Regulatory properties and subunit structure of chick embryo deoxycytidylate deaminase.

Evidence has been presented that chick embryo deoxycytidylate deaminase (EC 3.5.4.12) is capable of undergoing structural changes under a variety of conditions. Rate studies revealed that inhibition of the enzyme by ethylenediaminetetraacetate markedly diminishes at 10” and below. Although EDTA does not inhibit the deaminase at lo”, the inhibition by urea, N-ethyhnaleimide, and 5,5’dithiobis(Z-nitrobenzoic acid) is dramatically enhanced in its presence. End product inhibition by deoxythymidine 5’-triphosphate, shown to be both pH and concentration dependent, is completely eliminated at 10” by EDTA. The reversal of deoxythymidine 5’-triphosphate inhibition by deoxycytidine 5’-triphosphate is markedly sigmoidal, emphasizing the regulatory interaction of these ligands. Hill constants for the reaction of the enzyme with substrate in the presence of deoxycytidine 5’-triphosphate and deoxythymidine 5’-triphosphate are 1.0 and 4.0, respectively. The Hill constant for the homotropic reaction of deoxycytidine 5’-monophosphate with the deaminase at 30” is 4.0, whereas that at 10” is 2.8. The apparent Km value for the activation of the deaminase by deoxycytidine 5’-triphosphate is 2.12 X lop4 ITIM. The subunit structure of the deaminase is emphasized by its conversion from an &‘,z,” value of 6.78 in the presence of deoxycytidine 5’-triphosphate to a value of 3.63 S in the presence of deoxythymidine 5’-triphosphate. The latter form of the enzyme is inactive when assayed in the absence of mercaptoethanol. Deoxycytidine 5’-triphosphate and, to a lesser extent, deoxythymidine 5’-triphosphate and deoxyguanosine 5’monophosphate protect the deaminase rather strikingly from inactivation by trypsin. The low molecular weight, inactive form, of the enzyme is not protected under similar circumstances.