渔业科学进展  2023, Vol. 44 Issue (3): 97-110  DOI: 10.19663/j.issn2095-9869.20220213002
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引用本文 

张斐然, 李琳, 毕清竹, 卫育良, 梁萌青, 徐后国. 黄海海域5种底栖型海水鱼类组织脂肪酸组成特征分析[J]. 渔业科学进展, 2023, 44(3): 97-110. DOI: 10.19663/j.issn2095-9869.20220213002.
ZHANG Feiran, LI Lin, BI Qingzhu, WEI Yuliang, LIANG Mengqing, XU Houguo. Lipid Distribution and Fatty Acid Profile of Five Benthic Marine Fish Species in the Yellow Sea[J]. Progress in Fishery Sciences, 2023, 44(3): 97-110. DOI: 10.19663/j.issn2095-9869.20220213002.

基金项目

中国水产科学研究院基本科研业务费(2020TD48)和财政部和农业农村部:国家现代农业产业技术体系(CARS-47)共同资助

作者简介

张斐然,E-mail: zfr15981947065@163.com

通讯作者

徐后国,研究员,E-mail: xuhg@ysfri.ac.cn

文章历史

收稿日期:2022-02-13
收修改稿日期:2022-03-27
黄海海域5种底栖型海水鱼类组织脂肪酸组成特征分析
张斐然 1,2, 李琳 1,2, 毕清竹 1,2, 卫育良 2, 梁萌青 2, 徐后国 2     
1. 上海海洋大学水产与生命学院 上海 201306;
2. 中国水产科学研究院黄海水产研究所 山东 青岛 266071
摘要:本研究测定和评估了青岛地区5种底栖型海水鱼[(黄(Lophius litulon)、鲬(Platycephalus indicus)、长绵鳚(Zoarces viviparu)、角木叶鲽(Pleuronichthys cornutus)和带纹条鳎(Zebrias zebrinus)的形体指标、肌肉和肝脏的粗成分,以及肌肉、肝脏、脑、眼、皮肤、肠道等组织的脂肪酸组成,以全面评估底栖型代表鱼类的脂肪和脂肪酸组成特征。实验鱼每种鱼9尾,3尾一组混样作为一个重复。结果显示,在5种鱼中,肌肉脂肪含量整体偏低,黄和带纹条鳎肌肉脂肪含量尤其低(0.3%~0.4%),但黄、鲬和角木叶鲽具有较高的肝脏脂肪含量(19%~29%),而带纹条鳎和长绵鳚肝脏脂肪含量偏低(4%~5%)。脂肪酸组成方面,黄肌肉中n-3系列长链多不饱和脂肪酸(long chain-polyunsaturated fatty acid, LC-PUFA)含量显著高于其他鱼,尤其以DHA最为明显。黄背肌中还具有最高含量的18:1n-9。长绵鳚背肌脂肪酸组成最明显的特征是较低的16:0含量和较高的20:4n-6及EPA含量。肝脏脂肪酸中,鲬和角木叶鲽具有显著高的18:1n-9等单不饱和脂肪酸含量,但其DHA含量较低。在黄、带纹条鳎和长绵鳚的大多组织中均有较高的DHA、20:4n-6和22:5n-3,但EPA含量较低。研究表明,即使同为底栖型鱼类,不同种类间脂肪和脂肪酸组成差异也非常显著。黄具有显著高的n-3 LC-PUFA含量,脂肪酸营养价值较高。
关键词脂肪    脂肪酸            角木叶鲽    带纹条鳎    长绵鳚    
Lipid Distribution and Fatty Acid Profile of Five Benthic Marine Fish Species in the Yellow Sea
ZHANG Feiran 1,2, LI Lin 1,2, BI Qingzhu 1,2, WEI Yuliang 2, LIANG Mengqing 2, XU Houguo 2     
1. College of Fisheries and Life Sciences, Shanghai Ocean University, Shanghai 201306, China;
2. Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, China
Abstract: Fish are the main source of long-chain polyunsaturated fatty acids (LC-PUFA), in particular docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), which are nutritionally valuable to humans. Moreover, lipid and fatty acid distribution in fish tissues are highly diverse across species. The lipid distribution pattern across tissues determines fatty acid profile, one of the product quality indicators of fish. However, this diversity of lipid distribution and fatty acid profile is not fully known, although several studies have been conducted to compare the nutritive composition of muscle in different fish species in both freshwater and marine environments. This study aimed to comprehensively investigate the lipid distribution and fatty acid profile, as well as the somatic indexes and approximate composition of five benthic marine fish species, namely yellow anglerfish (Lophius litulon), flathead (Platycephalus indicus), eelpout (Zoarces viviparus), East Asian flatfish (Pleuronichthys cornutus), and zebra sole (Zebrias zebrinus). Fishes were purchased from the 15th Street Seafood Market of Qingdao, which were stored on ice since captured in an inshore fishery in Qingdao (located at 119°30'–121°00'E and 35°35'–37°09'N), and then delivered to the market from the fishing harbor in the same morning. After purchase, fishes were kept on ice and immediately taken to the laboratory. The experimental design consisted of nine fish of each species bulked in groups of three, comprising three replicates. First, the total weight and length of each fish were measured, and the liver and viscera weight