Preventive effects of amino acids and glutathione on the formaldehyde-induced denaturation of myofibrillar proteins

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1 Blackwell Science, LtdOxford, UK FISFisheries Science Blackwell Science Asia Pty Ltd 693June Formaldehyde-induced denaturation E Abe et al /j x Original Article605614BEES SGML FISHERIES SCIENCE 2003; 69: Preventive effects of amino acids and glutathione on the formaldehyde-induced denaturation of myofibrillar proteins Eriko ABE, 1 Kazunori HAYAKAWA, 1 Meiko KIMURA, 1 Ikuo KIMURA 2 AND Nobuo SEKI 1 * 1 Laboratory of Food Biochemistry, Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido and 2 Central Research Laboratory, Nippon Suisan Kaisha, Hachioji, Tokyo , Japan ABSTRACT: Formaldehyde (FA)-induced denaturation of myofibrillar proteins and its prevention were investigated by means of measuring the solubility, adenosine triphosphatase (ATPase) activity, and thermal gel formability of myofibrils and surimi proteins in the presence and absence of free amino acids and glutathione, reduced form. The addition of FA decreased the solubility of myofibrils in 0.5 M NaCl at ph 7.0 and 0 C depending on its concentration and incubation time. The solubility decrease was completely inhibited by the presence of equal, twofold, and threefold amounts of cysteine (Cys), glutathione, and histidine (His) to the amount of FA, respectively. Myofibrillar Ca- ATPase was markedly activated at the initial phase and then decreased later by the addition of FA. The K-ATPase was inactivated with an increase in the amount of FA. The FA-induced changes in both ATPase activities were inhibited in the presence of Cys and His. Thermal gel formability of surimi paste increased only in a short period after the addition of a low concentration of FA. Practically, FA inhibited the thermal gelation and setting effect through the inactivation of transglutaminase. In the presence of Cys, His or glutathione, a strong elastic surimi gel was produced because FA-induced detrimental effects were inhibited. KEY WORDS: cysteine, formaldehyde, glutathione, histidine, myofibril, surimi, trimethylamine-n-oxide, walleye pollack INTRODUCTION Trimethylamine-N-oxide (TMAO), a natural osmolyte, is present in most marine species, and accumulates abundantly in the various tissues of marine elasmobranchs, gadoid fishes, a few nongadoid fish, and molluscs. 1 3 It is demethylated to equimolar formaldehyde (FA) and dimethylamine by being catalyzed with an endogeneous enzyme, TMAOase, during storage above 0 C. During frozen storage, however, TMAO is degraded to FA and dimethylamine or to trimethylamine through nonenzymic pathways. 4,5 The produced FA has been suggested to cause a decrease in solubility and extractability of the myofibrillar proteins and to result in eventual detrimental changes in texture and functional properties, including viscosity, emulsifying ability, and gel-forming ability. This suggestion has been widely accepted although direct experimental evidence has been difficult to *Corresponding author: Tel: Fax: seki@fish.hokudai.ac.jp Received 24 June Accepted 26 November obtain and the mechanism through which FA affects the proteins is not fully understood. 6 In myosin isolated from cod and in hake and chicken actomyosins, the modification of a relatively small number of amino acid side chains by FA is accompanied by major alterations in solubility and adenosine triphosphatase (ATPase) activity. The denaturation would be mainly due to increased exposure of hydrophobic groups and this would lead to subsequent aggregation by non-covalent interactions. 7 9 It therefore follows that preventing or inhibiting the activity of this enzyme system and non-enzymic pathways leads to an improvement in quality and a greater stability of the fish muscle and surimi during storage. To be more precise, these purposes can be achieved by inhibiting the reactivity of FA or by reducing the amount of FA that can interact with the various muscle components. Free amino acids and glutathione are coexistent with myofibrillar proteins in muscle cells. Many studies have been reported on the reaction of FA with amino acids or proteins: FA is assumed to react with the a-amino group as well as some of the

