Effect of tetrahydropteridines on the monophenolase and diphenolase activities of tyrosinase

Abstract

This study explains the action of compounds such as 6-tetrahydrobiopterin, (6BH₄) and 6,7-dimethyltetrahydrobiopterin (6,7-di-CH₃BH₄) on the monophenolase and diphenolase activities of tyrosinase. These reductants basically act by reducing the o-quinones, the reaction products, to o-diphenol. In the case of the diphenolase activity a lag period is observed until the reductant is depleted; then the system reaches the steady-state. In the action of the enzyme on monophenol substrates, when the reductant concentration is less than that of the o-diphenol necessary for the steady-state to be reached, the system undergoes an apparent activation since, in this way, the necessary concentration of o-diphenol will be reached more rapidly. However, when the reductant concentration is greater than that of the o-diphenol necessary for the steady-state to be reached, the lag period lengthens and is followed by a burst, by means of which the excess o-diphenol is consumed, the steady-state thus taking longer to be reached. Moreover, in the present kinetic study, we show that tyrosinase is not inhibited by an excess of monophenol, although, to confirm this, the system must be allowed to pass from the transition state and enter the steady-state, which is attained when a given amount of o-diphenol has accumulated in the medium.

Keywords::

• Activation
• inhibition
• monophenol
• o-diphenol
• melanogenesis
• coenzymes, tyrosinase

Acknowledgements

This work was supported in part by grants from the MEC (Spain) Project BIO2006-15363, and from the Fundación Séneca/Consejería de Educación (Murcia), Projects 00672/PI/04, 07 BIO2005/01-6464 and BIO-BMC 06/01-0004. FGM has a fellowship from the MEC (Spain), Reference AP2003-0891. JLM has a fellowship from the MEC (Spain) Reference AP2005-4721.