Biologically active substances in fish skin mucus and their action
　

Fish skin acts as a primary barrier to protect the fish body from foreign enemies such as microorganisms. For that purpose, fish skin mucus bears peculiar substances with physiological functions, such as antibacterial and antimicrobial agents, enzymes, enzyme inhibitors, and toxins. This theme is related to various fields: namely, screening (treasure hunting) at first, then purification and identification of substances of interest, their structural elucidation, and characterization of their distribution, function, and action mechanisms. Furthermore, this issue can also be approached from a biological standpoint: How and where do fish produce these substances? How and when do fish use them? Why do fish bear them?

Occasionally, we behave as if we were fish and act to pursue studies from various perspectives.
　
　
　 　 　
　



Toxification and defense mechanisms of pufferfish
　
Why can pufferfish live with tetrodotoxin (TTX)? That remains a mystery even today. We specifically examine how pufferfish accumulate the toxin TTX. Previous studies have revealed that pufferfish accumulate TTX using a special technique. How does it store the taken-up TTX? Surprisingly, TTX is a toxin for pufferfish also. Some sophisticated mechanisms must be elaborated in pufferfish not to be poisoned by toxin. A possible approach is transforming the toxin to nontoxic one by binding it with biogenic components like proteins.

We have remained fascinated with the wonderful mechanisms of pufferfish. We are addressing this study with the dream of clarifying the mystery and producing a safe and nontoxic pufferfish from the viewpoint of food safety in the near future.

Pufferfish is not the only fish to have TTX: certain species of frogs, crabs, octopi, and snails do also. One can imagine easily that such animals bearing TTX also have TTX-binding proteins for self-defense. On the other hand, a very common crab called Asian shore crab, Hemigrapsus sanguineus, carrying no TTX, has TTX-binding proteins. What for? We hope to use this substance for development of a therapeutic prescription of TTX intoxication.
　
　
　



Inspection analysis of marine toxin
　
We have developed chemical analysis methods for TTX and paralytic shellfish toxins (PSTs), including high-performance liquid chromatography (HPLC), thin-layer chromatography-mass spectrometry (TLC-MS), the sensor method, affinity method, etc. These allow us to measure toxins specifically at high sensitivity, which has contributed to elucidate the distribution and transfer of toxins in nature, and the metabolism of toxin components. We have uncovered the following facts, using these analytical instruments: PSTs move along the food chain, which has led to toxification of a protochordata, Halocynthia roretzi, and a weed-eating snail, Haliotis tuberculata. Moreover, we have observed the micro-distribution of TTX in pufferfish hepatocytes.
　
　
　
　 　 　



Characterization of the structure and function of marine origin proteins
and related enzymes

　
We are conducting molecular level studies of the higher order structure, the structure formation mechanism, processing-characteristics expression mechanism, and structure-processing characteristics correlation between fish and shellfish myofibrillar proteins, and on the higher order structure and the catalytic function expression mechanism of fish-and-shellfish myofibrillar-related enzymes, by making full use of a protein-engineering approach. In particular, our genetic investigation is in progress on the species-specificity of the myosin molecule, a major structural protein of fish and shellfish muscles. Meanwhile, our study objectives about related enzymes are mainly related to properties of various endogenous proteases or transglutaminase of fish and shellfish muscles. Furthermore, we are conducting studies to modify and add processing characteristics to fish and shellfish myofibrillar proteins, and to transform them to make them highly applicable using a genetic engineering approach. Especially, we aim at the design of myosin molecule that has excellent processing characteristics for protein engineering modification based on structure-processing characteristics correlation. Our ultimate goal is to achieve complete quality control of fish meat pastes, such as kamaboko.
　
　





More efficient utilization of marine origin resources
　
Today, starfish are subjected to incineration disposal as totally useless organisms. However, in fact, they bear substances called saponin. We have demonstrated that saponin has antibacterial properties and high cholesterol-binding activity. We are also investigating its various properties to achieve the effective use of starfish. In addition, many marine resources have various substances that have yet to be clarified. We are also searching especially for anticancer drug substances from marine resources in this study. We have begun development of frozen surimi technology of underutilized resources, and effective use of giant squid.
　
　



　 　

Development of analysis / evaluation technology of the processing history
of marine food products
　
Fish stock classification using DNA technology has recently been gaining popularity, even though its technology has not been established yet. Today, judgment of toxic / noxious fishes, mainly pufferfishes, is performed using molecular approaches such as Polymerase Chain Reaction (PCR) amplification and Restriction Fragment Length Polymorphism (RFLP). We plan to establish a simplified identification method of various processed marine food products.