636 4. SULFHYDRYL REAGENTS 



ROLE OF SH GROUPS IN METABOLISM AND FUNCTION 



Cellular components containing SH groups may be conveniently grouped 

 in three categories: (1) low molecular weight thiols, such as the cofactors li- 

 poate, coenzyme A, and glutathione, or various amino acids and related 

 compounds, such as cysteine, homocysteine, 2-thiolliistidine, ergothioneine, 

 and thioglycolate, (2) nonenzyme proteins, probably including most of the 

 cytoplasmic proteins (e.g., those involved in movement, such as actomyosin, 

 ciliary proteins, and proteins of the mitotic spindle), plasma membrane pro- 

 teins, and structural proteins, and (3) enzymes of all types and catalyzing 

 every variety of reaction. Modification of or reaction with any of these SH 

 groups may directly or indirectly alter cellular metabolism and function. 

 Even reaction with nonenzyme protein SH groups may disturb metabolism, 

 because of the role such proteins may play in the structural organization of 

 the metabolic units or in the permeabilities of cells. In addition to the free 

 SH groups, many proteins and enzymes contain disulfide (S — S) groups 

 that, in the case of enzymes, are probably not involved directly in the ca- 

 talysis but in the structural stability. These disulfide groups can under cer- 

 tain circumstances be reductively cleaved to form free SH groups, with 

 simultaneous loosening of the protein structure, or can perhaps react di- 

 rectly with certain agents to form mercaptides. 



In the early days of interest in thiols, it was believed that SH reagents 

 altered metabolism, and were sometimes lethal, as a result of reaction with 

 glutathione or other low molecular weight thiols, but it was soon realized 

 that enzyme SH groups are a much more important site of attack. Even 

 today the role that such small thiols play in metabolism and the importance 

 of their reaction with SH reagents are not well understood, except in the 

 case of coenzyme A and lipoate. The ubiquitous glutathione plays at pres- 

 ent an indeterminate role in metabolism, except for its likely participation 

 in the reactions of phosphoglyceraldehyde dehydrogenase, glyoxalase, mal- 

 eate isomerase, maleylacetoacetate isomerase, formaldehyde dehydrogenase, 

 and indolylpyruvate tautomerase, and in transpeptidation and folate split- 

 ting. Low molecular weight thiols have also been supposed to regulate me- 

 tabolism by redox equilibria with enzyme SH groups, maintaining a cer- 

 tain fraction of these in the reduced or active state. 



The SH groups of enzymes have been considered to bind cofactors or 

 coenzymes to the apoenzyme, or to form acyl or phosphoryl complexes 

 with intermediates derived from substrates, or to function directly as redox 

 couples in electron transfer, but there is little evidence for any of these, as 

 likely as they may be. The SH group readily donates electron pairs and 

 thus is one of the most reactive enzyme groups with regard to the formation 

 of covalent bonds, so it would not be surprising if covalent intermediate 

 complexes occur. Whatever the role SH groups play in enzyme catalysis, 



