Abstract
A water-soluble, highly polar, heat-stable, small molecule has been isolated from cell-free extracts of the halotolerant green alga Dunaliella salina. This compound, soluble inhibitory factor (SIF), when added to in vivo light-activated, thylakoid membrane-bound preparations of the D. salina coupling factor 1 (CF1), causes a rapid inactivation of the ATPase activity. SIF must be in its oxidized form to inactivate the CF1 ATPase and probably functions by oxidizing the reduced form of the light-activated enzyme. SIF has been purified to homogeneity and characterized by UV-visible and IR absorption spectroscopy, 1H and 13C NMR spectroscopy, and mass spectrometry. SIF has five different kinds of nonexchangeable protons and seven different kinds of carbon atoms. Three of the carbon atoms and one proton are part of a heterocyclic (imidazole) ring. One carbon atom is a carbonyl (carboxylic acid). One carbon atom and three protons form a methyl group attached to the aromatic ring. One carbon atom and two protons are a methylene group, and one carbon atom (an alpha-amino carbon) is attached to a single proton. In addition, in its reduced form, SIF contains a thiol group attached to the heterocyclic ring. From high resolution mass spectrometry, the molecular weight of SIF was determined to be 401 (M + H+) and is consistent with the composition being C14H21N6O4S2. The UV absorption of SIF shows a large increase at 240 nm upon reduction. An effective difference extinction coefficient for this absorbance change has been calculated to be 6.84 meq/cm. A comparison of SIF with the oxidized form of ovothiol A (1-N-methyl-4-mercaptohistidine disulfide) shows the two compounds to be identical in all respects. In addition, ovothiol A disulfide is as effective as SIF in inhibiting the light-triggered, CF1 ATPase activity. It is concluded, therefore, that SIF and L-1-N-methyl-4-mercaptohistidine disulfide are identical.
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