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

Eight homologous and heterologous 125I-radioligand systems for the radioimmunoassay of progesterone were examined. Using an antiserum raised to 11alpha-hydroxyprogesterone 11-succinyl-bovine serum albumin, standard curves were set up with the homologous radioligands, 11alpha-hydroxyprogesterone 11-succinyl-[125I]-iodotyramine, -[125I]-iodohistamine and -[125I]-iodotyrosine methyl ester. Heterologous bridge systems were represented by progesterone-11alpha-oxycarbonyl-[125I]-iodotyrosine methyl ester and 11alpha-hydroxyprogesterone 11-phthalyl-[125I]-iodotyrosine methyl ester, and heterologous site systems by progesterone-3-(O-carboxymethyl)oxime-[125I]-iodotyramine, progesterone-12-(O-carboxymethyl)oxime-[125I]-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.