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New Biol. 1990 Feb;2(2):197-206.

Is apolipoprotein D a mammalian bilin-binding protein?

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  • 1Laboratory of Mathematical Biology, National Cancer Institute, Frederick, MD 21701.

Abstract

Human apolipoprotein D (APO-D) is a serum glycoprotein that has no sequence similarity with other apolipoproteins but rather belongs to the alpha 2-microglobulin superfamily whose other members transport small hydrophobic ligands in a wide variety of biological contexts. To investigate the ligand specificity of APO-D, we analyzed its relationship with the other members of this superfamily and constructed a detailed molecular model using the atomic coordinates of its most closely related homolog--insecticyanin from the tobacco hornworm, Manduca sexta. We studied the geometry of the binding pocket of APO-D and the topology of characteristic patches of both hydrophobic and polar side chains that also occur in crystal structures of insecticyanin and bilin-binding protein from the butterfly Pieris brassicae. From the data obtained we hypothesize that heme-related compounds may be more favorable ligands for APO-D than either cholesterol or cholesteryl ester. Preliminary experiments showed that purified human APO-D binds bilirubin in an approximately one-to-one molar ratio. These results suggest a new biological role for APO-D that is more congruent with its tissue distribution and evolutionary history.

PMID:
2083249
[PubMed - indexed for MEDLINE]
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