Tyrosinases of murine melanocytes with mutations at the albino locus.

Abstract

Tyrosinase (monophenol monooxygenase; monophenol, L-dopa:oxygen oxidoreductase, EC 1.14.18.1) is a key enzyme in the synthesis of melanin. Reduced levels of tyrosinase play an important role in albinism. The data described here show differences in the expression and characteristics of tyrosinase in cutaneous murine melanocytes grown in culture from normal wild-type strains (C/C); from three albino locus mutants: himalayan (ch/ch), chinchilla (cch/cch), and albino (c/c); and from the double-mutant heterozygous pink-eyed chinchilla (cchp/cp). Our results suggest that the diminished pigmentation in all mutants is due to abnormal posttranslational modification of the enzyme: the levels of mRNA for tyrosinase in wild-type, himalayan, and pink-eyed chinchilla melanocytes are similar; the himalayan mutation confers a deficiency in N-linked glycosylation, which results in an extremely unstable enzyme that is also temperature sensitive; the chinchilla and albino mutations confer susceptibility to proteolytic cleavage; the pink-eye dilution confers a reduction in the levels of immunoprecipitable tyrosinase, and what little enzyme there is fails to be translocated from the trans-Golgi network to melanosomes. The kinetics of activation and inhibition of the enzyme by the cofactor dopa are unique for the mutants tested and differ from those of tyrosinase from wild-type melanocytes. The findings support the conclusion that the albino locus in mice encodes the structural gene of tyrosinase.