Polymers containing groups of potential biological activity
The activity of many enzymes has been shown to depend on the presence of sulphhydryl groups13, not all of which are of equal reactivity nor of equal importance for the proper functioning of the enzyme. Three types of sulphhydryl groups are recognized depending on their reactivity with oxidizing, alkylating and mercaptide-forming reagents. These sulphhydryl groups are classified as freely reactive, sluggishly reactive and " masked ". The " masked" groups become freely reactive when the protein is denatured. It is believed that this is due to the unfolding of the peptide chains, thus making these hitherto inaccessible sulphhydryl groups freely available4 • Resistance to oxidation of some of the sulphhydryl groups of the enzyme is thought tobe due to the distance between sulphhydryl in the native protein, which prevents the formation of a disulphide bond. Barron has reported that for a series of dithiols, the greater the distance between the thiol groups the slower will be the oxidation 5• The presence of electronegative groups in a molecule was also found to influence the ease of oxidation 5, the effect being a decrease in the degree of dissociation of the -SH bond resulting in a decrease in the rate of oxidation. High energy irradiation of water containing dissolved oxygen has been shown to produce the oxidizing agents hydroxyl and hydroperoxyl radicals6 • Since thiols are easily oxidized by mild oxidizing agents, it was not surprising to find that ionizing radiation would rapidly oxidize biologically essential sulphhydryl groups and thus be detrimental to life6• However, reversal of inhibition and, further, protection on addition of sulphhydryl compounds such as cysteine and glutathione was shown to take place 710• The detoxification of hydroxyl and hydroperoxyl radicals is most probably accomplished by a rapid reaction of these radicals with the thiol groups, a reaction of relatively low activation energy. Thus, the efficiency of sulphhydryl compounds as protectors against the Iethai effects of radiation has been shown tobe related to their ease of oxidation11• Recently3 \ the suggestionwas made that darnage by irradiationwas due to the oxidation of biologically necessary cuprous ion to cupric. Protection