were recorded to calculate the viscerosomatic index (VSI), hepatosomatic index (HSI), and condition factor (CF). Subsequently, samples of muscle, liver, brain, eye, skin, subcutaneous adipose tissue around fins (only for East Asian flatfish and zebra sole), and intraperitoneal adipose tissue (only for flathead) were collected for lipid and fatty acid analysis. The dorsal muscle sampling position is behind the head on the right body side. The ventral muscle samples were collected under the dorsal muscle sampling spot. The proximate composition analysis of the dorsal muscle, ventral muscle, and liver (three individual samples per group) was performed according to the Association of Official Analytical Chemists standard methods. For the moisture assay, samples were oven-dried at 105 ℃ until they achieved a constant weight. The protein content was assayed by measuring nitrogen (N × 6.25) using the Soxhlet method, the lipids were analyzed using the Soxhlet method (petroleum ether extraction), and the ash by incineration at 550 ℃. The fatty acid compositions of all tissues were analyzed with gas chromatography (GC-2010, Shimadzu, Japan). The results showed that the flat body type of flatfish determines that the gut in these fish species cannot store high lipid contents in the liver or intraperitoneal adipose tissues. Like other flatfish, East Asian flatfish and zebra sole had lower HSI and VSI. In contrast, the HSI and VSI of yellow anglerfish and flathead were associated with high lipid contents in the gut. Additionally, the lipid distribution across tissues indicated that yellow anglerfish and flathead predominantly stored lipid in the liver and intraperitoneal adipose tissue, respectively. The muscle lipid content of all the five species was low, especially for yellow anglerfish and zebra sole (0.3%–0.4%). Yellow anglerfish, flathead, and East Asian flatfish had higher liver lipid content (19%–29%) than zebra sole and eelpout (4%–5%). Yellow anglerfish and flathead store lipid predominantly in the liver and intraperitoneal adipose tissue, respectively. The n-3 LC-PUFA content, especially DHA, in the muscle of yellow anglerfish was significantly higher than that of other species. The yellow anglerfish dorsal muscle also had the highest 18:1n-9 content. The relative abundances of different monounsaturated fatty acids (MUFA) and saturated fatty acids (SFA) in different fish species indicate a preference for energy storage and lipid mobilization. The most distinct fatty acid characteristics of the dorsal muscle of eelpout were low 16:0 content and high EPA content. For liver fatty acids, flathead and East Asian flatfish had significantly high MUFA contents, such as 18:1n-9, but their DHA contents were low. High DHA, 20:4n-6, and 22:5n-3 contents were consistently observed in nearly all yellow anglerfish, eelpout, and East Asian flatfish tissues, respectively, while low EPA content was observed for all of them. Among all five fish species, eelpout had a much higher ARA content than the other species in nearly all tissues. This could be due to the high ARA level in the food sources, namely algae. Yellow anglerfish had significantly high n-3 LC-PUFA content and thus high fatty acids nutritional value. This study revealed that even though these fishes are all benthic species, there are great differences in lipid and fatty acid composition among them.
Key words: Lipid    Fatty acids    Lophius litulon    Platycephalus indicus    Pleuronichthys cornutus    Zebrias zebrinus    Zoarces viviparus    

