What protein transports the majority of copper in the bloodstream?

Ceruloplasmin is the protein responsible for transporting the majority of copper in the bloodstream. It is a copper-carrying glycoprotein synthesized in the liver and plays a significant role in copper metabolism and homeostasis. In addition to transporting copper, ceruloplasmin also exhibits enzymatic activity as a ferroxidase, which aids in iron metabolism by oxidizing ferrous iron (Fe2+) to ferric iron (Fe3+), facilitating its binding to transferrin for transport.

Although other proteins such as transferrin and albumin can bind copper to some extent, they do not play a primary role in copper transport in the same way that ceruloplasmin does. Transcuprein, on the other hand, does transport copper but is present in much lower concentrations compared to ceruloplasmin. Therefore, ceruloplasmin is recognized as the major copper transport protein in the circulation, making it the correct answer to the question.