An αβ heterodimer of the F1-ATPase of Rhodospirillum rubrum was isolated by extraction of chromatophores with LiCl. Each αβ heterodimer contains one tightly bound ADP, which is released upon removal of medium Mg2+. The dimer can be reversibly dissociated by removal of Mg2+-ions. The αβ heterodimer restores both ATP-synthetic and hydrolytic activities to LiCl-treated chromatophores, saturation being achieved at approximately 2 mmol αβ · mol BChl−1. The heterodimer itself hydrolyses Mg-ATP with an activity distinct from RF1, being unaffected by azide or sulphite ions. The V max and K m (ATP) for this Mg2+-dependent activity were 110 ± 10 nmol · min−1 mg protein−1 and 100 ± 30 μM, respectively. The K m did not differ significantly from that of RF1.
