The use of synthetic linear tetrapyrroles to probe the verdin sites of human biliverdin-IXalpha reductase and human biliverdin-IXbeta reductase.
Many vertebrate species express two enzymes that are capable of catalysing the reduction of various isomers of biliverdin. Biliverdin-IXalpha reductase (BVR-A) is most active with its physiological substrate biliverdin-IXalpha, but can also reduce the three other biliverdin isomers IXbeta, IXdelta and IXgamma. Biliverdin-IXbeta reductase (BVR-B) catalyses the reduction of only the IXbeta, IXdelta and IXgamma isomers of biliverdin. Therefore, the activity of BVR-A can be measured using biliverdin-IXalpha as a specific substrate. We now show that the dimethyl esters of biliverdin-IXbeta and biliverdin-IXdelta are substrates for BVR-B, but not for BVR-A. This provides a useful method for specifically assaying the activity of both BVR-A and BVR-B in crude mixtures, using biliverdin-IXalpha for BVR-A and the dimethyl ester of either biliverdin-IXbeta or biliverdin-IXdelta for BVR-B. Human BVR-A has been suggested as a pharmacological target for neonatal jaundice. Because of the absence of a crystal structure with biliverdin bound to BVR-A, we have investigated indirect ways of examining tetrapyrrole binding. In the present study, we report that a number of sterically locked conformers of 18-ethylbiliverdin-IXalpha are substrates for human BVR-A, and discuss the implications for the biliverdin binding site. The oxidation of bilirubin-IXalpha ditaurate to biliverdin-IXalpha ditaurate is also described. We show that biliverdin-IXalpha ditaurate is a substrate for human BVR-A and discuss the possibility of using a competing substrate, which is reduced to a water soluble and excretable rubin, as a prototypic inhibitor of BVR-A.
Franklin
Edward M
EM
School of Biochemistry and Immunology, Trinity College, Dublin, Ireland. efrankli@tcd.ie
Browne
Seamus
S
Horan
Anne M
AM
Inomata
Katsuhiko
K
Hammam
Mostafa A S
MA
Kinoshita
Hideki
H
Lamparter
Tilman
T
Golfis
Georgia
G
Mantle
Timothy J
TJ
D001665:Binding Sites; D006801:Humans; D007700:Kinetics; D008024:Ligands; D015336:Molecular Probe Techniques; D015335:Molecular Probes / Q000138:chemical synthesis / Q000737:chemistry; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry / Q000378:metabolism*; D011166:Porphyrins / Q000378:metabolism*; D011485:Protein Binding; D013379:Substrate Specificity; D045725:Tetrapyrroles / Q000138:chemical synthesis / Q000737:chemistry*
D008024:Ligands; D015335:Molecular Probes; D011166:Porphyrins; D045725:Tetrapyrroles; C054069:verdin; D044925:Oxidoreductases Acting on CH-CH Group Donors; C019789:biliverdin reductase