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Matrices containing ligands which interact with catalytic or allosteric sites have been successfuly used for the purification of the allosteric enzyme dCMP-aminohydrolase. This enzyme is eluted specifically by a competitive inhibitor from a matrix coated with ligands which interact with the catalytic sites of the enzyme. The allosteric activator elutes the enzyme from a matrix coated with hydrazide-UTP which behaves as an allosteric inhibitor exploiting the allosteric conformational changes of dCMP-aminohydrolase.

The relation between sulphydryl groups and enzyme activity, and between sulphydryl groups and changes of conformation of the enzyme molecule caused by allosteric effectors has been studied in deoxycytidylate aminohydrolase with a homogeneous preparation of the spleen enzyme. Binding of 4 moles of p-chloromercuribenzoate per mole is sufficient to inactivate the enzyme. Three different time rates of inactivation by p-chloromercuribenzoate are obtained for the enzyme, for the enzyme · dTTP-Mg complex and for the enzyme · dCTP-Mg complex. The 4 thiol groups on the enzyme responsible for the inactivation by p-chloromercuribenzoate are not available to the action of 5,5′-dithio-bis(2-nitrobenzoic acid). Experiments of titration of the enzyme –SH groups by 5,5′-dithio-bis(2-nitrobenzoic acid) give additional support to the contention that at least three conformers are involved in the regulation of the activity of deoxycytidylate aminohydrolase. The data suggest that –SH groups have a very important role in catalysis and regulation of the enzyme.