EFFECTS OF THYROXINE AND RELATED COMPOUNDS ON LIVER MITOCHONDRIA 5 



inhibitors. Moreover, it is prevented by dinitrophenol, adenine nucleotides, 

 and in general apparently by conditions that pre\ent the accumulation of 

 high-energy intermediates within the mitochondria. Significantly, active 

 swelling is reversed by ATP, and this "contraction" of the mitochondrial 

 structure has been shown to be reflected in an extrusion of water from the 

 mitochondria, paralleled by a splitting of ATP. It has also been demon- 

 strated [29, 48] that the ATP-induced contraction is dependent on the 

 presence in the mitochondria of a specific protein fraction ; the symbol, 

 "M factor", has been used to denote this fraction. 



Parallel to the swelling, the mitochondria lose their endogenous content 

 of DPN. When DPX is added to such mitochondria in the presence of 

 ATP, a rebinding of the DPN to the mitochondrial structure takes place. 

 Whether the loss of DPN is a cause or a consequence of the swelling is not 

 quite clear, although recent investigations by Kaufmann and Kaplan [49] 

 would seem to indicate that the latter is the case. 



A further characteristic feature of the process of active swelling is that 

 it is not immediately accompanied by an uncoupling of phosphorylation 

 from respiration. Thus, mitochondria which have reached a state of swollen 

 structure following exposure to ageing in the presence of phosphate or 

 thyroxine are still capable of exhibiting an electron transport-coupled 

 phosphorvlation when DPN is added to restore respiration (with succinate 

 as substrate the situation seems to be somewhat more complicated [35], 

 owing probably to the recently disco\ered requirement of high-energy 

 phosphate for the oxidation of this substrate [50 54]). However, simul- 

 taneously with the swelling and the loss of DPN, or even preceding these 

 effects, the mitochondria lose the tight coupling between respiration and 

 phosphorylation, the former becoming independent of the presence of 

 orthophosphate and ADP. In such mitochondria, thus, coupled phos- 

 phorylation can take place, when phosphate and phosphate acceptor are 

 present, but respiration can proceed at maximal rate even in the absence 

 of these additions. It has been shown by Lehninger and associates [55, 56] 

 that this state of "loose-coupling" can be induced in intact mitochondria 

 not only by the above treatments but also by the addition of a protein 

 factor, called "R factor", which can be obtained from mitochondria 

 after disruption with sonic waves ; intact mitochondria thus seem to contain 

 this factor in an inactive state. "Loose-coupling" efi^ects can be induced in 

 mitochondria also by a number of common uncoupling agents if these are 

 added in low concentrations [57, 58]. 



Although thyroxine is not the only agent capable of enhancing active 

 swelling and related svmptoms in mitochondria besides inorganic 

 phosphate, calcium ions [t^^, 37, 59] and more recently phloridzine [60] 

 have been shown to exhibit similar eftects — several attempts have been 

 made to explain the primary mode of action of thyroxine in terms of these 



