WATSON ET AL : ETIOLOGY OF BURNT TUNA 



undesirable changes seen in burnt tuna. The lyso- 

 somal proteases, specifically cathepsins B, D, and 

 L, have an optimum pH of about 5 (Dahlmann et 

 al. 1984). However, recent work by Abe et al. 

 (1985, 1986) has shown that, intracellularly, tuna 

 muscle is well buffered and that a fall in extracel- 

 lular pH is not necessarily accompanied by an 

 equivalent fall in intracellular pH. Using is- 

 chemic rat gastrocnemius muscle, Hagberg 

 (1985) found that a 1 pH unit drop (7.30-6.36) in 

 extracellular pH was accompanied by a drop in 

 intracellular pH of only 0.4 unit (7.00-6.60). 

 Thus, the acidic intracellular environment that 

 would favor the action of the lysosomal proteases 

 probably is not present in burnt tuna. Also, with 

 the exception of calcium-activated neutral 

 protease, all other muscle proteases (cathepsins 

 B, D, L; alkaline serine protease; neutral trypsin- 

 like protease; and alkaline cysteinyl protease) de- 

 grade myosin (Dahlmann et al. 1984). Yet 

 Hochachka and Brill (1987) found burnt muscle 

 had no increase in 3-methyl-histidine, a specific 

 marker for myosin breakdown. Decomposition of 

 other myofibrillar proteins is, therefore, impli- 

 cated. 



Electronmicrographs of postmortem burnt tuna 

 muscle (Davie and Sparksman 1986) showed a 

 consistent, rapid disintegration of Z-discs and ir- 

 regularities in the sarcoplasmic reticulum (SR). 

 The changes in burnt muscle were not different in 

 kind from postmortem changes seen in unburnt 

 tissue, but were a result of a significant increase 

 in the rate of disintegration. Selected destruction 

 of the Z-discs, troponin and tropomyosin, and the 

 SR is characteristic of a pair of proteases known 

 as calcium-activated neutral proteases (CANP's) 

 (Sugita et al. 1984; Suzuki et al. 1984). These 

 proteases are cytoplasmic, ubiquitous, activated 

 by increased intracellular calcium levels, and ac- 

 tive at pH 5.5-8.0 (Sakamoto and Seki 1985; 

 Koohmaraie et al. 1986; Seki and Kimura 1986; 

 Zeece et al. 1986b). The intracellular pH most 

 likely found in burnt tuna muscle is, therefore, 

 more consistent with CANP action than with 

 lysosomal proteases whose activity requires a pH 

 closer to 4.5 (Hochachka and Brill 1987). In addi- 

 tion, while cathepsin D's activity is greatly re- 

 stricted at 15°C, CANP is still active at 5°C 

 (Koohmaraie et al. 1986; Zeece et al. 1986a). 



A new etiology of burnt tuna proposed by 

 Hochachka and Brill (1987) is summarized in Fig- 

 ure 2. Their hypothesis predicts that low intracel- 

 lular ATP concentrations lead to the leaking of 

 Ca"^^ into the cell and increases in intracellular 



INTENSE MUSCLE ACTIVITY 



I 



LACK OF 02 + ATP 



i 



METABOUC COLLAPSE OF MEMBRANE 



i 



RISE IN INTRACELLULAR CALCIUM 



i 



02 



i 



PSE 



i 



LLU 



i 



IN 



i 



ACTIVATION OF CANP 



i 



UNDESIRABLE MUSCLE TEXTURE 



Figure 2. — Biochemical reac- 

 tions involved in the development 

 of burnt tuna as proposed by 

 Hochachka and Brill (1987). 



Ca"^^ concentrations. These increases, in turn, ac- 

 tivate CANP, which specifically attacks troponin, 

 tropomyosin, SR, and mitochondria. The break- 

 down of the latter two intracellular organelles 

 releases more calcium into the cytoplasm, thus 

 further increasing the activity of CANP. 



The effect of brain and spinal cord destruction 

 on reducing the incidence of burnt tuna and of 

 similar muscle degradation seen in other fish spe- 

 cies can be explained by this new hypothesis, 

 which assumes the initial drop in ATP is the root 

 cause of the elevated intracellular calcium 

 (Amano et al. 1953; Fujimaki and Kojo 1953; 

 Konagaya and Konagaya 1978; Ikehara fn. 6). 

 Brain destruction maintains elevated muscle 

 ATP levels after capture (Boyd et al. 1984). Data 

 recently collected on the use of brain and spinal 

 cord destruction in large yellowfin tuna (Fig. 3) 



40 



I- 



I 35 

 m 



g 30 



m 



I 20 

 o 



Sl5 



z 

 o 



UJ 

 Q. 



10-- 



5- 



JAN 



MAR 



MAY JUL 



MONTH 



SEP NOV 



Figure 3. — Incidence of burnt tuna occurring in large yellowfin 

 tuna caught by 14 commercial handline fishermen operating 

 out of Hilo, HI, January-September 1984, and by 1 fisherman 

 who began using brain destruction in May. His incidence offish 

 becoming burnt dropped dramatically in spite of the normal 

 summer increased incidence seen in the catch of 14 other fisher- 

 men. 



369 



