PORPHYREXIDE AND PORPHYRINDIN 669 



at or near the active center of papain is in a high energy state, perhaps as a 

 thiol ester, and this may explain why it is resistant to porphyrindin. 



The reactive SH groups of urease are rapidly oxidized by porphyrindin 

 and the nitroprusside test becomes negative (Hellerman, 1939). However, 

 no inhibition occurs. Porphyrindin-treated enzyme is inhibited by jo-chloro- 

 mercuribenzoate, so that certain SH groups required for activity are resist- 

 ant to porphyrindin. If an excess of porphyrindin is used, inactivation oc- 

 curs slowly and is irreversible. Oxidation past the disulfide stage or oxida- 

 tion of other groups is possible. Since, p-chloromercuribenzoate protects the 

 enzyme from high concentrations of porphyrindin, it appears that SH groups 

 are indeed involved. It was established later that there are two types of SH 

 group in urease: reactive a groups not necessary for enzyme activity, and 

 less reactive b groups at the active center. Porphyrindin reacts with the 

 former but only slowly with the latter at higher concentrations (Hellerman 

 et al., 1943). If urease is denatured with guanidine, many more SH groups 

 appear and react with porphyrindin. 



Xanthine oxidase is inhibited readily by porphyrindin but this is not re- 

 versible with cysteine (Harris and Hellerman, 1956). The inhibition by o- 

 iodosobenzoate is also irreversible. This problem comes up repeatedly with 

 inhibitions by oxidants and seems on the surface to indicate that a simple 

 oxidation to the disulfide stage does not occur. However, it is also possible 

 that (1) the oxidation-reduction potential of the groups involved is such 

 that cysteine will not reduce them, (2) the enzyme structure is altered by 

 the formation of disulfide bonds, (3) oxidation past the disulfide stage has 

 occurred, or (4) the formation of intermolecular disulfide linkages prevents 

 the access of cysteine to the group. It is impossible to distinguish at this 

 time between these different possibilities. 



Effects on Tissue Function 



Porphyrindin at 0.37 0.75 mM produces an increase in the contractile 

 amplitude of the frog heart and this effect can last for as long as 90 min 

 (Mendez, 1946; Mendez and Peralta, 1947). Higher concentrations of 3.7- 

 7.5 mM bring about a progressive contracture, the heart stopping in systole 

 in around 30 min. The rate is simultaneously slowed. The atria continue to 

 beat after the ventricles have stopped. These effects can be prevented by 

 glutathione but not reversed, as expected. In these respects the heart re- 

 sponds to porphyrindin much as it does to other SH reagents. The site or 

 sites of action are not known, and it is useless to speculate since the relative 

 sensitivities of the possible enzymes involved are undetermined. 



A few miscellaneous and unrelated observations will be mentioned. The 

 short-circuit current and potential of frog skin are altered by oxidants and 

 reductants, such as quinones, dyes, and iodine, but there is little effect of 

 porphyrindin at 1 mM; the potential may drop temporarily but the cur- 



