The existence of apotyrosinase in the cytosol of Harding-Passey mouse melanoma melanocytes and characteristics of enzyme reconstitution by Cu(II).

This paper reports the effect of Cu(II) supplementation on the tyrosinase isozymes from Harding-Passey mouse melanoma. The dopa-oxidase activity of the microsomal and soluble isozymes is increased by incubation with Cu(II), whereas the activity of the unique 'in vivo' melanin-forming isozyme, bound to melanosomes, is not. Other divalent cations are ineffective in increasing the dopa-oxidase activity of tyrosinases. These results indicate the existence of a mixture of tyrosinase and apotyrosinase in the cytosol of melanocytes before reaching the melanosome. The paucity of Cu(II) in the cytosol could be one of the mechanisms of regulation contributing to avoid the formation of melanin outside the melanosome. Some kinetic characteristics of the enzymatic reconstitution of soluble and microsomal isozymes by Cu(II) are also studied, and the results suggest that the glycosylation of apotyrosinase during its maturation yields a conformational change favouring the binding of Cu(II) at the enzyme active site, by lowering the activation energy of the reconstitution reaction.