2 606 FISHERIES SCIENCE E Abe et al. side chain groups. 10,11 The principal initial reactions of FA with proteins under mild conditions are reversible. Formaldehyde is therefore bound to proteins at different levels known as 'reversible', 'acid labile', and 'acid resistant' based on the analysis of FA-treated proteins. 12,13 Glutathione also reacts with FA and produces a stable adduct. 14 It is therefore expected that coexistent amino acids and glutathione may prevent or inhibit the reactivity of FA to myofibrillar proteins. We decided to perform a study of the effect of amino acids and glutathione on FA-induced alterations in solubility, ATPase activity, and gelforming ability of fish myofibrils and surimi. MATERIALS AND METHODS Materials Live cultured carp Cyprinus carpio were decapitated, filleted and then stored at -60 C until used. Frozen walleye pollack Theragra chalcogramma surimi (SA grade) containing 170 mg protein/g, 4% sorbitol, 4% sucrose, and 0.2% polyphosphates was supplied from Nippon Suisan Kaisha (Tokyo, Japan). Formaldehyde and other chemicals were purchased from Wako Pure Chemical Industries (Osaka, Japan). Glutathione, reduced form, was used throughout the experiment. Preparation of myofibrils Carp dorsal white muscle was minced and washed with 0.1 M NaCl-20 mm Tris-HCl (ph 7.0) three times. The washed mince was homogenized in five volumes of the same buffer. After centrifuging, the precipitated myofibrils were further washed four times with the same buffer and passed through a layer of gauze to remove connective tissues. Formaldehyde modification of myofibrils Formaldehyde modification was carried out at a final myofibril concentration of 10 mg/ml in 0.1 M NaCl and 20 mm phosphate buffer (ph 7.0) or 20 mm Tris-HCl (ph 7.0) at varying concentrations of FA (0 10 mm) in total volume of 5 ml. Formaldehyde adjusted to 100 mm in 20 mm phosphate buffer (ph 7.0) or 20 mm Tris-HCl (ph 7.0) was added to the myofibril suspension. The mixture was stirred at 0 C for various periods. Formaldehyde concentration was determined by the Nash method. 15 Preparation of salted surimi paste Frozen surimi was thawed and chopped in a food cutter (Model MK-K7, Matsushita Electric Industry, Osaka, Japan) for 4 min at 4 C with 0.5 M NaCl, 5 mm CaCl 2, and 20 mm Tris-HCl (ph 7.0). Final protein concentration of the paste was adjusted to 90 mg/g. If necessary, FA was added and well mixed with the paste just before heating. Solubility of myofibrils The 5 ml of myofibril suspension (10 mg/ml) was centrifuged at 1400 g for 15 min at 2 C. To the precipitated myofibrils 4 ml of 0.6 M NaCl-20 mm phosphate buffer (ph 7.0) was added and then stirred for 3 h at 0 C. After stirring, the samples were centrifuged at g for 20 min. The protein concentration of supernatant was determined by the biuret method. 16 Protein solubility was expressed as the percent concentration of soluble protein in the sample. Assay of myofibrillar ATPase activity The Ca-ATPase activity of myofibrils was assayed at 25 C in a medium containing 0.2 mg/ml myofibrils, 0.5 M KCl, 5 mm CaCl 2, 1 mm ATP, and 25 mm Tris-maleate (ph 7.0). The K-ATPase was assayed at 25 C in 0.2 mg/ml myofibrils, 0.5 M KCl, 5 mm EDTA, 1 mm ATP, and 20 mm Trismaleate (ph 7.0). The inorganic orthophosphate liberated was determined by the method of Fiske and Subbarow. 17 Transglutaminase activity Transglutaminase activity in the muscle paste was measured by a modified method of Kishi et al. 18 as follows: monodansyl cadaverine (MDC) was added into the muscle paste at a final concentration of 2 mm containing 80 mg protein/g-paste, 0.5 M NaCl, 10 mm CaCl 2, and 20 mm Tris-HCl (ph 7.2). The mixture (3 g) was immediately incubated at 25 C for up to 60 min. A portion of the mixture (0.2 g) was taken out at given times and then mixed with a 5-mL sodium dodecylsulfate (SDS) urea solution (2% SDS-8 M urea-2% 2-mercaptoethanol-50 mm Tris-HCl at ph 8.0) containing 10 mm EDTA to terminate the reaction. The mixture was immediately heated at 100 C for 2 min and then stirred overnight at 25 C. Monodansyl cadaverine incorporated into the muscle proteins was determined on a fluorescence spectrophotometer

