664 5. OXIDANTS 



ase is inhibited 20% at 20°, 78% at 30°, and 93% at 40^ by 100 mM GSSG 

 after 4 hr incubation (Hopkins et al., 1938). Thus one might expect incon- 

 veniently slow reactions at low temperatures. Yet Barany (1959) ran his 

 experiments with ATPase at 0^, although up to 15 hr was sometimes re- 

 quired for satisfactory reaction. Increase in the pH favors the oxidation of 

 enzyme SH groups to disulfides, since the — S~ form presumably reacts 

 more readily. Hence, incubation of the enzyme with the disulfide at pH's 

 around 9 may be useful where possible. 



PORPHYREXIDE AND PORPHYRINDIN 



Porphyrindin was originally synthesized by Piloty and Schwerin (1901 a, 

 b, c; Piloty and Vogel, 1903) but only much later attracted attention, 

 when it was studied by Kuhn et al. (1934) as the first clearly demonstrated 

 double free radical, and shown by Kuhn and Franke (1935) to have one of 

 the highest oxidation-reduction potentials among organic substances. It was 

 introduced as a reagent for the determination of protein SH groups by Kuhn 

 and Desnuelle (1938) because of its high potential and applied particularly 

 by Greenstein (1938; Greenstein and Edsall, 1940; Greenstein and Jenrette, 

 1942) for this purpose. Meanwhile its synthesis was improved by Porter and 

 Hellerman (1939). For the past 20 years it has been used sporadically to 

 inactivate enzymes by oxidation of the SH groups. Possibly it has been 

 neglected in enzyme studies since, although it is probably not as specific 

 for SH groups as is o-iodosobenzoate, it is certainly more selective than 

 most oxidants and, furthermore, reacts more rapidly and more completely. 



Chemistry 



The structures of porphyrindin and porphyrexide may be written in sev- 

 eral different ways because of resonance. Both in the crystalline state are 

 paramagnetic, the values indicating one unpaired electron in porphyrexide 

 and two in porphyrindin (Kuhn et al., 1934). The paramagnetism, however, 

 increases with temperature (Miiller and Miiller-Rodloff, 1935), suggesting 

 equilibria between diamagnetic and paramagnetic forms. Thus the resonance 

 structures for porphyrindin may be written as: 



HoC O" ~0 CH, HjC O' b CH3 



1 L .11 I 1+ +11 



H3C-C— N N— C— CH3 H3C-C— N N-C— CH, 



I \\ //I \ ■\ /-I 



C— N=N— C -« >- C=N— N = C 



I / \ I I / \ I 



HN— C— N N— C = NH HN=C— N N— C = NH 



H H H H 



(diamagnetic) (paramagnetic) 



Porphyrindin 



