Mitochondrial ATP synthase complex: interaction of its F1 adenosine triphosphatase moiety with the heavy atom iodine

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Abstract
Studies were carried out to determine whether a simple electron-dense "heavy atom" like iodine could be introduced selectively into one or more of the subunits of the mitochondrial ATP synthase complex of rat liver. Surprisingly, very low amounts of iodine are incorporated into the isolated F1 moiety of this complex under conditions which result in a marked loss of catalytic activity. ATPase activity is inactivated in a concentration-dependent manner at pH 7.5 with half-maximal inactivation occurring at about 40 .mu.M iodine. A maximum of only 10 atoms of iodine are incorporated per F1 molecule under conditions where inhibition of ATPase activity is linearly related to iodine incorporation. The molecular size of F1 after iodination is unchanged, indicating that inactivation is due to modification of essential amino acid residues rather than subunit dissociation. Treatment of F1 with 20-50 .mu.M [125I]iodine followed sequentially by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography showed that the .beta. subunit is preferentially labeled. Significantly, about two atoms of iodine per .beta. subunit are incorporated. Some iodine amounting to less than 23% of the total radioactivity placed on the gels is recovered in the .alpha. and .gamma. subunits whereas no radioactivity is detected in the .delta. and .epsilon. subunits. Iodination of F1 appears to modify essential residues other than those involved in substrate or product binding per se. Thus, nucleotide binding to F1 is unaltered by iodine, and neither phosphate, MgADP, nor MgATP protects F1 against inhibition by this agent. Rather, loss of ATPase activity upon iodination appears to be associated with one or more pH-sensitive groups. It seems likely that these groups are tyrosine both because tyrosine is the principal amino acid involved in the iodination of proteins and because the pK of its phenolic hydroxyl groups is dramatically altered by iodination of the associated benzene ring. These studies represent the first attempt to introduce a heavy atom into an F1-ATPase preparation in a selective manner. The results show that at low concentrations iodine does react preferentially with .beta. subunits of the rat liver enzyme while inactivating the catalytic capacity of the intact complex. These findings may prove useful in future studies directed at understanding structural-functional relationships within ATP synthase complexes.