What Is Tyrosinase?

Tyrosinase is an enzyme that catalyses the rate-limiting step in melanin synthesis in animals. It helps converts the amino acid tyrosine to DOPA in particular cells called  stage II – IV melanosomes.  The enzyme is also found in many microorganisms and is often distinguished from mammalian variants. It may be one of the most widely found enzymes in living systems.

It is related in its activity to polyphenol oxidases and sometimes the terms are interchangeable although the activity of tyrosinase is usually much more specific.

The microbial tyrosinase (E.C. 1.14.18.1) is a member of the type-3 copper protein family.  The more general reaction catalysed is the hydroxylation of monophenols to the corresponding o-diphenols which is sometimes defined as monophenolase or cresolase activity, and then to o-quinones which is defined as diphenolase or catecholase activity. 

Industrial tyrosinases are used for the removal of phenols from wastewater and as a biosensor for detecting phenolics in effluent.

The enzyme is also associated with many browning reactions in fruit and vegetables which are often associated with spoiling but important in ripening. Whilst we tend to think of peroxidases and polyphenol oxidases as important catalysts in this process of browning, the tyrosinases certainly have their part to play. Mushrooms are very prone to browning and the main enzyme involved in the mushroom tyrosinase. Controlling this enzyme in food is important.

Assays for the enzyme still depend on colour reactions occurring which are easily measurable or by using using radioactive substrates followed by the release of tritiated water as in the assay of the mammalian enzyme (Pomerantz, 1964).

Mushroom Tyrosinase has been extensively studied in a variety of formats (Goodenough, 1978; Kumar & Flurkey, 1991).

Inhibitors Of Tyrosinase

As with polyphenol oxidases, the inhibitors are mainly:

• copper chelators
• reducing agents that convert quinones to phenolic compounds (Mayer, 1987). These work well until all the reducing agent is consumed.
• 4-hexylresorcinol (4HR)
• ascorbic acid and its derivatives
• bisulphites. The bisulphites modify the enzyme so that it loses reactivity.
• cysteine delays browning by forming addition compounds with any o-quinones formed 
• cinnamic acids and p-coumaric acid
• salicylhydroxamic acid
• tropolone (Kahn & Andrawis, 1985)

Many of the inhibitors are also potent carcinogens.

Anti-browning agents are regularly sought. The compound 4-hexylresorcinol is used to control black spotting in shrimp caused by tyrosinase activity as well as browning in potatoes, avocado and grape juice. Bisulphite washing is still more commonly accepted however. pasta is sometimes treated with these inhibitors because it too suffers from browning caused by excessive enzyme activity in durum wheat.

 A new antioxidant and anti-tyrosinase group of compounds called 1,2,4,-triazole hydrazones has recently been tested as antibrowning agents (Peng et al., 2020).

References