Steady-state binding of adenine nucleotides ATP, ADP and AMP to rat liver and adipose plasma membranes

Binding of native adenine nucleotides to rat liver and adipose plasma membr anes was studied under steady-state conditions using EDTA/Na for inhibition of ecto-nucleotidase activity. [H-3]-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, 2 31 +/- 36 and 2050 +/- 290 nmol/l for adipose membranes. The nucleotide-bin ding capacity (B-max) varied from 15 to 18 pmol/mg protein in the case of [ H-3]ATP and [H-3]ADP-binding studies and from 22 to 26 pmol/mg protein for [H-3]AMP-binding sites. Both 2-MeSATP and ADP inhibited [H-3]ATP-binding to membranes with respective IC50 values of 60 +/- 7 and 285 +/- 30 nM. Other purinergic agents suramin, Reactive blue 2, alpha,beta-MeATP and beta,gamm a-MeATP were less potent competitors of [H-3]ATP binding, whereas AMP, aden osine, GTP, UTP, and CTP did not cause any displacement effect at concentra tions of 10(-6)-10(-5) M. It is suggested that the described ATP/ADP-bindin g sites are linked to G protein-coupled P2Y receptors, whereas AMP-binding sites may represent a substrate-binding component of the membrane ecto-5'-n ucleotidase.