Instability of succinyl ester linkages in O2'-monosuccinyl cyclic AMP-protein conjugates at neutral pH

Chromatographic and immunological evidence is presented regarding the hydrolysis of the ester linkage of O2'-monosuccinyl cyclic AMP in neutral solutions. Such hydrolysis occurs whether the nucleotide derivative is present in free form in solution or conjugated through its succinyl carboxyl group via an amide bond to proteins. The latter process apparently occurs when succinyl cyclic AMP is conjugated to human serum albumin for use as an immunogen in the production of anti-cyclic AMP antibodies and when the derivative is coupled to the enzyme glucose-6-phosphate dehydrogenase (E.C. 1.1.1.49). The enzyme conjugate has been used in developing a homogeneous enzyme immunoassay for cyclic AMP. Inhibition of the catalytic activity of enzyme-cyclic AMP conjugates by anti-cyclic AMP antibody decreases with time, apparently due to the loss of cyclic AMP from enzyme-cyclic AMP conjugates stored in neutral solutions. In addition, the ability of free cyclic AMP to completely reverse the inhibition process decreases with time because of the presence of antibodies in the anti-cyclic AMP sera that apparently inhibit enzyme activity because of their binding specificity for the residual succinate-protein determinant sites of the enzyme conjugates. Lyophilization of the conjugates immediately after preparation helps to overcome the problem; however, in vivo hydrolysis of immunogens prepared with the succinyl cyclic AMP derivative may always occur. The consequence of this hydrolysis reaction and the subsequent formation of anti-succinyl-protein antibodies will be discussed with regard to existing RIAs for cyclic AMP and a new homogeneous enzyme immunoassay for the nucleotide.