Abstract
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.

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