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Proteolytic activities of splenic extract from three tuna species including skipjack tuna (Katsuwonus pelamis) , yellowfin tuna (Thunnus albacores) and tongol tuna (Thunnus tonggol) were studied. Optimal activity of splenic extract from all tuna species was at pH 9.0 and 55C when casein was used as a substrate. Among all species tested, yellowfin tuna showed the highest activity, followed by skipjack tuna and tongol tuna. The proteolytic activity was strongly inhibited by soybean trypsin inhibitor, TLCK and partially inhibited by ethylenediaminetetraacetic acid. E-64, N-ethylmaleimide, iodoacetic acid, TPCK and pepstatin A showed no inhibition. The effect of NaCl and CaCl 2 on proteolytic activity was also investigated. Activities continuously decreased as NaCl concentration increased, and no activity remained in the presence of 30% NaCl. On the other hand, activities increased as CaCl 2 concentration increased. The highest activity was obtained in the presence of 1 mM CaCl 2 . SDS-substrate gel electrophoresis revealed that major proteinases in splenic extract from different tuna species were different in apparent molecular weights and sensitivity to TLCK. Although the major activity bands 356 S. KLOMKLAO, S. BENJAKUL and W. VISESSANGUAN of all species were strongly inhibited by soybean trypsin inhibitor, varying sensitivity to TLCK probably implied the differences in binding characteristic of enzyme to substrate and/or inhibitors. The results suggest that major proteinases in spleen of all tuna species were trypsin-like serine proteinases.

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Partitioning of spleen proteinase from yellowfin tuna (Thunnus albacores) in an aqueous two-phase system (ATPS) was investigated. Phase compositions including PEG molecular mass and concentration as well as types and concentration of salts affected the protein partitioning. ATPS comprising PEG1000 (15% w/w) and magnesium sulfate (20% w/w) provided the best condition for the maximum partitioning of the proteinase into the top phase and gave a highest specific activity (47.0 units/µg protein) and purification fold (6.61). The yield of 69.0% was obtained.Under the same ATPS condition used, the partitioning of proteinase of splenic extract from three tuna species involving skipjack tuna, yellowfin tuna and tongol tuna were compared. The purity of splenic extract from all tuna species increased after ATPS process. Among all species tested, yellowfin tuna showed the highest purification fold, followed by tongol tuna and skipjack tuna, respectively. SDS-substrate gel electrophoresis revealed that the band intensity of major proteinase in ATPS fraction from all tuna species slightly increased with the concomitant decrease in band intensity of other contaminating proteins, indicating the greater specific activity of splenic extract. Therefore, ATPS was an effective method for partitioning and recovery of proteinases from tuna spleen.

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