The use of homologous and hetebologous 125 I-radioligands in the radioimmunoassay of progesterone

Eight homologous and heterologous¹²⁵ I-radioligand systems for the radioimmunoassay of progesterone were examined. Using an antiserum raised to 11α-hydroxyprogesterone 11-succinyl-bovine serum albumin, standard curves were set up with the homologous radioligands, 11α-hydroxyprogesterone 11-succinyl-¹²⁵I]-iodotyramine, -¹²⁵I]-iodohistamine and -¹²⁵I]-iodotyrosine methyl ester. Heterologous bridge systems were represented by progesterone-11α-oxycarbonyl-¹²⁵I]-iodotyrosine methyl ester and 11α-hydroxyprogesterone 11-phthalyl-¹²⁵I]-iodotyrosine methyl ester, and heterologous site systems by progesterone-3-(O-carboxymethyl)oxime-¹²⁵I]-iodotyramine, progesterone-12-(O-carboxymethyl) oxime-¹²⁵ I]-iodotyramine, and progesterone-20-(O-carboxymethyl) oxime-¹²⁵I]-iodohistamine. The preparation of the steroid derivatives and iodination by a two-phase method are described. The curves obtained from the homologous radioligands were relatively insensitive compared with a tritiated system, with the tyrosine methyl ester derivative providing a more sensitive assay than the corresponding tyramine or histamine analogues. The heterologous bridge systems gave more sensitive curves than the homologous tracers whilst the 3- and 12-(O-carboxymethyl) oxime derivatives of progesterone furnished curves as sensitive as the tritiated reference. Progesterone-20-(O-carboxymethyl)oxime-¹²⁵I]-iodohistamine was not bound by the antibody.