The Reactivity of ‐SH Groups in the ADP/ATP Carrier Isolated from Beef Heart Mitochondria

Abstract

The emergence of the reactivity of -SH groups associated with conformational changes was studied on the ADP/ATP carrier, as isolated in 3 different inhibitor-protein complexes. The bongkrekate-protein complex incorporates .apprx. 1 molecule more of N-ethylmaleimide than the carboxyatractylate-protein complex. After extensive denaturation by dodecylsulfate in urea, both inhibitor complexes exhibit 4 reactive .sbd.SH groups/subunit. Thus, 1 of 4 .sbd.SH groups per subunit was unmasked in the bongkrekate-protein complex. The interconversion from the bongkrekate-protein complex to the carboxyatractylate-protein complex is inhibited after the .sbd.SH groups have been blocked. The protein complex, isolated with the more easily dissociable atractylate, is used to demonstrate, by the emergence of the .sbd.SH groups, the transition into the m-state. This transition is specifically catalyzed by ADP and ATP. Using 2,2''-dinitro-5,5''-dithiodibenzoate, the appearance of the .sbd.SH groups on transition from the c-state to the m-state can be followed spectrophotometrically. The specificity for the catalyzing nucleotides is identical with that for transport. The Km for ADP and ATP is in the range of 1 .mu.M. The thiol groups of the isolated ADP/ATP carrier behave as in the mitochondrial membrane. The unmasking of .sbd.SH groups is in full accordance with the concept of 2 conformational states (c and m).