The properties of a mouse liver copper binding protein (CuBP) and human placental S-adenosylhomocysteine hydrolase (SAHH) were compared to test the hypothesis that CuBP is SAHH. CuBP and SAHH migrated identically on SDS-polyacrylamide gel electrophoresis gels, and their 48-kDa monomers both self-associate to tetramers. Human placental SAHH cross-reacted with polyclonal antibodies to mouse liver CuBP, and CuBP from mouse liver cross-reacted with two monoclonal antibodies to human placental SAHH. A third monoclonal antibody to human placenta SAHH reacted weakly with the mouse liver protein but well with CuBP from human lymphoblasts. NAD(+)-activated CuBP has high SAHH enzymatic activity. Moreover, human placental SAHH, like mouse liver CuBP, has a single high affinity copper binding site per 48-kDa subunit. Thus, the data confirm that CuBP is SAHH, and SAHH is proposed to be a bifunctional protein with roles in sulfur-amino acid metabolism and copper metabolism. The copper binding activity of SAHH is proposed to play a significant role in the intracellular distribution of copper, and SAHH enzymatic activity may influence copper metabolism through its role in cysteine biosynthesis from methionine.