3 Formaldehyde-induced denaturation FISHERIES SCIENCE 607 Fig. 1 Effect of formaldehyde (FA) concentration on the solubility of myofibrils.carp myofibrils (10 mg/ml) were incubated with various concentrations of FA at 0 C for up to 6 h in solution containing (a) 0.1 M NaCl-20 mm phosphate buffer or (b) 0.1 M NaCl-20 mm Tris-HCl at ph 7.0. After incubation, myofibrils were centrifuged and then the precipitate was solubilized with 0.5 M NaCl-20 mm phosphate buffer (ph 7.0) with stirring for 3 h at 0 C. Formaldehyde concentration: ( ) 0 mm; ( ) 1 mm; ( ) 2 mm; ( ) 5 mm; ( ) 10 mm. (Shimadzu RF-500, Kyoto, Japan) with an excitation of 350 nm and an emission of 480 nm. Thermal gelation and rheological measurements The surimi pastes with and without FA were spread to a thin layer in vacuum to remove bubbles and then stuffed into a cell ( cm 3 with a mm 3 blade) for a dynamic rheological measurement. Dynamic rheological properties of the surimi pastes during thermal gelation were measured using a Rheolograph-Sol (Toyo Seiki Seisakusho, Tokyo, Japan) as reported by Ni et al. 19 Storage (G ) and loss (G ) moduli were monitored continuously at a fixed frequency of 3 Hz as a function of temperature from 5 to 80 C at a heating rate of 2 C/min. Analyses were carried out in duplicate and average values were used in figures. Sodium dodecylsulfate polyacrylamide gel electrophoresis Samples for SDS-polyacrylamide gel electrophoresis (PAGE) were solubilized with SDS urea solution by heating at 100 C for 2 min and continuous stirring at 25 C overnight. Sodium dodecylsulfate- PAGE was performed by the method of Laemmli 20 using 5.0% and 7.5% polyacrylamide gels. RESULTS AND DISCUSSION Effect of formaldehyde concentration on the solubility of myofibrils Myofibrils were treated with various concentrations of FA at 0 C for up to 6 h in 0.1 M NaCl- 20 mm phosphate or 0.1 M NaCl-20 mm Tris-HCl buffers at ph 7.0 (Fig. 1). The solubility of myofibrils in 0.5 M NaCl decreased with an increase in FA concentration and decreased more rapidly in phosphate buffer than in Tris-HCl buffer. The addition of FA to 10 mm decreased the salt solubility to the lowest value within 15 min in both buffers. We used phosphate buffer throughout this experiment for higher sensitivity of myofibrils to FA. Effect of amino acids and glutathione on the formaldehyde-induced solubility change of myofibrils The addition of FA at the final concentration of 10 mm to myofibrils decreased the solubility to the bottom as shown in Fig. 1. When FA was added to myofibrils in the presence of some kinds of amino acids and glutathione, however, the solubility change was effectively inhibited at neutral ph (Fig. 2). An equal molar ratio of Cys to that of added FA inhibited completely a decrease in the solubility of myofibrils. Twofold and three-fold amounts of glutathione and His also completely inhibited the solubility decrease, respectively. The other amino acids such as serine, lysine, glutamic acid, and glycine had no preventative effect on the FA-induced solubility change. These results indicated that added FA preferentially reacted with Cys, His, and glutathione rather than with myofibrillar proteins. As a result, the amount of FA reacting with the myofibrillar proteins was reduced in the reaction system. To confirm the reaction ratio of FA to glutathione, the reaction was initiated by mixing 4 mm glutathione with various concentrations of FA in 0.1 M NaCl-20 mm phosphate buffer (ph 7.0) at 0 C. It is reported that the level of absorbance of glutathione at 240 nm decreases after addition of FA due to the formation of S-(hydroxymethyl)glu-

4 608 FISHERIES SCIENCE E Abe et al. Fig. 2 Effects of additives on the solubility of carp myofibrils in the presence of formaldehyde (FA).Carp myofibrils (10 mg/ml) were incubated with 10 mm FA in the solution containing 0.1 M NaCl-20 mm phosphate buffer (ph 7.0) in the presence of various amounts of added amino acids and glutathione for 4 h at 0 C. Each additive without FA had no effect on the solubility of myofibrils. ( ) Solubility of myofibrils without FA and additives; ( ) Cys; ( ) His; ( ) glutathione; ( ) Ser; ( ) Lys; ( ) Glu; ( ) Gly. Fig. 3 Reaction of glutathione and formaldehyde (FA).The absorbance of 4 mm glutathione was measured at 240 nm after the addition of various amounts of FA in 0.1 M NaCl-20 mm phosphate buffer at ph 7.0. Time after the addition of FA: ( ) 120 s; ( ) 160 s; ( ) 200 s. tathione. 