Mat 3, 1918] 



SCIENCE 



427 



might aid in throwing light on the nature 

 of the grouping. A systematic study of the 

 factors which caused inactivation of the 

 esterase and lipase was therefore under- 

 taken. The results were presented in de- 

 tail elsewhere. 



Inactivation of the lipase and esterase 

 preparations was brought about by acids, 

 bases, neutral salts, alcohols, acetone, esters 

 and heat. 



The different ways in which these prep- 

 arations may be inactivated make it appear 

 at first sight as if different reactions occur 

 in the inactivations. If, however, a defi- 

 nite chemical group is responsible for a 

 definite enzyme action, it might perhaps be 

 more reasonable to assume that inactiva- 

 tion follows a definite reaction. The prepa- 

 rations used were essentially protein in 

 character. There was no evidence that a 

 dehydration, or loss of the elements of 

 water, caused inactivation. Some of the 

 reactions indicated that a possible hydrol- 

 ysis might be a cause of inactivation. 

 With proteins, hj'drolysis is generally 

 taken to occur with the — CO — NH — 

 group, the peptide linking, which goes over 

 into the carboxyl and amino groups. Ex- 

 periments with all the inactivations in no 

 case showed an increase in the formol titra- 

 tion such as would be expected in this re- 

 action, and, therefore, makes the assump- 

 tion of such a hydrolysis improbable. Co- 

 agulation of the material accompanied 

 some of the inactivations. This physical 

 change alone does not appear satisfactory 

 as an explanation; some change in chem- 

 ical structure unquestionably must accom- 

 pany or produce the physical phenomenon. 

 Furthermore, the lipase material in sus- 

 pension in water showed the same activity 

 as in 1.5 normal sodium chloride solution 

 when tested immediately. 



The explanations of the chemical changes 



accompanying inactivation so far suggested 

 are not satisfactory. The reagents used 

 are simple. It is difficult to conceive of a 

 very deep-seated chemical reaction taking 

 place under so many different conditions, 

 none of a complex nature. The only chem- 

 ical change which appears probable under 

 these conditions is that involving a simple 

 rearrangement within the molecule, such 

 as a tautomeric change involving the 

 change in position of a hydrogen atom. 

 In considering the structure of proteins it 

 is evident that such a rearrangement is 

 possible in the peptide linking. 



The hypothesis to be suggested is that 

 the active grouping of the esterase and 

 lipase preparations is of the enol-lactim 

 structure, — C(OH):=N — , the specifici- 

 ties being dependent in part upon the 

 groups combined with the carbon and ni- 

 trogen, and that inactivation consists pri- 

 marily in a rearrangement to the keto- 

 lactam group, — CO — NH — . 



This hypothesis was tested in several dif- 

 ferent ways. It has been found that in tau- 

 tomeric substances, the presence of alkali 

 in solution favors the enol form of com- 

 pounds showing such tautomerism, while 

 acid favors the existence of the keto form. 

 The hydrolytic actions of some simple 

 dipeptides on esters at different hydrogen- 

 ion concentrations would, therefore, be evi- 

 dence bearing on this point, the alkaline 

 solutions presumably favoring the enol- 

 lactim structure. In order to find the ac- 

 tions exerted by the amino-carboxyl groups 

 of the peptide, the hydrolytic actions of a 

 number of amino acids on different esters 

 were determined at the same hydrogen-ion 

 concentrations. The actions of the dipep- 

 tides and amino acids were also measured 

 with the amino-carboxyl groups actions 

 masked by the hydrogen of the carboxyl 

 group being replaced by the ethyl group. 



