Turnover in humans of iodine- and cobalamin-labeled transcobalamin I and of iodine-labeled albumin.

Purified transcobalamin I labeled with radioactive iodine and cobalamin was used for turnover studies in 17 humans. Mean fractional catabolic rate was 0.15 d-1 and mean distribution ratio was 0.57. No difference was observed between the turnover in a control group and in patients with vitiligo or pernicious anemia. The plasma curves of the iodine and the cobalamin label were identical, indicating that cobalamin was liberated from transcobalamin I only when this protein was degraded. It was further concluded that transcobalamin I was of minor importance for the transport of cobalamin from the gut to the tissues. The turnover of transcobalamin I was correlated to the turnover of albumin in 9 patients studied with both proteins.