for reductase activity.

Uptake of both metal ions was measured with radioactive analogs. [3]

All in all, these data are really not that convincing that the Steap and DCyb1 reductases are really doing that much for copper absorption.

References

1. Gulec, S., & Collins, J. F. (2014). Molecular mediators governing iron-copper interactions. Annual review of nutrition, 34, 95–116. PMC free article
2. Wyman S, Simpson RJ, McKie AT, Sharp PA. Dcytb (Cybrd1) functions as both a ferric and a cupric reductase in vitro. FEBS Lett. 2008 Jun 11;582(13):1901-6. Free article
3. Ohgami R. S., Campagna D. R., McDonald A., Fleming M. D. (2006) The Steap proteins are metalloreductases. Blood 108, 1388–1394 PMC free article