14 This absorbance change has been used to estimate the reaction of FA and glutathione. Figure 3 shows that the initial absorbance of glutathione at 4 mm sharply decreases with the addition of FA up to 2.2 mm. This result showed that FA was required at approximately half the molar concentrations of that of glutathione to form the stable adduct. That is, the reactivity of FA could be lost by the addition of twice the amount of glutathione as shown in Fig. 2. It was therefore supposed that FA reacted preferentially with glutathione rather than with myofibrillar proteins. Cysteine reacts with FA over a wide range of ph to form thiazolidine-4- carboxylic acid, which is remarkably stable against both acid and alkaline. 21 Kitamoto and Maeda reported that His had a high reactivity with FA at ph and yielded a single reaction product, 1,2,5,6-tetrahydropyrido-3,4-imidazole-6- carboxylic acid. 11 All these reaction products from FA with Cys, His and glutathione are stable. Carp myofibrils were treated with FA at 0 C for 4 h and then Cys, His or glutathione was added to the myofibrillar suspension. After standing for 1 h, the solubility of myofibrils was measured (Fig. 4). Although the added Cys, His, and glutathione Fig. 4 Effect of cysteine (Cys), histidine (His) and glutathione on the solubility of formaldehyde (FA)-treated myofibrils.carp myofibrils (10 mg/ml) were incubated with 10 mm FA in 0.1 M NaCl-20 mm phosphate buffer (ph 7.0) for 4 h at 0 C and then various amounts of Cys, His or glutathione were added. ( ) Solubility of myofibrils without FA and additives; ( ) Cys; ( ) His; ( ) glutathione.

5 Formaldehyde-induced denaturation FISHERIES SCIENCE 609 Fig. 5 Effect of formaldehyde (FA) on the myofibrillar adenosine triphosphatase (ATPase) activity.carp myofibrils (10 mg/ml) were treated with various concentrations of FA in 0.1 M NaCl-20 mm phosphate buffer (ph 7.0) at 0 C for 10 min. The ATPase activity was assayed at 25 C in a medium containing myofibrils (0.2 mg/ml), 0.5 M KCl, 1 mm ATP and 25 mm Trismaleate (ph 7.0) in the presence of 5 mm CaCl 2 (, Ca- ATPase) and 5 mm EDTA (, K-ATPase). Fig. 6 Change in Ca-adenosine triphosphatase (ATPase) activity during formaldehyde (FA) treatment.carp myofibrils (10 mg/ml) were treated with 5 mm FA in 0.1 M NaCl-20 mm phosphate buffer (ph 7.0) at 0 C prior to ATPase activity measurement. The inserted figure shows SDS-PAGE patterns of myofibrils before and after treatment with FA. HC, myosin heavy chain; HC 2, myosin heavy chain dimer; Ac, actin. recovered the FA-induced solubility decrease, the higher concentration was required as compared with the case of coexistence with myofibrils prior to the addition of FA. The results suggest that the initial reactions of FA with myofibrils may be reversible because the adducts formed are dissociated by the addition of Cys, His, and glutathione. The irreversible reaction of FA with myofibrils is supposed to be comparatively slow at 0 C and neutral ph judging from the reversible solubility occurring even after several hours. Effect of cysteine and histidine on the formaldehyde-induced denaturation of myofibrillar ATPase Myofibrillar Ca-ATPase and K-ATPase activities are good indices for the denaturation of myosin and actomyosin. The addition of FA to myofibrillar suspension greatly increased Ca-ATPase activity and decreased K-ATPase activity depending on the increase in FA concentration (Fig. 5). The Ca-ATPase activity reached a plateau after 10 h and then gradually decreased after 30 h of treatment with 5 mm FA at 0 C (Fig. 6). Sodium dodecylsulfate-page patterns of myofibrillar proteins hardly changed during up to 24 h of FA treatment (Fig. 6). The results suggested that FA denatured myofibrillar proteins without the formation of covalent cross-links at the initial stage. After 48 h, however, a small amount of myosin heavy chain dimer was formed. Because the changes in ATPase activities, namely an increase in Ca-ATPase activity and a decrease in K-ATPase activity, were similar to the case of oxidative and chemical modifications of SH 