Steady-State Binding of Adenine Nucleotides ATP, ADP and AMP to Rat Liver and Adipose Plasma Membranes

Abstract

Binding of native adenine nucleotides to rat liver and adipose plasma membranes was studied under steady-state conditions using EDTA/Na for inhibition of ecto-nucleotidase activity. [³H]-labelled ATP, ADP and AMP are able to interact with specific binding sites with respective K_d values of 88 ± 9, 278 ± 29 and 495 ± 40 nmol/l for liver membranes; and of 64 ± 7, 231 ± 36 and 2050 ± 290 nmol/l for adipose membranes. The nucleotide-binding capacity (B_max) varied from 15 to 18 pmol/mg protein in the case of [³H]ATP and [³H]ADP-binding studies and from 22 to 26 pmol/mg protein for [³H]AMP-binding sites. Both 2-MeSATP and ADP inhibited [³H]ATP-binding to membranes with respective IC₅₀ values of 60 ± 7 and 285 ± 30 nM. Other purinergic agents suramin, Reactive blue 2, α,β-MeATP and β,γ-MeATP were less potent competitors of [³H]ATP binding, whereas AMP, adenosine, GTP, UTP, and CTP did not cause any displacement effect at concentrations of 10^-6-10^-5 M. It is suggested that the described ATP/ADP-binding sites are linked to G protein-coupled P2Y receptors, whereas AMP-binding sites may represent a substrate-binding component of the membrane ecto-5′-nucleotidase.