Protein Complexes as Biochemically Active Systems 181 



establishing the role of morphological structures in biochemical processes in 

 the cell attaches, among other problems, to the study of the whole series of 

 problems relating to the interaction of proteins and Upids. 



Model experiments were staged by the author of the present paper to study 

 the interaction of certain proteins with hydrophobic substances of a Hpid nature; 

 their interaction, by virtue of the specific features in the chemical structure of 

 the constituents, is due chiefly to van der Waals' forces, and does not cause 

 substantial changes in the energy characteristics of the protein molecule. In 

 earlier papers [21] it was pointed out that egg albumin, when mixed with 

 crystaUine ergosterol at 37-40 °C, forms a complex containing one sterol mole- 

 cule for every two protein molecules. Further investigations revealed that the 

 protein-sterol complex is stable within a definite pH range (4-5) close to the 

 isoelectric point of the protein. To ascertain what factors influence the formation 

 and stability of protein-sterol complexes, we investigated the action of various 

 denaturing agents. It was shown [22] that the addition of guanidine or urea to 

 the initial protein solution inhibits its ability to form a complex with sterol, while 

 their addition to the already formed complex does not lead to its decomposition 

 in these same conditions, provided the optimal range of pH for complex forma- 

 tion is retained. It was also found [23] that the addition of 0-25% of cysteine to 

 the solution stabilizes the complex to ultra-violet irradiation, which in the ab- 

 sence of cysteine decomposes the complex. From these experiments we drew 

 the conclusion that the abihty of the protein to form complexes with sterol 

 depends on the preservation of its native structure, and that the complexes of 

 egg albumin with ergosterol are more stable with respect to denaturing influences 

 than the initial protein is. 



In subsequent papers [24-25] we established, in conformity with the facts 

 known concerning natural lipoproteins, that the lability of artificial protein- 

 lipid complexes is not only determined by the state of the protein molecule, but 

 also depends on the condition of the sterol molecule. More specifically, it was 

 estabhshed that if a solution of the complex is kept in contact with air for several 

 hours, it is spht into its initial components. The decomposition of the complex 

 in these conditions was found to be due to sterol oxidation, which can be cata- 

 lysed by copper ions, just as in the case of natural hpoproteins. This pointed to 

 a certain similarity between our artificial protein-lipid complexes and the lipo- 

 proteins isolated from organisms; the similarity is in composition and stability 

 conditions, which helps to understand the factors controlling the transformations 

 of these important compounds in the organism. It should be noted that we 

 established formation of the above-mentioned protein-sterol complexes by 

 means of two independent methods : by studying the isotherms of compressi- 

 bility of the protein in a unimolecular layer on 5% ammonium sulphate solution 

 and by the method of paper electrophoresis. 



It was particularly interesting, in relation to the problem we are studying, to 

 obtain complexes of Hpid substances with enzyme proteins and to investigate 

 the changes in enzyme activity resulting from complex formation. For this 

 purpose we prepared complexes of trypsin with ergosterol according to the method 

 described earher. The formation of these complexes was determined on the basis 



