136 GENERAL BIOCHEMISTRY 



factor. The ribonucleic acids must also be involved. Ribonuclease 

 acts only on ribonucleic acid. Systems treated with this enzyme lose 

 their ability to synthesize other enzymes. When radioactive isotopes 

 are used to follow the formation of an enzyme, ribonucleic acids are 

 also formed. Anything blocking the synthesis of ribonucleic acids 

 blocks enzyme synthesis. 



The current hypothesis concerning the control of enzyme synthesis 

 runs as follows. Deoxyribonucleic acids carry the hereditary chemical 

 information necessary for precise specification of enzyme structure 

 from mother cell to daughter, thus providing inheritance. The 

 information thus available is somehow transferred through ribonu- 

 cleic acid into enzyme structure with ribonucleic acid and protein 

 synthesized at about the same time. 



It is not yet known whether the nucleic acids serve a catalytic role, 

 take part in energy transfers, or function only as patterns or templates. 

 Clearly they are involved somehow, almost certainly in the last 

 capacity but perhaps in others also. 



Precursors 



In several cases enzymes are synthesized and secreted in inactive 

 forms called precursors. These proteins are not really enzymes, of 

 course, because they display no catalytic activity but do give rise to 

 enzymes under special conditions. It is difficult to see why cells should 

 make precursors for some enzymes and not others, rather than merely 

 forming the active enzymes in all cases. Perhaps the precursors are 

 evolutionary anachronisms. Or perhaps they are required at times or 

 places when the active enzymes woidd injure cells. Many of the 

 known precursor proteins convert into proteolytic enzymes which 

 might conceivably cause the destruction of essential protein com- 

 ponents in the cells synthesizing these specialized materials. 



Pepsinogen is a well-known precursor system. It is secreted by the 

 giant or chief cells into the stomach, where the acid secreted by 

 other cells helps convert the precursor into active pepsin. Any pepsin 

 already present catalyzes this conversion, which involves splitting oft 

 a peptide fragment. The molecular weights of the various molecules 

 are as follows: pepsinogen, 42,000; pepsin, 36,000; peptide, 6,000. 



Pepsmogen > pepsm + peptide 



pepsin 



Several of the enzyme-precursor systems are important in the diges- 

 tive processes of mammals and birds. Others are believed to participate 



