Functional activity of intrinsic factor measured by using solubilized receptor protein

The traditional radioimmunoassay for gastric intrinsic factor, in which this protein is measured on the basis of immunoreactivity rather than function, is of no value for identifying intrinsic factor that binds cobalamin but does not bind to the ileal receptor site, or for detecting animal intrinsic factor, which does not cross react with human intrinsic factor. Accordingly, we have applied a radioassay for the intrinsic factor receptor protein to measure the functional activity of intrinsic factor in gastric juice. The receptor protein reagent was partly purified from guniea pig ilea and its interaction with intrinsic factor--CN[57Co]-cobalamin was determined by precipitation with sodium sulfate at a final concentration of 150 g/L. Results of this assay were comparable with results obtained for intrinsic factor by radioimmunoassay. The receptor protein did not bind immunoreactive intrinsic factor that was functionally abnormal. This functional radioassay for intrinsic factor is not species specific and will be of value when specific antiserum to intrinsic factor is not available and when cobalamin malabsorption is to be evaluated in patients who are secreting normal amounts of immunoreactive intrinsic factor.