SOLUBLE AND MEMBRANE-ASSOCIATED HUMAN LOW-AFFINITY ADENOSINE BINDING-PROTEIN (ADENOTIN) - PROPERTIES AND HOMOLOGY WITH MAMMALIAN AND AVIAN STRESS PROTEINS

A low-affinity adenosine binding protein has recently been distinguished from the adenosine A2 receptor and purified from human placental membranes. Soluble human placental extracts contain an adenosine binding activity that has properties similar to those of the membrane low-affinity adenosine binding protein. The binding protein was purified from soluble human placental extracts 134-fold to 89% purity with a Bmax of 2.5 nmol/mg. It comprises 0.7–0.9% of the soluble protein. The major purified soluble protein has a subunit molecular mass of 98 kDa and a Stokes radius identical with that of the membrane-bound adenosine binding protein. Competition analysis of the soluble protein revealed similar affinities and an identical potency order for displacement of 5′-(N-ethylcarbamoyl) [2,8−3H]adenosine ([3H]NECA) as follows: NECA > 2-chloroadenosine > adenosine > (R)-N6-(2-phenylisopropyl)adenosine. The soluble binding protein was more acidic than the membrane binding protein as revealed by a comparison of the elution properties during ion exchange chromatography. A second form of soluble adenosine binding activity comprised 17% of the major form and had a charge similar to that of the membrane binding protein, a smaller Stokes radius, and a subunit molecular mass of 74 kDa. Carbohydrate composition analysis revealed that the major soluble form has 4.3% carbohydrate by weight as compared to the membrane-associated form, which has 5.5% carbohydrate by weight. The amino-terminal sequence of each protein is identical and has identity with sequences in other proteins as follows: the glucose-regulated protein (GRP94) from hamster fibroblasts, 83% homology; the endoplasmic reticulum protein from murine plastocytoma tissue (ERp99), 94% homology; the murine tumor rejection antigen precursor (GP96), 94% homology; the chicken heat shock protein (HSP108), 87% homology. The relationship between the membrane and soluble low-affinity adenosine binding proteins is not known at present. The homology of the amino terminus to several stress-induced proteins indicates that the low-affinity adenosine binding proteins have a highly conserved amino-terminal sequence and may be human heat shock proteins. We propose the name adenotin to describe these unique low-affinity adenosine binding proteins. © 1990 American Chemical Society. All rights reserved.