Km OF TYROSINASE IN MICROSOMES IS HIGHER THAN THAT IN MELANOSOMES

Abstract

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 O.6mM 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 O.06mM, 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.

Effects of 4‐Tertiary Butylphenol on the Tyrosinase Activity in Human Melanocytes

“…4-TBP showed an inhibition of tyrosine hydroxylase in a dose-dependent manner. This K, calculated for cultured human melanocytes is relatively lower than that reported using human tissue (i.e., 150.0 x M) (Pomerantz and Ances, 1975) or mouse melanoma (i.e., -30.0 x lo-" M) (Tomita et al, 1982;Jara et al, 1988). The variation in reported values for the K, of mammalian tyrosinase for tyrosine may be related to either the availability of the co-factor DOPA or inhibitors that may promote or retard, respectively, the tyrosine hydroxylase activity in the various biological systems being evaluated (Hearing and Jimenez, 1987).…”