1 located on the surface of myosin head, we supposed that FA modified the same amino acid residue. The effects of the coexistence of Cys or His with myofibrils on the change in Ca-ATPase activity are shown in Fig. 7. The FA-induced increase in Ca- ATPase activity was inhibited, although the effect was slightly lower than that for the solubility recovery as shown in Fig. 2. The presence of His also inhibited the increase in Ca-ATPase activity induced by FA but the effect was lower than that of Cys. The FA-induced inactivation of K-ATPase was also inhibited by the presence of Cys or His (Fig. 8). However, the recovery of K-ATPase activity was low even in the presence of a high concentration of Cys and His.

6 610 FISHERIES SCIENCE E Abe et al. Fig. 7 Effect of cysteine (Cys) and histidine (His) on the Ca-adenosine triphosphatase (ATPase) activity of formaldehyde (FA)-treated myofibrils.carp myofibrils (10 mg/ml) were reacted with 10 mm FA in 0.1 M NaCl-20 mm phosphate buffer (ph 7.0) at 0 C in the presence of various concentrations of Cys (left) or His (right) as shown above. Fig. 8 Effect of cysteine (Cys) and histidine (His) on the K-adenosine triphosphatase (ATPase) activity of formaldehyde (FA)-treated myofibrils.carp myofibrils (10 mg/ml) were reacted with 10 mm FA in 0.1 M NaCl-20 mm phosphate buffer (ph 7.0) at 0 C in the presence of various concentrations of Cys (left) or His (right) as shown above. Effect of formaldehyde on the thermal gelation of surimi paste The gelling process of the salted walleye pollack surimi paste was monitored by measuring the changes in storage modulus (G ) and loss modulus (G ). G represents the elastic component and G is the viscous component of the sample. The temperature of the paste was linearly increased from 5 to 25 C and kept at 25 C for 120 min for setting. After setting, the temperature of the paste increased to 80 C at a heating rate of 2 C/min

7 Formaldehyde-induced denaturation FISHERIES SCIENCE 611 (Fig. 9). During the heating after setting, G of the control paste increased and reached 2700 Pa at 80 C. When 1 mm FA was added to the paste and mixed well just before heating, G of the paste greatly increased and reached >3000 Pa at 80 C (line 0 in Fig. 9). G was kept at a higher value than that of the control during setting and heating. The high G value at 80 C indicated water exudation from the heated gel. Thus, the strong rigid gel was formed. However, when FA-added surimi paste was allowed to stand cold at 0 C for 1 8 h prior to heating, G and G of the paste lowered gradually during the standing. G of the heated gel to 80 C was greatly reduced depending on the duration of standing time. The gel-forming ability of the paste containing 1 mm FA was completely lost after 8 h at 0 C. The addition of 5 mm FA also decreased the gel-forming ability of the paste (Fig. 10; line f). To prevent the FA deterioration in thermal gelation, either 5 mm Cys, 15 mm His or 10 mm glutathione was added to surimi paste before the addition of 5 mm FA. The concentration of the additives was adopted based on the results as shown in Fig. 2. These additives increased the G of the pastes before heating and recovered completely the G value of heated gels to that of the control gel at 80 C. The G values before and after heating were almost recovered by the addition of these additives. The walleye pollack surimi paste contains a Ca 2+ -activating transglutaminase, which catalyzes cross-linking of myosin heavy chains during setting, and forms a strong and elastic cooked gel Sodium dodecylsulfate-page (Fig. 11) shows the effect of adding FA to surimi paste on the crosslinking of myosin heavy chains during setting at 25 C. In the absence of Ca 2+, most of the myosin heavy chains remained as a monomer even in the Fig. 9 Effect of added formaldehyde (FA) on the gelation of walleye pollack surimi paste.walleye pollack surimi paste containing 90 mg protein/g, 0.5 M NaCl, 5 mm CaCl 2, and 20 mm Tris-HCl (ph 7.0) with 1 mm FA was allowed to stand for 0, 1, 2, and 8 h at 0 C and then heated to 80 C at 2 C/min including setting, which was adopted at 25 C for 120 min. The numerals in the figure indicate the standing time prior to heating. Co, control surimi paste without FA.

