Most of our knowledge of the mammalian
tyrosinase related
protein (TRP) activities is derived from studies using murine
melanoma models, such as B16 or Cloudman S-91 melanocytes. Owing to the high degree of homology between the murine and human
enzymes, it has been assumed that their kinetic behaviour could be similar. However, the
protein sequences at the
metal binding sites of the murine and human
enzymes show some differences of possible functional relevance. These differences are more significant in the
metal-A site than in the
metal-B site. By using three human
melanoma cell lines (HBL, SCL, and BEU), we have studied the catalytic abilities of the human melanogenic
enzymes in comparison to those obtained for the counterpart murine
enzymes isolated from
B16 melanoma. We have found that TRP2 extracted from all cell lines show
dopachrome tautomerase activity, although the activity levels in human malignant melanocytes are much lower than in mouse cells. Reconstitution experiments of the human
enzyme indicate that TRP2 has Zn at its
metal binding-sites. Although mouse
tyrosinase does not show DHICA
oxidase activity, and this step of the melanogenesis pathway is specifically catalyzed by mouse TRP1, the human
enzyme seems to recognize carboxylated
indoles. Thus, human
tyrosinase could display some residual DHICA
oxidase activity, and the function of human TRP1 could differ from that of the murine
protein. Attempts to clarify the nature of the
metal cofactor in TRP1 were unsuccessful. The
enzyme contains mostly Fe and Cu, but the reconstitution of the enzymatic activity from the
apoprotein with these
ions was not possible.