Discussion 97 



enzyme. I don't say that cases previously quoted do not involve qualita- 

 tive chan<yes but simply that they are not at all good examples, for that 

 reason. One wants a purified enzyme in which one can characterize 

 tilings like the molecular weight and the specific enzyme activity. Such 

 criteria are much better than thermostability. 



Davis: I think that at a sufiiciently refined level of analysis one can 

 never answer the question of whether or not a temperature-sensitive 

 mutant, which produces an apparently temperature-sensitive enzyme, 

 is really producing an altered enzyme. One can always ask the question : 

 What is a true enzyme and what is an enzyme with something attached 

 to it ? It is true that changes in the environment of a protein, such as 

 solubilization or even changes in the concentration of various solutes in 

 the medium, can change its thermostability. Thus, Dr. Maas and I 

 observed that the thermostability of pantothenate synthase, whether 

 from the wild-type or from the temperature-sensitive mutant, is several 

 degrees higher in the intact cell than in the extract. However, I think 

 that the results of the mixture experiment excluded the possibility that 

 the difference in thermostability of the two extracted enzymes depended 

 on differences in their respective environments; for in this experiment 

 the fraction that had been contributed by the mutant underwent 

 irreversible denaturation, just as when the mutant extract was tested by 

 itself, while the other fraction of the total enzyme activity remained 

 unchanged. Therefore, in such a mixture, whatever the factor may be 

 that makes one of the enzymes more stable, it is not present in an excess 

 that can affect the other. It is a stoichiometric change in the enzyme. 



Pollock: Even if you did get an association between some relatively 

 quite small molecular weight compound which conferred thermostability 

 on the enzyme at an earlier stage, it might well be irreversible and not 

 be affected by the subsequent extract. 



Davis: This would be a different enzyme. 



Pollock: I quite accept your point here. I would only say that it is 

 very easy to change the thermostability of enzymes by treatment. It is 

 not the best criterion. 



Slonimski: Emil Smith, in his work on papain, showed that you can 

 chop off something like 18 amino acids without- changing the enzyme 

 activity. Therefore, you can have a partial protein molecule without 

 any detectable change, as judged by the usual criteria of enzymic activity. 

 It will be interesting to see whether, in such a case, there is a change in 

 thermostability. 



Lederberg: There is really a much more fundamental difficulty in 

 trying to distinguish between qualitative and quantitative changes. The 

 cell produces what it chooses, not necessarily all that it can produce. 

 Even in the case of the human haemoglobin the wild genotj^DC is perfectly 

 capable of making foetal haemoglobin and does so during embryonic life. 

 But in the course of normal development there is a transition to the 

 formation of typical adult haemoglobin. There are certain mutations 

 which prevent this changeover and the result is that in the adult indi- 

 vidual of mutant genotype you then get large amounts of foetal haemo- 

 globin instead. If you did not know all the developmental details you 



DRUG EES. — 4 



