CHAPTER 4 



SULFHYDRYL REAGENTS 



A substance which can react with sulfhydryl groups and thus alter en- 

 zymic, metabolic, or functional processes is generally called a sulfhydryl 

 reagent. Such substances represent a very important group of inhibitors 

 and have been used extensively to determine if enzymes or metabolic reac- 

 tions depend in any way on intact sulfhydryl groups. In addition, they are 

 often used to estimate the number and reactivity of sulfhydryl groups on 

 proteins, or to histochemically localize the sulfhydryl groups in cells or tis- 

 sues. The next few chapters will be concerned with sulfhydryl reagents, and 

 in this chapter we shall discuss several general aspects of inhibition result- 

 ing from modifications of sulfhydryl groups and some of the problems en- 

 countered in work with these substances. This is one phase of inhibition 

 that has recently received considerable attention, and several reviews cover- 

 ing certain aspects of the problem are available. The articles by Boyer (1959) 

 and Putnam (1953), and the books "Glutathione" (1954) and "Sulfur in 

 Proteins" (1959), are particularly recommended. 



The terminology to be adopted attempts to follow the most recent usage. 

 The sulfhydryl group (= mercapto group) will for brevity be designated 

 as an SH group. Compounds containing SH groups will be designated as 

 thiols (elsewhere occasionally called sulfhydryl compounds or mercaptans). 

 A sulfhydryl reagent wiU be termed an SH reagent. The designation as a 

 sulfhydryl enzyme has often been meant to imply that the catalytic activity 

 of the enzyme is dependent on SH groups, i.e., that the SH groups actually 

 participate in the enzyme reaction. As Boyer (1959) has pointed out, not 

 a single enzyme has been definitely shown to involve protein SH groups 

 in the catalysis, and we shall see that the inhibition of an enzyme by an 

 SH reagent does not prove that the SH groups are functional. Hence, a 

 more practical definition of a sulfhydryl enzyme at the present time is an 

 enzyme that shows a loss of activity when some or all of its SH groups 

 are modified. 



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