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Abstract
SUMMARY: Tyrosinase in crude cell-free preparations from two strains of Neurospora crassa 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 TS 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 TL shows qualitatively similar behaviour but is quantitatively different with regard to stabilization by Na+ and K+.
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