666 5. OXIDANTS 



Spiro analogs of both porphyrexide and porphyrindin were synthesized 

 by Porter and Hellerman (1944) and found to have high oxidation-reduc- 



O" O 



N— C— NH / \ N=C— N=N— C=N, 



C— NH ' ' C— NH HN— C 



II II II 



NH NH NH 



Spiroporphyrexide Spiroporphyrindin 



tion potentials. The Ef^ at pH 7 for spiroporphyrexide is + 0.69 v, the po- 

 tential increasing at lower pH's. The spiroporphyrindin is quite insoluble 

 and may be more polymerized than indicated. An interesting aspect of spiro- 

 porphyrexide is that it does not inhibit urease, despite its high potential, 

 indicating possible steric effects of the cyclohexyl ring. This is a good exam- 

 ple that not only is oxidation-reduction potential important in the oxida- 

 tion of SH groups on enzymes but that structural configuration is a factor, 

 as in any inhibition. 



Oxidation of Thiols and Amino Acids 



Porphyrexide and porphyrindin react very rapidly with thiols at neutral- 

 ity and the end-point is generally quite sharp; cysteine, glutathione, and 

 cysteinylcysteine are titrated quite comparably (Greenstein, 1938). The oxi- 

 dation of cysteine at pH 7.2 is complete within 3-60 sec and glutathione is 

 oxidized only slightly more slowly (Brand and Kassell, 1940). No reaction 

 under ordinary conditions is seen with cystine, cysteate, tryptophan, hydro- 

 xyproline, histidine, methionine, serine, phenylalanine, or threonine. Tyro- 

 sine, however, is oxidized slowly with the formation of a pink-orange color, 

 the reaction taking around 30 min for completion (2 equivalents of porphy- 

 rindin per mole of tyrosine) at pH 7.2 and 0°. Tyrosine and other phenols 

 are oxidized more rapidly in alkaline solutions and at higher temperatures, 

 but in most cases the reaction is much slower than the oxidation of SH 

 groups (Greenstein and Edsall, 1940). At pH 7.33 and 25^ the oxidation of 

 tyrosine may be fairly rapid, and even tryptophan may be slowly reacted 

 (half-reaction time around 2 hr) (Barron et al., 1941). Porphyrindin can 

 also oxidize a variety of other substances, such as ascorbate or thiamine 

 (Kuhn and Desnuelle, 1938), and in complex systems or cellular prepara- 

 tions the effects may not be due entirely to SH group oxidation. 



Oxidation of Proteins 



Kuhn and Desnuelle (1938) showed that native ovalbumin does not react 

 with porphyrindin (in common with other SH reagents) but that following 



