THE CHEMICAL NATURE OF DIFFERENTIALS 577 



precursor rather than in the enzyme which induces this reaction. This applies 

 also as far as the species specificity of the enzymes and their precursors is 

 concerned. As Herriot, Bartz and Northrop have shown, swine pepsinogen 

 can be converted only into swine pepsin and chicken pepsinogen into chicken 

 pepsin, irrespective of the species character of the enzyme which serves as 

 catalyst of this reaction. 



A very marked organ and perhaps also organismal specificity of enzymes 

 has been found in Limulus (Loeb and Bodansky). In this species, urease 

 occurs in the bodyfluid, muscle, and even in the eggs. Moreover, a urease is 

 present in the amoebocyte tissue prepared from the amoebocytes of the body- 

 fluids. This enzyme has been found so far only in Limulus and not in any 

 of the arthropods which have been examined for its presence. But in 

 Limulus the urease obtained from amoebocytes differs from that found in the 

 other organs or tissues in that the amoebocyte-tissue enzyme combines with 

 various kations, and the degree of its activity depends upon the kind of 

 kation with which it is combined ; but heavy metal combinations of the 

 enzyme are inactive, probably because they induce denaturation. This urease 

 represents therefore, in all probability, a metal protein combination. If the 

 enzyme obtained from various organs of^ Limulus is injected into the body- 

 fluids of this animal in sufficient quantity, urea is transformed into ammonium 

 carbonate, a substance which is toxic and lethal. 



Similar in certain respects to the action of enzymes is that of some viruses, 

 such as the virus of tobacco mosaic disease, which has been found to be a 

 crystalline nucleoprotein (Stanley), and bacteriophage, which is also a 

 nucleoprotein, according to Northrop. With this interpretation accord the 

 experiments of Bronfenbrenner and Kalmanson which have made it very 

 probable that bacteriophages do not multiply as bacteria do, but are con- 

 tinually newly formed by the type of bacteria in which they originated. On 

 the other hand there should be considered also the strong indication that 

 bacteriophage occurs in association with larger particles of various sizes by 

 which the phage has been adsorbed, and that the active agent represents a 

 smaller molecule (Bronfenbrenner). Both the viruses and the phages are 

 specific in their action as far as the character of the substratum is concerned. 

 Thus, bacteriophages act primarily only on the bacteria in which they origi- 

 nated, or on nearly related microorganisms. The phages derived from dif- 

 ferent bacteria can also be distinguished by immunological methods. To a 

 certain extent, an adaptation of bacteriophages to new hosts may take place. 

 Likewise, bacteria and yeasts may produce new enzymes in response to 

 altered substratum (Euler, Dubos) on which they are cultivated. 



In passing from cell protoplasm to enzymes and viruses, we compare the 

 most complex substances with other substances which are less complex and 

 less specific. A further step leads to the hormones, some of which are still 

 proteins, while others represent relatively simple organic substances. In 

 different species the production and distribution of various hormones may 

 be different, and some of the complex hormones seem to possess a certain 

 degree of organismal-specificity (insulin, pituitary hormones), but the large 



