Km OF TYROSINASE IN MICROSOMES IS HIGHER THAN THAT IN MELANOSOMES

Km values of tyrosinase solubilized from melanosomes and microsomes from Harding-Passey mouse melanomas, consisting of rough and smooth surface membranes were found to be 0.6 mM and 1.3 mM, respectively. The maximum reaction velocities of the two enzymes from the melanosomes and microsomes were similar. In comparison with the tyrosine level in human plasma, reported to be approximately 0.06 mM, the Km value for microsomal tyrosinase is very high. Therefore it is likely that the tyrosinase reaction occurs at a extremely low level in the smooth and rough membranes of the melanocyte. This may be one possible reason why melanin formation does not take place in vivo in rough and smooth surface membranes in the melanocytes, despite the fact that tyrosinase activity has been detected in microsomes in vitro after isolation from melanoma tissue.