ABSTRACT
The size of molecules able to be measured in immunoassays where antibody is encapsulated within semipermeable microcapsules is restricted by the pore size of the membrane. This study was performed to determine the approximate molecular weight cut-off of this membrane. Permeability was assessed by measuring which labelled hormones were able to enter and bind their respective microencapsulated antibody. Hormones with molecular weights of less than 4000 (angiotensin II, thyroxine, 17-hydroxyprogesterone, progesterone, testosterone and androstenedione) passed freely through the pores but larger molecules, with molecular weights in excess of 10 000 (parathyroid hormone, human GH, TSH) could not. Insulin, with a molecular weight of 6000 (approximate minimum diameter 3·5 nm), had restricted entry while the next smallest hormone tested, the 1–34 amino acid portion of parathyroid hormone (molecular weight 4000; diameter 1·8 nm), was able to bind encapsulated antibody, suggesting that the pore diameter is between 1·8 and 3·5 nm. It can now be predicted that the method is able to measure compounds with a diameter within this range and with a molecular weight below 6000. Microcapsules may be useful for improving specificity of assays where a cross-reactant is too large to penetrate the membrane.
J. Endocr. (1987) 115, 47–51
