Metabolism of IgG in type II mixed essential cryoglobulinaemia--autologous cryoprecipitated and normal homologous IgG are incorporated into complexes and metabolized in vivo at similar rates.

The metabolism of autologous cryoprecipitated and normal homologous IgG was studied in four patients with type II mixed essential cryoglobulinaemia. 131I-autologous IgG purified from each patient's cryoglobulin (Cryo-IgG), and 125I-pooled normal homologous IgG (N-IgG) were studied simultaneously to compare the extent of their incorporation into complexes with IgM in vitro and in vivo, and their turnover in vivo. A proportion of each preparation of IgG was incorporated into macromolecular complexes in vitro and in vivo in all patients, the Cryo-IgG only slightly more so than N-IgG. Results of the turnover studies were heterogeneous, but the common finding was the absence of any significant difference in the metabolism of Cryo-IgG and N-IgG. In two patients the fractional catabolic rates (FCR) of Cryo-IgG and N-IgG were increased and in one they were normal. The fourth patient was also hypogammaglobulinaemic (IgG 0.52 mg/ml) and it was shown that in vivo virtually all his IgG was combined with IgM; despite this the FCR of both types of IgG was reduced. These results suggest (1) that the IgG component of the cryoglobulins in these patients is unlikely to differ significantly from normal IgG and (2) that, contrary to expectation, complexed IgG is not necessarily rapidly eliminated from the circulation.