8 612 FISHERIES SCIENCE E Abe et al. Fig. 10 Effect of added cysteine (Cys), histidine (His), and glutathione on the gelation of walleye pollack surimi paste in the presence of formaldehyde (FA).Walleye pollack surimi paste containing 90 mg protein/g, 0.5 M NaCl, 5 mm CaCl 2, 20 mm Tris- HCl (ph 7.0), and 5 mm FA with either 5 mm Cys (C), 15 mm His (H), or 10 mm glutathione (g) was allowed to stand for 1 h at 0 C and then heated to 80 C at 2 C/min including setting at 25 C for 120 min. Co, control surimi paste without FA; f, with 5 mm FA without additives. presence of added FA, except for the addition of 10 mm FA, suggesting that FA only slightly mediated myosin cross-linking at low concentration and short duration. In contrast, in the presence of Ca 2+ the myosin heavy chain decreased and crosslinked regardless of the addition of FA. However, the remaining myosin heavy chain monomer increased with an increase in added FA. The results demonstrated that the added FA inhibited the transglutaminase-mediated myosin cross-linking in the surimi paste. Added FA promoted a strong gel formation with increased G value at 80 C due to the accelerated aggregation of surimi proteins for a short period after the addition of FA (Fig. 9). It is well known that protein denaturation and aggregation are prerequisites for the formation of a three-dimensional gel network. 29 However, long standing with FA induced the excess aggregation or severe denaturation of surimi proteins and then resulted in a decrease in gel-forming ability in addition to the inactivation of transglutaminase. Seki et al. have reported that the denatured surimi protein forms a weak and fragile gel even in the presence of active transglutaminase that is able to cross-link both native and denatured myosins. 28 In the present paper we described that Cys, His, and glutathione effectively protected the FAinduced aggregation of myofibrillar proteins and loss of the gel-forming ability of surimi paste. The protective mechanism is thought to act in the muscle of marine fish during storage because these compounds are always contained in the muscle cells. If a large amount of these compounds can be incorporated into the muscle tissues, especially into minced muscle and surimi where FA is produced from TMAO, FA-induced muscle protein denaturation may be more effectively protected.

