Abstract
Proteins of approximately 10,000 daltons (presumably metallothionein) and greater than 75,000 daltons bound ^64^Cu when this metal was added to fibroblast lysates. Treatment with either 2‐mercaptoethanol or the disodium salt of ethylenediamine tetraacetic acid demonstrated that the high molecular weight copper‐binding proteins in lysates prepared from both normal and Menkes fibroblasts exhibited a relatively low affinity for copper compared to the 10,000 dalton protein(s). No difference was detected in the affinity of the low molecular weight protein(s) of normal and Menkes fibroblast lysates for copper. The amount of ^64^Cu bound to the 10,000 dalton protein(s), however, was approximately two to three times greater in lysates prepared from Menkes fibroblasts than from normal fibroblasts. Mixing experiments indicated that the increased binding of ^64^Cu to the 10,000 dalton protein(s) in lysates of Menkes fibroblasts did not result from the deficiency of a factor that effects the cleavage of copper from this protein(s), from the presence of a soluble inhibitor, or from the lack of an activator. In addition, the use of lysates, rather than whole cells, demonstrated that the observed differences in copper binding between the normal and the Menkes fibroblasts were not caused by an abnormality in the membrane transport of copper in the mutant cells. Thus the findings suggest that the increased accumulation and the reduced efflux of copper previously observed in cultured Menkes fibroblasts result either from an increased amount of the 10,000 dalton copper‐binding protein(s) or from an increased capacity of this molecule(s) for copper.