125I-labelled hormone?

Wolfgang Schechinger novalidaddress at nurfuerspam.de
Tue May 16 14:35:33 EST 2006

[reply moved to a thread of its own]

Dear Ngo Thi,

125-Iodine is very convenient to use because it easily may be
introduced into proteins (it goes to aromatice side chains, mainly the
tyrosines) by a chemical reaction (eg Na125I / Chloramine-T, you might
search the web for radioiodination) without disturbing the function of
the molecule too much. Then 125I easily may be detected with a gamma
counter, the radiation may easily be shielded with lead foil and 125I
decays pretty fast, so it's no long-term radioactive waste.

Example 1:

In a radioimmunoassay, antibody against a hormone (eg. insulin) is
immobilized. Then different dilutions of the sample, mixed with 125I
labelled insulin are placed in the wells. After extensive washing, the
bound radioactivity is determined. In this assay, labeled and
unlabelled insulin complete for the binding sites. After some
calculations and regression analysis, the actual insulin concentration
whis is present in is obtained.

Example 2:

One might set up an immunoassay for determining the amount of a hormone
receptor in a lysate by coating a surface with an antibody against the
receptor. Then, the lysate is added. After a wash, radiolabelled ligand
(i.e. hormone) is added and after washing, the bound radioactivity is
counted and gives you an idea how many receptors are present in your

HTH Wolfgang

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