Characterization of [3H]Guanine Nucleotide Binding Sites in Brain Membranes

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Abstract
[3H]GTP [guanosine triphosphate] and [3H]GMP‐PNP [guanosine 5′‐(β,8‐imino)triphosphate, a nonmetabolized analog of GTP] have been utilized as ligands to characterize binding sites of guanine nucleotides to rat brain membranes. Binding of both [3H]GTP and [3H]GMP‐PNP is saturable, with respective KD values of 0.76 and 0.42 μM. The number of binding sites for GMP‐PNP (4 nmol/g) is three times greater than for GTP (1.5 nmol/g). This discrepancy is caused by rapid degradation of GTP to guanosine by brain membranes, which can be partially prevented by addition of 100 μM‐ATP. The binding of [3H]guanine nucleotides is selective, with approximately equipotent inhibition by GTP, GDP, and GMP‐PNP (at 0.2‐1.0 μM), but no inhibition by other nucleotides at 100 μM concentrations. The binding sites for guanine nucleotides in brain membranes appear not to be associated with microtubules, since treatments that reduce [3H]colchicine binding by 65% have no effect on [3H]GTP binding. [3H]Guanine nucleotide binding is widely distributed in various organs, with highest levels in liver and brain and lowest levels in skeletal muscle. The characteristics of these binding sites in brain show specificity properties of sites that regulate neurotransmitter receptors and adenylate cy‐clase.