M. N. SWARTZ, N. O. KAPLAN AND M. E. FRECH 69 



not been purified sufficiently to determine whether the two enzymes have 

 the same or different inhibitor proteins. Inorganic pyrophosphate is 

 needed to protect the 5' nucleotidase just as it is needed to protect the 

 pyrophosphatase against heat denaturation. In contrast, there appears 

 to be no evident protective co-factor for the 'heat-activated' DPNase of 

 Mijcohacterium. Also, the heat-labile enzyme alcohol dehydrogenase is 

 not rendered heat-stable by boiling in the presence of pyrophosphate. 



The mechanism by which pyrophosphate affords protection is not at 

 all clear. Inorganic pyrophosphate is a substrate for the Proteus pyrophos- 

 phatase, but it is split at an extremely slow rate. Perhaps, just as the 

 substrate glucose protects hexokinase against inactivation by trypsin, 

 pyrophosphate may analogously protect the Proteus pyrophosphatase 

 against heat. However, other substrates for this enzyme afford no pro- 

 tection. Also, this would not explain the protective effect for the 5' nucleo- 

 tidase. 



SUMMARY 



Several 'heat-activated' bacterial enzymes have recently been described. 

 The 'activation' has been dependent on the destruction by heat of an in- 

 hibitor protein that is more temperature-labile than the enzyme involved. 

 The most interesting aspect of these relationships may lie in the role of 

 various factors in stabilizing or labilizing proteins to the effects of heat. 

 In the case of the Proteus DPN pyrophosphatase, inorganic pyrophosphate 

 or another as yet unidentified compound in extracts of Proteus will protect 

 against heat inactivation of the enzyme. In the case of the Mycobacterium 

 DPNase, an unidentified factor in crude extracts renders the inhibitor 

 protein sensitive to heat. 



The enzymes themselves, once freed of the inhibitors, are not 'heat- 

 activated'. They exhibit the usual Qio that one might expect with enzymes. 

 Thus, the 'activation' is a function of the effect of temperature on a bound 

 inhibitor and not of the effect of temperature in 'exciting' the enzyme 

 molecule itself. 



REFERENCES 



1. Allen, T. H., 0. M. Ray and J. H. Bodine. Enzymes in orthopteran ontogenesis. 

 VI. Autocatalytic nature of in vivo formation of protyrosinase. Proc. Soc. Exper. 

 Biol, cfc Med. 39: 549, 1938. 



2. Baker, H., S. H. Hutner and H. Sobotka. Nutritional factors in thermophily. 

 Ayin. New York Acad. Sc. 62: 349, 1955. 



3. Bodine, J. H. and L. D. Carlson. Enzymes in ontogenesis (Orthoptera). X. The 

 effects of temperature on the activity of the naturally occurring and other acti- 

 vators of protyrosinase. J. Cell. & Comp. Physiol. 16: 71, 1940. 



4. Bodine, J. H. and T. H. Allen. Enzymes in ontogenesis (Orthoptera) XV. Some 

 properties of protyrosinases. /. Cell. & Comp. Physiol. 18: 151, 1941. 