鱼类是人类食用长链多不饱和脂肪酸(LC-PUFA),尤其是二十二碳六烯酸(DHA, 22:6n-3)和二十碳五烯酸(EPA, 20:5n-3)的主要来源。LC-PUFA在脂质代谢调节和预防心脑血管疾病方面具有重要的生理功能,它们在神经、视觉发育以及免疫调节等生理过程中也发挥着重要作用(Maillard et al, 2018; Xu et al, 2021; Yamagata, 2021; Yu et al, 2021)。

鱼类中脂质和脂肪酸的组织分布在不同的物种中,具有高度多样性(Takama et al, 1994)。组织间的脂质分布模式决定了鱼的许多脂质相关特征,包括一些与鱼肉品质直接相关的特征,如脂肪酸分布、脂质代谢物、胆固醇含量和鱼肉质地等(Katikou et al, 2001)。然而,尽管已经有一些研究比较了不同鱼种(包括淡水和海水鱼种)肌肉的营养成分,但鱼体内脂质分布和脂肪酸组成的多样性还没有被完全了解。目前,已在一些物种之间进行脂质分布模式的比较,主要集中于鲑鳟鱼类,如大西洋鲑鱼(Salmo salar)和虹鳟鱼(Oncorhynchus mykiss)以及常见的淡水物种,如罗非鱼(Oreochromis mossambicus)、鲤鱼(Cyprinus carpio)、鲶鱼(Silurus asotus)和印尼拟松鲷(Datnioides pulcher) (Katikou et al, 2001; Nanton et al, 2007; Wu et al, 2015; Ren et al, 2018; Khieokhajonkhet et al, 2019; 宋红梅等, 2020)。本实验室最近的一项研究也揭示了3种具有不同脂质储存模式的瘦肌型海水鱼类即大菱鲆(Scophthalmus maximus)、红鳍东方鲀(Takifugu rubripes)和花鲈(Lateolabrax japonicus)的不同脂质分布情况。

本研究旨在研究青岛地区(黄海) 5种底栖海水鱼类的脂肪和脂肪酸组成以及形体指标和粗成分等。本研究以黄(Lophius litulon)、鲬(Platycephalus indicus)、长绵鳚(Zoarces viviparu)、角木叶鲽(Pleuronichthys cornutus)和带纹条鳎(Zebrias zebrinus)作为实验对象。这些鱼类味道鲜美、广受欢迎,但它们的脂质和脂肪酸组成还没有被报道。更重要的是,这5种鱼均为底栖型海洋鱼类,本研究进一步阐明鱼类中脂质和脂肪酸分布的多样性,将为揭示底栖型鱼类脂质和脂肪酸代谢生理提供基础数据。

1 材料与方法 1.1 实验材料与样品处理

本实验用鱼在山东省青岛市十五大街农贸市场购买,分别为黄(图 1A)、鲬(图 1B)、长绵鳚(图 1C)、角木叶鲽(图 1D)和带纹条鳎(图 1E),其营养级分别为4.1、4.6、3.2、3.5和3.2,数据来源于相关参考文献和FishBase (Froese et al, 2019; 杨纪明, 2001)。在青岛的近海渔场(119°30ʹ~121°00ʹE和35°35ʹ~37°09ʹN)捕获的鱼在当天早上置于冰上从渔港运送到市场(这5种鱼目前在国内无养殖,全部为近海捕捞野生种群)。购买后,实验鱼被保存在冰上并立即转移到实验室,然后收集组织样本。每个品种的9尾鱼被用于取样,其中,3尾被随机分为一组作为一个重复。采样前,测量每条鱼的重量和长度,同时记录肝脏和内脏的重量,以计算脏体比(VSI)、肝体比(HSI)和肥满度(CF)。

图 1 本研究的5种底栖型海水鱼类 Fig.1 The five benthic marine fish species investigated in this study A: 黄; B: 鲬; C: 长绵鳚; D: 角木叶鲽; E: 带纹条鳎。 A: Yellow anglerfish; B: Flathead; C: Eelpout; D: East Asian flatfish; E: Zebra sole.

