[125I]N6-(3-iodo-4-hydroxyphenyl)isopropyladenosine: the use of the diastereomers as ligands for adenosine receptors in rat brain.

The diastereomers of N6-(4-hydroxyphenyl)isopropyladenosine (HPIA) were synthesized, radioiodinated and used as ligands for adenosine receptors in rat brain membranes. In contrast to the S-isomer, specific binding (measured by displacement with 10 microM N6-R-phenylisopropyladenosine (PIA] of [125I]N6-[R-(3-iodo-4-hydroxyphenyl)isopropyl] adenosine ([125I]R- IHPIA) required about 2 h for equilibrium but bound with a higher affinity. Scatchard analysis of binding data were compatible with the existence of single binding site with Kd values of 0.7 nM (R-isomer) and 10 nM (S-isomer), and maximal binding of 228 and 237 fmol/mg, respectively, at 30 degrees C. Computer-based non-linear curve fitting resulted in estimates of 0.67 and 7.5 nM, and of 208 and 173 fmol/mg, for the R- and S-isomer, respectively. Dissociation of the R-isomer in the presence of 10 microM PIA was biphasic, both phases being increased in the presence of 100 microM Gpp(NH)p. The ratio of the rate of dissociation for the slower phase, k2, and the rate of association, k1, yielded a further estimate for Kd of 0.3 nM. The specific binding of [125I]R-IHPIA was displaced by adenosine receptor agonists with the following order of decreasing potency: N6-[R-(phenyl)-isopropyl]adenosine = N6-[R-(4-hydroxyphenyl)isopropyl]adenosine greater than 5'-N-ethylcarboxamidoadenosine greater than 2-chloroadenosine greater than N6-[S-(phenyl)isopropyl]adenosine = N6-[S-(4-hydroxyphenyl)isopropyl]adenosine which is typical of binding to Ri/A1 type receptors. Receptor antagonists displaced this binding in the following order of decreasing potency: 8-(p-hydroxyphenyl) theophylline greater than 8-phenyltheophylline greater than theophylline.(ABSTRACT TRUNCATED AT 250 WORDS)