168 Bernard D. Davis 



lead to the formation of qualitatively altered enzymes, and 

 the presence in bacteria of mutable stereospecific permeation 

 systems. Since these developments are so recent it may be 

 profitable here to consider their experimental basis as well as 

 their application, as yet limited, to the problem of drug 

 resistance. 



Qualitatively altered enzymes 



While evolutionary considerations have long made it 

 evident that mutations must be able to affect the nature as 

 well as the amount of the various enzymes (and other proteins) 

 formed by an organism, this phenomenon was first demon- 

 strated less than ten years ago by Pauling and co-workers 

 (1949), and in mammals rather than microbes. They showed 

 that patients with sickle-cell anaemia, a hereditary disease, 

 formed haemoglobin with different electrophoretic mobility 

 from that of normal human haemoglobin. Since then a number 

 of different kinds of human haemoglobin have been discovered. 



Shortly thereafter a mutational change in the nature of a 

 protein was shown for an enzyme, and in micro-organisms, by 

 Maas and Davis (1952) working with pantothenate synthase 

 in Escherichia coli, and by Horowitz and Fling (1953) working 

 with tyrosinase in Neurospora crassa. In each case the mutant 

 studied was a temperature-sensitive one: i.e. it lacked the 

 enzyme in question when grown at ordinary temperatures, 

 but formed it when grown at low temperatures. When the 

 enzyme was extracted from the cells grown at low temperature, 

 it was found to differ strikingly from the corresponding enzyme 

 of the wild-type strain in one respect: it was irreversibly 

 denatured at moderate temperatures which did not denature 

 the wild-type enzyme. When tested at low temperature the 

 enzymes were indistinguishable in other respects : the reaction 

 catalysed, cofactors required, pH optimum and Michaelis 

 constant. Evidence for the thermal instability of the mutant 

 pantothenate synthase is presented in Fig. 1 (from Maas and 

 Davis, 1952), in which the triangles represent for the wild- 

 type extract, and the circles for mutant (99-lt) extract, the 



