SUMMARY: Tyrosinase in crude cell-free preparations from two strains of is markedly activated by gentle heating or by exposure to sodium ion, urea, or certain organic nitrogenous bases. The tyrosinase activity of non-activated extracts increases during assay, whereas activated enzyme displays the ‘reaction inactivation’ kinetics characteristic of many copper oxidases. Activation by Na is antagonized by K and is readily reversible. Urea activation is also reversible, but the activation due to heating appears to be irreversible. The thermostability of tyrosinase derived from strain T is increased greatly by Na (and less effectively by K) and the half-life of thermal inactivation at 59° varies between wide limits, depending on the ionic environment. The relatively heat-labile tyrosinase of strain T shows qualitatively similar behaviour but is quantitatively different with regard to stabilization by Na and K.


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