Metabolism of human immunoglobulin D (IgD).

Abstract

The turnover of radioiodine-labeled IgD was studied in 28 subjects with a variety of diseases. IgD was isolated from 2 sources, a patient with abundant IgD and a selective deficiency of other immunoglobulins and a patient with D myeloma. These preparations were labeled with I125 or I131 and useds in 34 studies. Six patients received both preparations. The serum IgD level was found to be directly related to the rate of synthesis. Individuals differ markedly in the rate of IgD synthesis. Differences as great as 60-fold were found in a group of 27 subjects without neoplasms of IgD-synthesizing cells. A patient with D myeloma synthesized the protein at a rate 160 times greater than the median rate for 27 other subjects. The median rate of IgD synthesis was 0.4 mg/kg per day. This is about 1 to 2% of the synthetic rates for IgG and IgA (and about 10% of the IgM synthetic rate). The median serum level of IgD (0.023 mg/ml in 27 subjects studied) was only about 0.2% of the median IgD level. The difference between relative serum levels (1: 465) and synthetic rates (1:1.00) for IgD: IgG refects the more rapid rate of IgD catabolism. The biologic half-life of IgD is shorter than that of other immuno-globulins. The median half-life for IgD was 2.8 days. This contrasts with the tl/2 of IgM (5 days), IgA (6 days), and IgG (23 days). The fractional rates of catabolism of intravascular IgD (0.37) and IgA, however, are similar. The other immunoglobulins are catabolized at slower rates. Studies in 3 patients with disordered metabolism of IgA or IgG revealed normal IgD metabolism, indicating that IgD is metabolized independently from IgA or IgG. The fractional catabolic rate of IgD appears to be altered by the serum IgD concentration. Those subjects with high serum IgD levels tend to have relatively low fractional catabolic rates. This concentration-catabolism relationship for IgD is the opposite of that noted for IgG, where high serum levels are associated with relatively rapid IgG catabolism. IgD is largely (73%) in the intravascular compartment. This contrasts with other 7 S immunoglobulins, for only 40 to 50% of the total body IgG and IgA is in the intravascular compartment.