Comparison of human and porcine thyroid membranes for radioreceptor assay of bovine thyrotrophin and thyrotrophin-binding inhibiting immunoglobulins

Abstract

A detailed comparison between the use of human and porcine thyroid membranes for the radioreceptor assay (RRA) of bovine TSH (bTSH) and thyrotrophin-binding inhibiting immunoglobulins (TBIIg) is reported. Bovine thyroid membranes were also investigated but were found to be far less satisfactory than either human or porcine thyroid membranes. The affinity constant (Ka) of the interaction of bTSH with porcine thyroid membranes (Ka = 3·3 × 109l/mol) measured by Scatchard analysis was higher than with human thyroid membranes (Ka = 2·1 × 108l/mol). Porcine thyroid membranes were more sensitive for the assay of bTSH (detection limit 30 μu., half-maximal inhibition 0·3 mu.) than human thyroid membranes (detection limit 200 μu., half-maximal inhibition 7·4 mu.). Preincubation of membranes from either species with immunoglobulin rich in long-acting thyroid stimulator (LATS) inhibited the saturable binding of 125I-labelled TSH to a greater extent than did normal immunoglobulin. The binding of 125I-labelled TSH to porcine membranes was more sensitive to the inhibitory effect of LATS-immunoglobulin and was also less affected by normal immunoglobulin than was binding to human thyroid membranes. When assayed with each type of membrane preparation there was good correlation between the RRA of immunoglobulins prepared from patients with Graves's disease and from normal subjects (n = 18) (r = 0·85, P <0·001, n = 73). The incidence of positive TBIIg in untreated Graves's disease was greater for porcine than for human thyroid membranes.