20
compounds into quinones and the quinones are then transformed into coloured melanoids (Kim et al. 2000; Marshall et al. 2000; Villamiel et al. 2006).
Melanosis is a natural post-mortem process originating from the polymerization
of phenols in water-insoluble black pigments (melanin). Polymerization is catalysed by polyphenoloxidase, an enzyme complex (tyrosinases and catecholoxidases) found in almost all organisms. As soon as the shrimp is taken out of the
water, the melanosis begins with the reaction of the atmosphere with oxygen or by
the enzyme reaction during storage. The enzymatic browning reaction is catalysed
by the phenoloxidase, monophenoloxidase, diphenoloxidase and tyrosinasecatalysed enzyme. The development of physiological functions of aquatic organisms is based on the important contributions of polyphenoloxidases.
Polyphenoloxidases are important in the hardening of the outer shell after shell
replacement in shellfish. It is also responsible for wound healing (Corzo-Martinez
et al. 2012). The substrate of the polyphenoloxidase enzyme is phenolic compounds.
Phenolic compounds (substrates) are secondary metabolites synthesized during the
metabolism of aromatic amino acids in tissues (Hernandez-Romero et al. 2006).
These compounds as food components are important in many aspects such as their
functions in human health, taste and odour formation, colour formation and change,
antimicrobial and antioxidative effects, roles in enzyme inhibition, (Marshall
et al. 2000).
Polyphenoloxidase enzyme catalyses two basic reactions: hydroxylation (cresolase activity) and oxidation (catecholase activity). By using phenolic substances and
oxygen as substrates, they hydroxylate the existing hydroxyl groups by zooming to
the o-position. It then oxidizes diphenols to o-benzoquinones, which are then oxidized by non-enzymatic mechanism and oxidized to dark-coloured products, melanins (Nicolas et al. 2003; Kim et al. 2000; Martínez-Alvarez et al. 2005b). This
initial reaction is called monophenolase activity and transforms tyrosine into 3,
4-hydroxyphenylalanine (l-DOPA). l-DOPA is then oxidized to o-quinones which
are called PO diphenase activity (Aladaileh et al. 2007). Difenoloksidaz is activity
which catalyzes oxidation of o-diphenolic compounds to o-quinone and initiates the
formation of brown coloured compounds (Cemeroglu et al. 2004; Villamiel
et al. 2006).
The distribution of PPOs in different body regions of crustaceans is different.
They are usually present in the cuticle, in particular on the inner surface of the chromatophores; however, these enzymes have been reported in the hepatopancreas,
muscle and hemolymph of Penaeus monodon (Kim et al. 2000). It is generally
reported that melanosis occurs in the carapace of cephalothorax, in the caudal region
and mainly in the abdomen regions where cuticle segments are combined and cuticle joins with pleopods (Montero et al. 2001a; Nirmal and Benjakul 2012; Ogawa
et al. 1984). Darkening starts from the head-thorax region where the polyphenoloxidase activity is highest, and gradually spreads to the abdomen and tail (Zamorano
et al. 2009).
The severity of melanosis formation in crustaceans has been reported to vary
with species due to differences in substrate and enzyme concentration (Benjakul
et al. 2005; Nirmal and Benjakul 2012). In some species PPO activity are faster than
2 Crustacean Shellfish
compounds into quinones and the quinones are then transformed into coloured melanoids (Kim et al. 2000; Marshall et al. 2000; Villamiel et al. 2006).
Melanosis is a natural post-mortem process originating from the polymerization
of phenols in water-insoluble black pigments (melanin). Polymerization is catalysed by polyphenoloxidase, an enzyme complex (tyrosinases and catecholoxidases) found in almost all organisms. As soon as the shrimp is taken out of the
water, the melanosis begins with the reaction of the atmosphere with oxygen or by
the enzyme reaction during storage. The enzymatic browning reaction is catalysed
by the phenoloxidase, monophenoloxidase, diphenoloxidase and tyrosinasecatalysed enzyme. The development of physiological functions of aquatic organisms is based on the important contributions of polyphenoloxidases.
Polyphenoloxidases are important in the hardening of the outer shell after shell
replacement in shellfish. It is also responsible for wound healing (Corzo-Martinez
et al. 2012). The substrate of the polyphenoloxidase enzyme is phenolic compounds.
Phenolic compounds (substrates) are secondary metabolites synthesized during the
metabolism of aromatic amino acids in tissues (Hernandez-Romero et al. 2006).
These compounds as food components are important in many aspects such as their
functions in human health, taste and odour formation, colour formation and change,
antimicrobial and antioxidative effects, roles in enzyme inhibition, (Marshall
et al. 2000).
Polyphenoloxidase enzyme catalyses two basic reactions: hydroxylation (cresolase activity) and oxidation (catecholase activity). By using phenolic substances and
oxygen as substrates, they hydroxylate the existing hydroxyl groups by zooming to
the o-position. It then oxidizes diphenols to o-benzoquinones, which are then oxidized by non-enzymatic mechanism and oxidized to dark-coloured products, melanins (Nicolas et al. 2003; Kim et al. 2000; Martínez-Alvarez et al. 2005b). This
initial reaction is called monophenolase activity and transforms tyrosine into 3,
4-hydroxyphenylalanine (l-DOPA). l-DOPA is then oxidized to o-quinones which
are called PO diphenase activity (Aladaileh et al. 2007). Difenoloksidaz is activity
which catalyzes oxidation of o-diphenolic compounds to o-quinone and initiates the
formation of brown coloured compounds (Cemeroglu et al. 2004; Villamiel
et al. 2006).
The distribution of PPOs in different body regions of crustaceans is different.
They are usually present in the cuticle, in particular on the inner surface of the chromatophores; however, these enzymes have been reported in the hepatopancreas,
muscle and hemolymph of Penaeus monodon (Kim et al. 2000). It is generally
reported that melanosis occurs in the carapace of cephalothorax, in the caudal region
and mainly in the abdomen regions where cuticle segments are combined and cuticle joins with pleopods (Montero et al. 2001a; Nirmal and Benjakul 2012; Ogawa
et al. 1984). Darkening starts from the head-thorax region where the polyphenoloxidase activity is highest, and gradually spreads to the abdomen and tail (Zamorano
et al. 2009).
The severity of melanosis formation in crustaceans has been reported to vary
with species due to differences in substrate and enzyme concentration (Benjakul
et al. 2005; Nirmal and Benjakul 2012). In some species PPO activity are faster than
2 Crustacean Shellfish