VARIATIONS IN BIOCHEMICAL REACTIONS 293 



regard any given bacterial species as equipped with an armoury of enzymic 

 mechanisms of the kind discussed in Chapter 3. Some of these may perhaps 

 readily be lost by disuse, and as readily regained if called into activity by appropriate 

 stimuli. Others may, perhaps, be easily adapted to deal with some substrate 

 that does not differ too greatly from that which the enzyme naturally attacks. 

 Others, again, will depend on more constant and fundamental cell mechanisms ; 

 these will be lost, regained, or altered only as the result of some deep-seated variation 

 in cell structure. 



It is indeed impossible to draw any hard and fast line between those changes in 

 fermentative ability that arise as temporary adaptations to environment, and those 

 that may be regarded as variations of a more permanent kind, though certain 

 instances can be assigned with some confidence to one category or the other. 



Adaptive and Constitutive Enzymes. — The enzyme response of a bacterium to 

 a substrate may fall into one of two classes. The enzyme may be produced only 

 in the presence of the substrate or it may be produced whether the substrate is 

 there or not. Thus, Wortmann (1882) described a bacterium that produced 

 amylase in a starch medium, but none in a starch-free medium, and contrasted 

 it with a yeast, which produced invertase whether sucrose was present or not. 

 For these two classes Karstrom (1930, 1937) proposed the names " adaptive " 

 and " constitutive " enzymes. For example, the production of molecular hydrogen 

 from glucose or formic acid by Bact. coli was found to be due to two " adaptive " 

 enzymes, one a glucose hydrogenlyase, the other a formic hydrogenlyase (Stephenson 

 and Stickland 1932, 1933, Yudkin 1932). Not only were the enzymes adaptive, 

 but they were apparently formed in resting, as well as in reproducing cells. Some 

 protoplasmic growth, however, appeared to be necessary, since in the absence of 

 nutrient broth no enzymes were formed (see also Stephenson and Yudkin 1936). 

 The adaptation disappeared in the absence of the stimulating substrate, and was 

 therefore not heritable. 



Enzyme formation may also be conditioned by the presence of materials which 

 are unrelated to the substrate, presumably because they are needed for enzyme 

 synthesis (see Jacoby 1916, 1917, 1918, Passmore and Yudkin 1937). Quastel 

 (1937) developed this concept by postulating that enzymes themselves were meta- 

 bolites whose rates of formation and destruction follow the same physicochemical 

 laws as those controlling the metabolism of other metabolites in the cells ; the 

 presence of substrate, therefore, might or might not be the particular condition 

 necessary for the production of an enzyme. In his view, the adaptive and con- 

 stitutive enzymes represent the limits of variability of cellular enzymes, the 

 constitutive having the least, the adaptive the greatest range under different 

 environmental conditions. 



Euler and Cramer (1913) were able to stimulate the production of invertase 

 in a yeast by the addition of fructose or glucose, and also by mannose. It appears, 

 therefore, that not only substrates, but the products of hydrolysis of substrates 

 and chemicals related to the substrate act as stimulants of adaptive enzymes. 

 According to Yudkin (1938) the Mass Law serves to reconcile many of these pheno- 

 mena of enzyme adaptation. If it is assumed that the enzyme and its precursor 

 are in a state of equilibrium in the cell, with the precursor predominating and the 

 enzyme in undetectable amounts, a substrate, its hydrolytic products, or a related 

 substance will, by combining with the enzyme, shift the equilibrium so that more 

 enzyme is formed from the precursor. The mere presence of the substrate is 



