ADAPTIVE AND CONSTITUTIVE ENZYMES 95 



organisms had doubled in numbers, which certainly seems 

 to rule out selection as the mechanism in this case. In 

 another experiment washed suspensions of Esch. coli were 

 added to a tryptic digest plus formate, and the production 

 of enzjTne (as measured by evolution of hydrogen in a 

 Barcroft apparatus) was followed. Initially there was no 

 enzyme, but it began to appear after forty-five minutes 

 and reached a maximum value in 150 minutes, during 

 which time there was less than a 5 per cent, increase in 

 the number of organisms ; the hydrogen production in 

 the same time increased by more than a thousandfold. 



Other examples of adaptive enzymes are hyaluronidase 

 (the " spreading factor " produced by Clostridium 

 welchii) which is only formed when its substrate, hyalu- 

 ronic acid, is present and the enzyme, formed by a soil 

 organism, which hydrolyses the specific polysaccharide 

 of Type III pneumococcus. 



The results of experiments on adaptive and constitutive 

 enzymes depend largely on the time for which the 

 organism is allowed to grow or to remain in the medium. 

 The amounts of some enzymes change considerably with 

 time (that is, with the state of growth of the organism), 

 whilst others remain more or less constant in amount. 

 Thus an organism examined after twelve hours' gro\\i:h 

 may have quite a different set of enzymes from that 

 which it possesses after, say, seventy-two hours. When 

 grown on a particular substrate an organism usually 

 tends to maintain a normal concentration of the corres- 

 ponding enzyme for a considerably longer time than it 

 ordinarily does, and to maintain that concentration after 

 the activity of most of the other enzymes has fallen off. 

 Hence the apparent increase in activity of an enzyme 

 in presence of its substrate may not always be due to a 

 true increase in production of the enzjnne but to a 

 contrast with the low values of the other enzymes. This, 

 of course, will be particularly marked if old cultures are 

 examined ; if the cultures are examined during the 



