The turnover of 57Co-labeled cyanocobalamin bound to cobalamin binding proteins in patients with chronic myelogenous leukemia.

Individual plasma specimens from six patients with chronic myelogenous leukemia (CML) in chronic phase were incubated with [57Co]cyanocobalamin and injected into patients to study the turnover of cobalamin bound to haptocorrin and transcobalamin. The [57Co] radioactivity bound to haptocorrin and transcobalamin was determined after separating the proteins by adsorption to insolubilized antibodies to haptocorrin. The distribution of the radioactivity on the different forms of cobalamin and on haptocorrin isoproteins were determined. The fractional catabolic rate of hapatocorrin (0.102 to 0.158 d-1) and transcobalamin (3 to 29 d-1) was of the same magnitude as previously reported for a reference population. During the first 24 hours of the turnover, no change of the cyanocobalamin form was registered, indicating that conversion of cyanocobalamin to the coenzyme forms of cobalamin does not take place in the circulation. The haptocorrin isoprotein pattern changes in alkaline direction during the first 5 hours indicated that the glycoprotein was desialinated during circulation. From the calculated turnover parameter, the possible competition between transcobalamin and haptocorrin for the transport of cobalamins in CML is estimated to be of minor clinical significance. In conclusion, the increased plasma concentration of haptocorrin in CML is due to an increased liberation of haptocorrin and is not due to a decreased turnover of the protein.