随后,取肌肉、肝脏、脑、眼、皮肤、肠道、鳍条周围皮下组织(仅角木叶鲽和带纹条鳎)和腹腔内脂肪组织(仅鲬)的样品进行脂肪和脂肪酸分析。其中,背部肌肉取样位置为将鱼头朝向右,取背部右下方肌肉作为背部肌肉,取腹部相应肌肉作为腹部肌肉。其中,黄、角木叶鲽和带纹条鳎背肌取样大小为4 cm×2 cm×1 cm,鲬和长绵鳚背肌取样大小为3.5 cm× 1.5 cm×1.0 cm。长绵鳚未取得腹部肌肉。所有组织样品立即用液氮冷冻,在分析前储存在–76 ℃。

1.2 组织粗成分和脂肪酸的测定方法

使用AOAC的标准方法,对背肌、腹肌和肝脏(每3尾混一组)的粗成分(%湿重)进行了分析。水分测定为105 ℃烘干至恒重;粗蛋白测定采用凯氏定氮法;粗脂肪测定采用索氏抽提法;粗灰分测定为马弗炉550 ℃灼烧法。肌肉脂肪含量采用氯仿甲醇法进行测定。

所有组织的脂肪酸用气相色谱仪(GC-2010,岛津,日本)进行测定。使用冷冻干燥机将肌肉冷冻干燥24 h,每尾鱼一份组织样品,每种鱼三尾鱼的组织进行混样,然后在72 ℃水浴条件下依次用氢氧化钾–甲醇和氯化氢–甲醇进行甲酯化,用正己烷萃取。取上清液上机测定,采用二氧化硅毛细管柱(SH-RT-2560, 100.00 m×0.25 mm×0.20 μm)和火焰离子检测器。色谱柱升温程序:从150 ℃到200 ℃,升温速率为15 ℃/min;然后,以2 ℃/min的速度从200 ℃升到250 ℃。进样器和检测器的温度均为250 ℃。结果表示为每种脂肪酸相对于总脂肪酸的百分比(%TFA)。

1.3 计算和统计分析

脏体比(VSI)=内脏湿重/鱼体重×100;

肝体比(HSI)=肝脏湿重/鱼体重×100;

肥满度(CF)=鱼体重/(鱼体长3)×100。

所有数据均在SPSS 16.0中进行单因素方差(one-way ANOVA)分析。符合方差齐性检验后,采用Duncan′s检验进行多重比较,计算结果以平均值±标准误(Mean±SE)的形式表示。P < 0.05为显著差异。

2 结果 2.1 形态指标

HSI的差异(P < 0.05)为黄和鲬 > 角木叶鲽 > 带纹条鳎和长绵鳚,VSI的差异(P < 0.05)为黄 > 鲬 > 角木叶鲽和长绵鳚 > 带纹条鳎(表 1)。CF的差异(P < 0.05)为角木叶鲽 > 黄 > 带纹条鳎和鲬 > 长绵鳚。

表 1 5种鱼的形体指标(平均值±标准误) Tab.1 Somatic parameters of five fish species (Mean±SE)
2.2 组织粗成分 2.2.1 粗脂肪

通过粗脂肪结果可知(图 2),鲬的背肌粗脂肪显著高于其他组(P < 0.05),且其他组间无显著差异(P > 0.05)。黄的肝脏粗脂肪含量最高,且依次显著高于鲬、角木叶鲽(P < 0.05),带纹条鳎和长绵鳚肝脏粗脂肪显著低于其他各组(P < 0.05)。各种鱼的腹肌粗脂肪含量无明显差异(P > 0.05),但在角木叶鲽中具有最大值。

图 2 5种鱼的背肌、腹肌和肝脏的粗脂肪含量(湿重%, 平均值±标准误) Fig.2 Crude lipid content in dorsal muscle, ventral muscle and liver of five fish species (% wet weight, Mean±SE) 不同上标字母的同一组织的数据有显著差异(P < 0.05),下同。 Data bars for a same tissue not sharing a same superscript letter were significantly different (P < 0.05), the same as below.
2.2.2 粗蛋白

