Tightly bound 2-azido-adenine nucleotides at catalytic and noncatalytic sites of the rat liver F1 ATPase label adjacent tryptic peptides of the beta subunit.

UV irradiation of rat liver F1 ATPase, previously exposed to Mg2+ and [beta, gamma-32P]-2-azido-ATP and separated from medium nucleotides, covalently modifies two tyrosine residues in adjacent tryptic peptides of the beta subunit. This results from the occupancy by 2-azido-ATP or 2-azido-ADP of two distinct types of nucleotide binding sites, the catalytic and noncatalytic sites. The two modified peptides are identical to the ones modified by 2-azido-adenine nucleotides in the beef heart F1 ATPase. Both catalytic and noncatalytic sites are labeled when the ATPase is exposed to [beta-32P]-2-azido-ADP in the presence or the absence of 5'-adenylyimidodiphosphate (AMP-PNP), showing that two distinct types of ADP binding sites are present on the liver enzyme. Similar incorporation of 2-azido-adenine nucleotides is obtained when membrane-bound rat liver F1 ATPase is incubated with Mg2+ and [beta, gamma-32P]-2-azido-ATP.