A comparison of the properties of two forms of tyrosinase from Neurospora crassa

Abstract

The differences between a thermostable (T s ) and thermolabile (T l ) form of tyrosmase from Neurospora crassa have been studied by means of a spectrophotometric technique using ascorbate or ferrocyanide. No significant differences in substrate specificity have been found in a survey of over 30 substances which included mono-, di-, tri-, and conjugated phenols. Catechol was found to be a substrate, in contrast to previous findings. Michaelis-Menten constants for the two forms of enzyme acting on l-tyrosine and l- and d-hydroxyphenylalanine (Dopa) also were found to be similar, as were the pH optima on l-Dopa and phenyl-4-catechol, and the response to several inhibitors. On the other hand, the activity of the thermolabile enzyme was lost more rapidly than that of the thermostable one after incubation for 5–15 min. in high concentrations of urea and formamide. These data are interpreted as support for the conclusion that the two forms of the enzyme differ mainly in their secondary and tertiary structure and not in their active centers.