TETRATHIONATE 699 



suiting from 0.2 mM and 93% from 0.6 mM (Gordon and Quastel, 1948). 

 The only clear-cut demonstration of reaction with enzyme SH groups is 

 that of phosphoglyceraldehyde dehydrogenase (Pihl and Lange, 1962). Here 

 tetrathionate inhibits as well as /)-chloromercuribenzoate, i.e., when 3 moles 

 of inhibitor are reacted per mole of enzyme, the activity is reduced to zero 

 in both cases. Also the inhibition is fully reversible with thiols. However, 

 as mentioned above, the reaction does not appear to be a simple oxidation, 

 but involves the formation of sulfenyl thiosulfate groups. The enzyme is 

 very sensitive, since 0.005 mM inhibits completely (enzyme = 0.0005 mM) 

 within 5 min. 



In view of the potent inhibition of phosphoglyceraldehyde dehydrogenase 

 one might anticipate tetrathionate to be a glycolytic inhibitor. Goffart and 

 Fischer (1948) attempted to demonstrate a Lundsgaard effect in muscle, 

 i.e., a typical contracture such as produced by iodoacetate and certain other 

 SH reagents. Following injection into rabbits, the extremities become weak 

 but the muscles remain elastic and the reflexes normal; if the gastrocne- 

 mius is stimulated, it does not go into contracture. Intraarterial injection 

 produces a temporary contracture (or at least some inhibition of relaxation). 

 Injection into frogs does not give an iodoacetate-like effect and the isolated 

 frog rectus abdominis muscle gives only a temporary contracture-like reac- 

 tion. It is doubtful if true contractures are observed, and in any case the 

 tetrathionate concentration must be quite high. It is possible that the phos- 

 phoglyceraldehyde dehydrogenase in the muscle is protected by a perme- 

 ability barrier to the doubly charged inhibitor, and by the presence of NAD 

 and substrate on the enzyme. MacLeod (1951) found inhibition of glycolysis 

 in human spermatozoa, but the tetrathionate concentration was 10 mM and 

 the inhibition progressed very slowly, so that even after 3 hr the glycolysis 

 is not completely depressed (around 50%). The motility decreases simul- 

 taneously with the reduction in glycolysis. One of the pitfalls of reversibility 

 experiments is well illustrated here, for when cysteine is used there is a rapid 

 toxic effect on the spermatozoa, this being due to the cystine arising as the 

 result of the oxidation by tetrathionate. 



Tetrathionate is reduced to thiosulfate by reaction with SH groups and 

 it has been supposed that the rapid conversion into thiosulfate in rabbits 

 and dogs is due to this (Gilman et al., 1946). While this must be true in part, 

 there is some evidence for enzyme systems catalyzing this reaction. Thus in 

 various bacteria tetrathionate is readily reduced, while in others no reaction 

 at all occurs (Pollock and Knox, 1943). Postgate (1956) has isolated cell-free 

 systems reducing tetrathionate, thiosulfate, and sulfite from the anaerobe 

 Desulfovibrio desulfuricans, the cytochrome system acting as an electron 

 carrier for the tetrathionate reductase. Indeed, it is likely that tetrathio- 

 nate can be oxidized through the cytochrome system in most cells. Thus 

 tetrathionate must generally be rather labile in most biological situations. 



