[35S]ATP gamma S binding sites in the purified heart sarcolemma membrane

Purified heart sarcolemma membranes were found to bind a slowly hydrolyzable analogue of ATP [35S-labeled adenosine 5'-(gamma-thio)triphosphate [( 35S]ATP gamma S)] in a specific manner and exhibited two apparent affinity sites. The high-affinity site had a dissociation constant (KD) of 4.7-8.3 nM [maximum binding (Bmax) = 9.5-18.4 pmol/mg protein], whereas the low-affinity site had a KD of 655-1,257 nM (Bmax = 812-2,955 pmol/mg protein). Like ATP, other nucleotides such as GTP, UTP, ITP, and CTP were effective in displacing [35S]ATP gamma S binding. Although crude membrane preparations from different tissues also exhibited both high- and low-affinity sites for [35S]ATP gamma S, KD values for the high affinity sites were severalfold higher than that for the purified heart membranes. It is proposed that the high-affinity binding site for nucleotides may represent the ATP receptor in the heart cell membrane.