Biochemical properties of human urinary megakaryocyte colony-stimulating factor and erythropoietin: the role of sulfhydryl groups and disulfide bonds.

The labeling of cystine residues with [1-14C]iodoacetic acid showed that urinary preparations from patients with aplastic anemia contained 3.06 X 10(-9) mol of sulfhydryl groups and 2.90 X 10(-7) mol of half-cystine as disulfide bonds in the native state, and 6.36 X 10(-7) mol in the denatured state per absorbance unit of protein, respectively. Sulfhydryl reagent-treated proteins retained full activity of megakaryocyte colony-stimulating factor (Meg-CSF) and erythropoietin (Epo), except with DTNB-treated protein. Reduction-carboxymethylation and reduction-mercuration resulted in complete loss of Meg-CSF and Epo activities, suggesting that one of the essential chemical groups of Meg-CSF and Epo is a disulfide bond. Reduction of disulfide bonds at neutral pH revealed that Meg-CSF is less susceptible to reduction than Epo. Reactivation occurred by spontaneous reoxidation in most of the reduced Meg-CSF (92.6%) and part of the reduced Epo (22.1%). These molecular behaviors may reflect differences in the spatial configurations of Meg-CSF and Epo.