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.

Pepsin from the stomach of albacore tuna, skipjack tuna, and tongol tuna was characterized. Pepsin from all tuna species showed maximal activity at pH 2.0 and 50 degrees C when hemoglobin was used as a substrate. Among the stomach extract of all species tested, that of albacore tuna showed the highest activity (40.55 units/g tissue) (P < 0.05). Substrate-Native-PAGE revealed that pepsin from albacore tuna and tongol tuna consisted of 2 isoforms, whereas pepsin from skipjack tuna had only 1 form. The activity was completely inhibited by pepstatin A, while EDTA (ethylenediaminetetraacetic acid), SBTI (soybean trypsin inhibitor), and E-64 (1-(L-trans-epoxysuccinyl-leucylamino)-4-guanidinobutane) exhibited negligible effect. The activity was strongly inhibited by SDS (sodium dodecyl sulfate) (0.05% to 0.1%, w/v). Cysteine (5 to 50 mM) also showed an inhibitory effect in a concentration dependent manner. ATP, molybdate, NaCl, MgCl(2), and CaCl(2) had no impact on the activity. When tuna pepsin (10 units/g defatted skin) was used for collagen extraction from the skin of threadfin bream for 12 h, the yield of collagen increased by 1.84- to 2.32-fold and albacore pepsin showed the comparable extraction efficacy to porcine pepsin. The yield generally increased with increasing extraction time (P < 0.05). All collagen obtained with the aid of tuna pepsin showed similar protein patterns compared with those found in acid-solubilized collagen. Nevertheless, pepsin from skipjack tuna caused the degradation of alpha and beta components. All collagens were classified as type I with large portion of beta-chain. However, proteins with molecular weight (MW) greater than 200 kDa were abundant in acid-solubilized collagen.

Eflects of setting temperature, time, and addition of porcine plasma protein (PPP) on gel propeflies of sunmi from bigeye snapper (Priacanthus tayenus) were investigated. Breaking force and deformation of the surimi gels increased as the setting time and temperature increased. l2e gel preincubaled at 35C far 90 min in the presence of 0.5% PPP, followed by cooking at WC for 20 min showed the marimurn force and deformation. The decrease in solubility of the resultant suwari and kumaboko gels in solution containing sodium dodecyl sulfate, urea and 8-mercaptoethanol suggested that gel enhancement was mainly mediated through the formation of nondisulfide covalent bonds catalyzed by both transglutaminase ( T h e ) in fish muscle and porcine plasma. Addition of PPP slightly decreased the whiteness of the kamaboko gels.

Discoloration and lipid deterioration of farmed giant catfish (Pangasianodon gigas) muscle during 14 d refrigerated storage were investigated. Lipid deterioration, lipolysis, and lipid oxidation in both dorsal and ventral muscles increased as storage time increased. A progressive formation of primary lipid oxidation products monitored by the increase in conjugated dienes (CD) was observed (P < 0.05) and the increase in thiobarbituric reactive substances (TBARS), an index of secondary lipid oxidation products, was noticeable throughout the storage (P < 0.05). The pH of both dorsal and ventral muscles tended to increase as storage time continued (P < 0.05). A gradual increase in free fatty acid (FFA) formation was found within the first 10 d of refrigerated storage (P < 0.05), suggesting hydrolysis induced by lipases and phospholipases. However, a sharp decrease in FFA content was observed at the end of storage. Refrigerated storage also resulted in changes in redness index of both dorsal and ventral muscles. These changes were coincidental with the changes in metmyoglobin content. Therefore, the discoloration and lipid changes in giant catfish muscle during refrigerated storage depended on the muscle type and might be related to the difference in composition between dorsal and ventral muscles.

Composition and some properties of muscle from two species of bigeye snapper, P. tayenus and P. macracanthus, were investigated. Both species had a similar composition with the same myofibrillar protein content. However, muscle proteins from P. tayenus had higher thermal stability than those from P. macracanthus, as indicated by the higher enthalpy for transitions as well as the lower inactivation rate constant (KD). Upon 15 days of iced storage, natural actomyosin Ca2*‐ATP ase and Mg2+‐Ca2+‐ATPase activities decreased, whereas Mg2+‐EGTA‐ATPase activity increased, suggesting the denaturation of myosin, actomyosin and troponin/tropomyosin complexes, respectively. Increased surface hydrophobicity and decreased sulfhydryl groups indicated the denaturation possibly occurred via hydrophobic interaction and disulfide formation. Heading and eviscerating offish retarded the denaturation and physicochemical changes of proteins during iced storage. The results indicated that a rapid and proper post harvest handling was of importance to maintain the muscle quality of bigeye snapper.