Validation of radioimmunoassay for human lactalbumin in the serum by testing the endogenous antibodies interference.

For re-establishing the value of human lactalbumin as a functional marker of normal and pathological activity of the breast a sensitive and specific radioimmunoassay was established with the prior important control of the interference of endogenous antibodies. The specificity of the assay was assessed by the absence of interference from other proteins in milk or in breast cyst fluid, various hormones and tumor markers. Bovine lactalbumin showed incomplete and weak cross-reactivity. By an enzymoimmunoassay method it was shown that all the 222 human sera studied contain IgG immunoglobulins which bind bovine and human lactalbumin with greater reactivity of children's serum and without relationship to the blood groups. The maximum affinity constant of these endogenous immunoglobulins determined by the radioimmunoassay method is 4.5 times greater for bovine (Kd = 18 X 4(-11) M) than for human (Kd = 4 X 10(-11) M) lactalbumin. These endogenous anti-lactalbumin immunoglobulins caused no interference in the radioimmunoassay as shown by the complete correlation between the concentrations of human lactalbumin previously incubated and added to sera containing high-affinity antibodies and those measured directly in the radioimmunoassay. This lack of interference was explained by the higher (22-fold) affinity constant of the rabbit antiserum against human lactalbumin (Kd = 9 X 10(-12) M). The study of endogenous antibodies by the two enzymes and radioimmunoassay methods is needed before assessing and using a radioimmunoassay of human lactalbumin in serum.