Regulation of tue activity of adenos1ne deaminase bound to normal and transformed chick embryo fibroblasts

Binding of adenosine deaminase (SS-ADA) to adenosine deaminase complexing protein (ADCP) in solution and to normal and RSV-transformed chick embryo fibroblasts (CEF), as well as ils effect on SS-ADA activity, were studied SS-ADA was labeled with either pyrene sulfonyl chloride, fluorescein isothiocyanate or 125-iodine These modifications resulted in minor changes in catalytic activity and binding capacity In solution, Py-SS-ADA bound to ADCP with a Kd value of 14.1 +/- 21 nM ADCP inhibited ADA catalytic activity by circa 40% when present in a 10 I molar ratio of ADCP:SS-ADA. Binding of F1TC-SS-ADA to CEF was specific, with a Kd value of 64 +/- 6 nM Binding of [ 125I]SS-ADA to CEF showed that RSV-transformation resulted in increased number of ADA specific binding sites (P<0.02) but did not affect Kd values (5.93 +/- 1.84 x 1000000 sites/cell, Kd = 317 +/103 nM, in normal CEF, and 14.39 +/- 2 3 I x 1000000 sites/cell, Kd = 238 +/- 92 nM, in RSV-transformed CEF; n=5). ADA activity was inhibited upon binding to CEF, with a larger inhibitory effect in transformed cells. A distinctive regulatory role of ADCP on ADA activity in solution and when bound to CEF is thus suggested.