Cascading Mechanism Triggering the Activation of Polyphenol Oxidase Zymogen in Shrimp Litopenaeus vannamei After Postmortem and the Correlation with Melanosis Development.

Melanization is the major deterioration in organoleptic quality of shrimp Litopenaeus vannamei during cold storage and leads to dramatical reduction of commercial value. So far, the biochemical cascade mechanism triggering and accelerating melanosis progression remains unclear. Herein, this investigation aimed to monitor the melanosis development in L. vannamei during 5 days' cold storage at 4 °C and to explore the role of serine protease (SP) in zymogen activation of polyphenol oxidase (PPO) by application of low field nuclear magnetic resonance (LF-NMR), transmission electron microscopy (TEM), and immunohistochemistry (IHC). The results showed that melanosis development was positively associated with the increase of PPO and SP activities, and the utmost melanization was observed at cephalothorax segment, following with the telson and abdomen. Besides, LF-NMR relaxometry revealed both a dramatical reduction in trapped water component T22 and a significant increase in free water component T23. Meanwhile, the histopathological findings of hepatopancreas tissue demonstrated the progressive disruption in cytoarchitecture. Along with the increase of T23 and cytoarchitecture disruption, SP and Ca2+ were arbitrarily disseminated in hepatopancreas tissue. In addition, a heat map analysis revealed that there was a highly positive relationship between melanosis development and the aberrant elevation of PPO and SP activities, ratio of T22 components, cytoarchitecture disruption level, and dissemination status of SP and Ca2+. Altogether, the biochemical cascade events for melanosis development of L. vannamei during cold storage could be sketched out; i.e., the cytoarchitecture disruption, in combination with the driving force of free water molecule migration, was greatly favorable for aggregation of SP, Ca2+, and PPO zymogen, following with aberrant activation of PPO zymogen and initiation of melanization. These data provide new insights into the biochemical cascade mechanism of melanosis development in L. vannamei during cold storage and pave new ways for target controlling of melanization at initial stage. [ABSTRACT FROM AUTHOR]