The metabolism of IgE. Studies in normal individuals and in a patient with IgE myeloma.

IgE metabolic turnover studies with purified radioiodinated IgE were performed in normal individuals and in a patient with IgE myeloma. The validity of the turnover studies was established in several ways, including comparisons of radioiodinated IgE turnover with the turnover of endogenously labeled 14C-IgE and with the turnover of infused unlabeled IgE in a patient with hypogammaglobulinemia. The geometric mean serum IgE concentration in 73 normal adults was 96 ng/ml with a 68% confidence interval of 24 to 386 ng/ml. Metabolic turnover studies in 10 control individuals disclosed a geometric mean total circulating IgE of 4.1 mug/kg, a mean percentage of the total exchangeable IgE in the intravascular space of 41%, a mean half-time of survival of IgE of 2.7 days, a mean fractional catabolic rate of 94% of intravascular pool per day, and a geometric mean synthetic rate of 3.8 mug/kg/day. IgE has the lowest synthetic rate and highest fractional catabolic rate of the five major classes of immunoglobulin molecules. In contrast to these normal values, a patient with IgE myeloma had a serum IgE comcentration of 42 mg/ml, a total circulating IgE of 1.7 g/kg, and a synthetic rate of 270 mg/kg/day. Furthermore, although the synthetic rate was vastly increased, the survival time was prolonged and the fractional catabolic rate was decreased to 5.1 days and 16 to 22% of the intravascular pool per day, respectively. These data are compatible with the concept that IgE is catabolized in part by a mechanism common to all immunoglobulin classes and in part by a unique mechanism not available to other immunoglobulins. At very high IgE serum concentrations, such as those encountered in patients with IgE myeloma, the unique mechanism would be saturated and only the catabolic pathways available to all immunoglobulins would be available to IgE.