粗蛋白结果如图 3所示,其中,鲬的背肌和腹肌粗蛋白含量显著高于(P < 0.05)其他组,黄的背肌和腹肌粗蛋白含量显著低于(P < 0.05)其他组,各种鱼在肝脏粗蛋白含量方面无显著差异(P > 0.05)。

图 3 5种鱼的背肌、腹肌和肝脏的粗蛋白含量(湿重%, 平均值±标准误) Fig.3 Crude protein content in dorsal muscle, ventral muscle and liver of five fish species (% wet weight, Mean±SE)
2.2.3 水分

水分含量如图 4所示,黄和长绵鳚背肌水分含量显著高于(P < 0.05)其他组,鲬的背肌水分含量显著低于(P < 0.05)其他组。然而,腹肌水分含量中,黄腹肌水分含量依次高于带纹条鳎、角木叶鲽和鲬,且各组鱼间出现显著差异(P < 0.05)。带纹条鳎肝脏水分含量最高,且显著高于(P < 0.05)除长绵鳚之外的其他各组,黄肝脏水分含量最低。

图 4 5种鱼的背肌、腹肌和肝脏的水分含量(湿重%, 平均值±标准误) Fig.4 Moisture content in dorsal muscle, ventral muscle and liver of five fish species (% wet weight, Mean±SE)
2.3 组织脂肪酸 2.3.1 背部肌肉

背肌的脂肪酸组成如表 2 (图 5图 6)所示。对于饱和脂肪酸(SFA),鲬背肌中总饱和脂肪酸(ΣSFA)含量最高,特别是16:0和14:0。相比之下,长绵鳚的16:0含量最低。

表 2 5种鱼背肌脂肪酸组成(%TFA, 平均值±标准误) Tab.2 The fatty acid composition in dorsal muscle of five fish species (% TFA, Mean±SE)
图 5 5种鱼类背肌中一些重要的脂肪酸组成 Fig.5 Some important fatty acid composition in dorsal muscle of five fish species
图 6 5种鱼的ΣSFA、ΣMUFA、ΣPUFA和ΣHUFA的相对含量 Fig.6 Relative content of ΣSFA, ΣMUFA, ΣPUFA, and ΣHUFA of five fish species SFA: 14:0, 16:0, 18:0 and 20:0; MUFA: 16:1n-7, 18:1n-9, 20:1n-9; PUFA: 18:2n-6, 18:3n-6, 18:3n-3; HUFA: 20:2n-6, 20:4n-6, 20:5n-3, 22:5n-3, 22:6n-3.

对于单不饱和脂肪酸(MUFA),黄和长绵鳚的18:1n-9含量远高于其他鱼,而鲬的16:1n-7含量最低。

的花生四烯酸(ARA,20:4n-6)含量明显高于鲬和角木叶鲽(P < 0.05)。黄的DHA含量比其他鱼种高得多,角木叶鲽的22:5n-3含量高,但EPA含量低。

2.3.2 腹部肌肉

腹肌的ΣSFA无明显差异,但角木叶鲽的16:0含量明显高于黄和带纹条鳎(表 3)。鲬和角木叶鲽的16:1n-7含量明显高于黄和带纹条鳎。其中,角木叶鲽的MUFA含量比其他鱼低。与背肌相似,黄的DHA含量远高于其他鱼种,而角木叶鲽的22:5n-3含量高,但EPA含量低。

表 3 5种鱼腹肌脂肪酸(%TFA, 平均值±标准误) Tab.3 The fatty acid composition in ventral muscle of five fish species (% TFA, Mean±SE)
2.3.3 肝脏

不同鱼种之间的ΣSFA没有显著差异(表 4)。鲬的MUFA含量最高,特别是18:1n-9和16:1n-7,但这些脂肪酸的含量在带纹条鳎和长绵鳚中低得多。长绵鳚的ARA含量是黄和带纹条鳎的近4倍多,是鲬和角木叶鲽的40倍。DHA含量在黄中是最高的,其次是长绵鳚。与在肌肉的情况类似,角木叶鲽的肝脏中22:5n-3含量高,但EPA含量低。

表 4 5种鱼肝脏脂肪酸(%TFA, 平均值±标准误) Tab.4 The fatty acid composition in liver of five fish species (% TFA, Mean±SE)
2.3.4 脑

