Kinetic analysis of melanogenesis by means of Agaricus bisporus tyrosinase

Melanins are a heterogeneous group of polymers formed by enzymatic reactions in vegetable tissues that contain phenolic or polyphenolic molecules. Recent studies have discovered some beneficial properties of melanins on health, making that not only its elimination should be reconsidered, but also its addition could be proposed to functional food of new creation. A further knowledge about the kinetic mechanism of melanogenesis is required prior to its possible industrial utilization. In this work, the kinetics of melanogenesis from 4-methylcatechol using mushroom tyrosinase and measuring the absorbance of the solution has been analyzed. The reaction pathway has been divided in two steps, and a mathematical expression has been developed to describe each one of them. The first one, an enzymatic reaction from the o-diphenol to colorless intermediate products. The second one, a polymerization of these intermediates leading to melanin chains. These expressions allow describing melanin formation as a function of reaction time, including some important parameters such as the extinction coefficient. In addition, the effect of pH and substrate concentration has been assayed in melanogenesis from two kinds of tyrosinase substrates: monophenolic (l-tyrosine) and o-diphenolic (4-methylcatechol).