Studies on a radioimmunoassay for ovine follicle stimulating hormone: interference by ovine albumin

Summary A previously documented radioimmunoassay for ovine follicle stimulating hormone has been examined as to its suitability for measurement of the hormone in sheep plasma. In addition, the behaviour of two other antisera to ovine follicle stimulating hormone, and various standards, has been investigated. Little cross‐reaction with other protein hormones was seen but when sheep plasma, at dilutions 1/25‐1/100 was included in the radioimmunoassay, it caused complete displacement of ovine follicle stimulating hormone‐ 125 I from all three antibodies; even at dilutions In the range 1/250‐1/5,000 incomplete displacement was observed. Addition of ovine albumin to the assay in amounts equivalent to plasma levels produced similar degrees of displacement of the tracer antigen. Plasma from other species did not show this effect. When ovine albumin‐ 125 I and ovine follicle stimulating hormone‐ 125 I were reacted separately with antibodies to each, in the presence and absence of excess unlabelled follicle stimulating hormone and albumin, each labelled protein combined to some extent with antibodies to the other. Excess of either of the unlabelled antigens caused displacement of both follicle stimulating hormone‐ 125 I and albumin‐ 125 I from their homologous antisera. Gel filtration studies suggested that no complex was formed between ovine follicle stimulating hormone and ovine albumin. The must likely explanation of this phenomenon is that the highly purified labelled follicle stimulating hormone contains some labelled albumin and that the antisera contain anti‐albumin. Alternatively, the albumin and ovine follicle stimulating hormone may show some intrinsic immunological cross‐reactivity. Interference by albumin renders the radioimmunoassay of ovine follicle stimulating hormone, using several different antisera inapplicable to the measurement of follicle stimulating hormone in ovine plasma.