Purification and Identification of the Factor Capable of Converting Ca2+-ATPase into Mg2+-ATPase Present in Rhodospirillum rubrum Chromatophores1

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Abstract
It is known in the case of the photosynthetic bacterium Rhodospirillum rubrum that the ATPase solubilized from acetone powder of chromatophores shows activity in the presence of Ca2+, but not in the presence of Mg2+ (Ca2+-ATPase), whereas chromatophores show ATPase activity in the presence of either cation. The present study deals with the purification and identification of the factor capable of converting Ca2+-ATPase into Mg2+-ATPase (conversion factor) present in chromatophores. 1. The conversion factor was effectively solubilized from chromatophores in 50% acetone, and purified by chromatography on columns of silica gel and Sephadex LH-20. 2. The purified conversion factor was found to contain unsaturated fatty acids (C16:1 and C18:1) by infrared spectroscopy, proton magnetic resonance spectroscopy, thin layer chromatography, and gas liquid chromatography. The molar ratio of the C16:1 to the C18:1 acids was approximately 3 : 2. 3. Activity for the conversion of Ca2+-ATPase into Mg2+-ATPase (conversion activity) was investigated among authentic fatty acids (C14 to C20). All unsaturated fatty acids other than elaidic acid (C18:1δ9, trans) showed conversion activity. The activity with palmitoleic acid (C16:1 δ9) was highest, being about 6 times as high as the activity with vaccenic acid (C18:1 delta;11) or oleic acid (C18:1 δ9). 4. Under optimum conditions, the conversion activity with purified conversion factor was the same as that with palmitoleic acid, indicating that the naturally occurring conversion factor is palmitoleic acid.

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