各物种间脑部脂肪酸组成与肝脏非常相似,带纹条鳎的16:0含量较低,鲬的16:1n-7含量较高,长绵鳚的16:1n-7含量较低,长绵鳚的ARA含量远高于鲬和角木叶鲽,黄的DHA含量最高,其次是长绵鳚,以及带纹条鳎的22:5n-3含量较高,但EPA含量较低(表 5)。

表 5 5种鱼脑脂肪酸(%TFA, 平均值±标准误) Tab.5 The fatty acid composition in brain of five fish species (% TFA, Mean±SE)
2.3.5 眼

带纹条鳎的16:0含量较低,长绵鳚的ARA含量较高,以及角木叶鲽的22:5n-3含量高,但EPA含量低,这些在其他组织中的差异在眼中也同样被发现(表 6)。然而,对于16:1n-7,角木叶鲽中的含量比黄高得多。角木叶鲽在眼睛中的DHA含量最高,其次是黄和长绵鳚,与鲬和带纹条鳎相比,其含量显著较高。

表 6 5种鱼眼脂肪酸(%TFA, 平均值±标准误) Tab.6 The fatty acid composition in eye of five fish species (% TFA, Mean±SE)
2.3.6 皮肤

在皮肤中,黄的ΣSFA,特别是18:0,比其他鱼高(表 7)。鲬16:1n-7含量远高于黄和长绵鳚。ARA依旧在长绵鳚中含量最高。在角木叶鲽和带纹条鳎这两种鲆鲽鱼类中观察到较高的22:5n-3,但EPA含量较低。带纹条鳎的DHA含量显著低于其他鱼类(P < 0.05)。

表 7 5种鱼皮肤脂肪酸(%TFA, 平均值±标准误) Tab.7 The fatty acid composition in skin of five fish species (% TFA, Mean±SE)
2.3.7 鲆鲽鱼鳍附近的皮下脂肪组织和鲬腹腔内脂肪组织

角木叶鲽和带纹条鳎的皮下脂肪组织分别以高16:0和低DHA含量为特征。鲬腹腔内脂肪组织的特点是18:1n-9和EPA含量高(表 8)。

表 8 角木叶鲽和带纹条鳎鳍条附近皮下脂肪组织、鲬腹腔内脂肪组织的脂肪酸组成(%TFA, 平均值±标准误) Tab.8 The fatty acid composition in the subcutaneous adipose tissue around the fin of P. cornutus and Z. zebrinus, intraperitoneal adipose tissue of P. indicus (% TFA, Mean±SE)
3 讨论 3.1 形态指标和脂肪含量

脂质在各组织中的含量可以反映在形体指标上。与其他鲆鲽鱼类一样,角木叶鲽和带纹条鳎的HSI和VSI较低。鲆鲽类的扁平体型决定了这类鱼的肝脏及腹腔内的脂肪组织不能储存较多的脂质,正如在被广泛研究的大菱鲆中观察到的一样(Wu et al, 2015; Xu et al, 2021)。相反,它们在鳍条周围的皮下脂肪组织中具有相对较高的脂肪含量(Wu et al, 2015; Xu et al, 2021)。这就是只对角木叶鲽和带纹条鳎进行鳍条周围皮下脂肪组织采样的原因。相比之下,黄和鲬的HSI和VSI与腹腔中的高脂肪含量有关。从各组织的脂质含量来看,黄和鲬主要是将脂肪分别储存在肝脏和腹腔内的脂肪组织。

一般来说,硬骨鱼有4种典型的脂质储存模式。在这4种模式中,脂质主要分别储存在腹腔内脂肪组织(鲤鱼)、腹腔内脂肪组织+肌肉(大西洋鲑)、肝脏[红鳍东方鲀和大西洋鳕鱼(Gadus morhua)]和皮下脂肪组织(大菱鲆)(Katikou et al, 2001; Kaneko et al, 2016; 毕清竹等, 2020; Xu et al, 2021)。本实验室对红鳍东方鲀进行多年研究,发现其在脂肪酸积累和利用方面具有特殊之处(Xu et al, 2019ab; Yu et al, 2019)。然而,这些特点似乎很少与生态位有关。红鳍东方鲀和黄都是底栖型鱼类,且同样将脂质主要储存在肝脏中,但大西洋鳕却不是。

