THE RADIOIMMUNOASSAY OF OXYTOCIN AND VASOPRESSIN

An antibody might recognise elements unrelated to the antigen against which it was raised, and unrelated molecules might cross-react.

“In some cases, an antibody might recognise elements unrelated to the antigen against which it was raised. Antibodies are sensitive to differences in amino acid sequence, although they recognise the structure of a molecule, not its amino acid sequence per se, and so unrelated molecules might cross‐react 76, 79.”

Bioassays are less specific than immunoassays because of the cross-reactivity of vasopressin and oxytocin with their respective receptors.

“However, bioassays are less specific because of the cross‐reactivity of vasopressin and oxytocin with their respective receptors 76.”

In 1973, Chard noted that the best immunoassays for vasopressin were no more sensitive than the best bioassays; the same is still true today for most radioimmunoassays and most enzyme-linked immunoassays (ELISAs).

“In 1973, Chard 76 noted that best immunoassays for vasopressin were no more sensitive than the best bioassays; the same is still true today for most radioimmunoassays and most enzyme‐linked immunoassays (ELISAs).”

Problems in immunoassays can come from factors such as heparin or low pH or high osmolality that interfere with the antigen-antibody reaction.

“Problems can come from factors such as heparin or low pH or high osmolality that interfere with the antigen–antibody reaction.”

There were considerable difficulties in developing immunoassays for vasopressin and oxytocin because they have a low molecular weight, are poor immunogens, making high-affinity antisera difficult, and circulating levels were clearly very low.

“There were considerable difficulties in developing immunoassays for vasopressin and oxytocin. Because these have a low molecular weight, they are poor immunogens, making the preparation of high‐affinity antisera difficult, and circulating levels were clearly very low.”

Antibodies do not recognise the whole molecule, so an immunoassay may overestimate hormone concentrations by measuring fragments of no physiological significance.

“Antibodies do not recognise the whole molecule, and so an immunoassay may overestimate hormone concentrations by measuring fragments of no physiological significance 76.”

Close attention was given to extraction and concentration procedures in developing immunoassays for vasopressin and oxytocin.

“Thus, close attention was given to extraction and concentration procedures 76.”

When oxytocin is incubated in late-pregnancy plasma, there is a rapid loss of biological activity but after 2 h oxytocin immunoreactivity exceeds bioactivity by three-fold; the explanation is that oxytocinase degrades labelled oxytocin, reducing label binding to antibody and leading to overestimation.

“When oxytocin is incubated in this plasma, there is a rapid loss of biological activity but, after 2 h, oxytocin ‘immunoreactivity’ exceeds bioactivity by three‐fold. The obvious explanation is that oxytocinase in the sample also degrades the labelled oxytocin. The consequence will be that less label will bind to antibody, leading to overestimation of the amount of oxytocin 76.”