9 Formaldehyde-induced denaturation FISHERIES SCIENCE 613 REFERENCES Fig. 11 Effect of formaldehyde (FA) on the cross-linking of myosin during setting. Walleye pollack surimi paste containing 90 mg protein/g, 0.5 M NaCl, and 20 mm Tris- HCl (ph 7.0) with either (a d) 5 mm CaCl 2 or (e h) 5 mm EGTA was incubated for setting at 25 C for 2 h in the presence of various concentrations of FA. a, 0 mm FA; b, 1 mm FA; c, 5 mm FA; d, 10 mm FA; e, 0 mm FA; f, 1 mm FA; g, 5 mm FA; h, 10 mm FA. Sodium dodecylsulfate polyacrylamide gel electrophoresis was performed on a 7.5% polyacrylamide gel. HC, myosin heavy chain; Ac, actin. However, it should be noted that Cys and glutathione are effective reductants promoting both the TMAOase and non-enzymic degradation of TMAO to FA and dimethylamine; if they are added to the muscle containing TMAO, higher amounts of FA will be produced. 4,5 Furthermore, Cys is known as a major precursor of H 2 S, the main components of the peculiar odor of retorted kamaboko. 30 Therefore, much still remains to be done before these compounds can be put to practical use for the prevention of FA denaturation of muscle proteins. In contrast, we feel that His may be useful in preventing the FA-induced protein denaturation because there is no adverse effect in the muscle and surimi during cold storage and subsequent processing. Fish muscle, especially that in red-flesh fish, contains a large amount of free His, which is present more than TMAO. Even if FA is produced from TMAO, it will immediately bind to His. Therefore, FA-induced protein denaturation may be effectively protected in these scombroid fish muscles. 1. Dyer WJ. Amines in fish muscle. VI. Trimethylamine oxide content of fish and marine invertebrates. J. Fish. Res. Bd. Can. 1952; 8: Tokunaga T. Trimethylamine oxide and its decomposition in the bloody muscle of fish. I. TMAO, TMA, and DMA contents in ordinary and bloody muscles. Nippon Suisan Gakkaishi 1970; 36: Harada K. Studies on enzyme catalyzing the formation of FA and dimethylamine in tissues of fish and shells. J. Shimonoseki Univ. Fish. 1975; 23: Kimura M, Seki N, Kimura I. Purification and characterization of trimethylamine-n-oxide demethylase from walleye pollack muscle. Fish. Sci. 2000; 66: Kimura M, Seki N, Kimura I. Enzymic and nonenzymic cleavage of trimethylamine-n-oxide in vitro at subzero temperatures. Nippon Suisan Gakkaishi 2002; 68: Sotelo CG, Pineiro C, Perez-Martin RI. Denaturation of fish proteins during frozen storage: Role of formaldehyde. Z. Lebensm. Unters. Forsch. 1995; 200: Ang JF, Hultin HO. Denaturation of cod myosin during freezing after modification with formaldehyde. J. Food Sci. 1989; 54: Careche M, Li-Chan ECY. Structural changes in cod myosin after modification with formaldehyde or frozen storage. J. Food Sci. 1997; 62: Careche M, Cofrades S, Carballo J, Colmenero FJ. Emulsifying and gelation properties during freezing and frozen storage of hake, pork, and chicken actomyosins as affected by addition of formaldehyde. J. Agric. Food Chem. 1998; 46: Fraenkel-Conrat H, Olcott HS. Reaction of formaldehyde with proteins. V. Cross-linking of amino groups with phenol, imidazole, or indole groups. J. Biol. Chem. 1948; 174: Kitamoto Y, Maeda H. Reevaluation of the reaction of formaldehyde at low concentration with amino acids. J. Biochem. 1980; 87: Galembeck F, Ryan DS, Whitaker JR, Feeney RE. Reaction of proteins with formaldehyde in the presence and absence of sodium borohydride. J. Agric. Food Chem. 1977; 25: Tome D, Kozlowaki A, Mabon F. Carbon-13 NMR study on the combination of formaldehyde with bovine serum albumin. J. Agric. Food Chem. 1985; 33: Sanghani PC, Stone CL, Ray BD, Pindel EV, Hurley TD, Bosron WF. Kinetic mechanism of human glutathionedependent formaldehyde dehydrogenase. Biochemistry 2000; 39: Nash T. The colorimetric estimation of formaldehyde by means of the Hantzsch reaction. Biochem. J. 1953; 55: Gornall AG, Bardawill CJ, David MM. Determination of serum proteins by means of biuret reaction. J. Biol. Chem. 1949; 177: Fiske CH, Subbarow Y. The colorimetric determination of phosphorus. J. Biol. Chem. 1962; 66: Kishi H, Nozawa H, Seki N. Reactivity of muscle transglutaminase on carp myofibrils and myosin B. Nippon Suisan Gakkaishi 1991; 57:

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