无论脂质分布模式如何,虽然肝脏和眼可以加工成鱼油产品并被人类间接食用。肌肉仍是人类食用的主要部分。在研究的5种底栖型鱼类中,肌肉脂质含量都很低(˂2%)。这与中上层鱼类不同,中上层鱼类在游泳过程中会在肌肉中储存大量的脂质,如大西洋鲑鱼(7.4%~18.3%)、太平洋蓝鳍金枪鱼(Thunnus orientalis) (7.5%~20.1%)和玉筋鱼(Ammodytes personatus) (4.13%~6.99%) (Tsukamasa et al, 2007; Ornholt-Johansson et al, 2017; 刘胜男等, 2022)。相反,它们的蛋白(鲬、角木叶鲽和带纹条鳎)或水分(黄和长绵鳚)含量反而高。鲬、角木叶鲽和带纹条鳎的蛋白含量与大西洋鲑(9.3%~24.1%)、太平洋蓝鳍金枪鱼(23.28%~25.41%),沙丁鱼(Sardinella gibbosa) (15.43%~22.76%),欧洲鳀(Engraulis encrasicolus) (16.94~17.24%)和竹荚鱼(Trachurus japonicus) (16.82%~28.12%) 相当(Tsukamasa et al, 2007; Kaya et al, 2010; Bagthasingh et al, 2016; Kim et al, 2016; Ornholt-Johansson et al, 2017)。黄肌肉的水分很高,使肌肉非常柔软,这是由于其特有的捕食方式形成的,黄几乎不游泳,因此不需要坚硬的肌肉。这种特殊的肌肉质地在一些地区如日本非常受欢迎。

3.2 脂肪酸组成

最明显的特征是在几乎所有组织中都有非常高的DHA含量。已经广泛证明,鱼类的脂肪酸组成密切反映了其食物中的脂肪酸组成(Xu et al, 2020a)。然而,这5种鱼是在相同的地方捕获的,并占据类似的生态位。因此,这些物种之间的食物选择应该是相似的。考虑到营养级的影响,肉食性鱼类应该比杂食性鱼类和草食性鱼类含有更多的n-3 LC-PUFA (Mauro et al, 2015; 苏程程等, 2021)。然而,黄的营养级甚至低于鲬,但它的DHA含量比鲬高。因此,食物结构应不是黄DHA含量高的主要原因,可能主要是由于这种鱼在生理过程中对DHA的需求较高。考虑到海洋肉食性鱼类一般不能自己生物合成DHA (Tocher, 2003; Tocher et al, 2019),黄中可能存在一种选择性的DHA累积机制。然而,这种有趣的机制仍然是未知的。无论如何,高DHA含量对人类是有益的。DHA在预防心脑血管疾病、脂质代谢调节、生殖和免疫方面的积极作用已被广泛报道(Maillard et al, 2018; Xu et al, 2021; Yamagata, 2021; Yu et al, 2021)。因此,高DHA含量的黄会是一个不错的消费选择。

关于其他LC-PUFA,研究发现角木叶鲽的22:5n-3含量较高,但EPA含量较低,这是值得探究的。我们最近的一项研究显示,海水鱼类中具有高含量的22:5n-3,可能是来源于EPA的延长(Xu et al, 2020a)。考虑到大多数海洋鱼类缺乏有效表达Elovl2和Δ6去饱和酶的能力,22:5n-3进一步生物转化为DHA是有限的(Tocher et al, 2006; Geay et al, 2010; Castro et al, 2016)。本研究表明,角木叶鲽可能有相对较强的将EPA延长为22:5n-3的能力。此外,上述研究显示,鱼类肌肉中的22:5n-3与盐度呈正相关(R2=0.324; P<0.001)。显然5种鱼之间没有盐度等级。因此,需要进一步研究角木叶鲽的C22:5n-3含量高的具体原因。

在5种鱼中,长绵鳚在几乎所有组织中的ARA含量都比其他鱼种高很多。与DHA和EPA相比,食物中ARA的重要性相对被忽视。然而,特别是在过去的20年中,人们越来越关注ARA在鱼类营养和人类健康中的需求和功能(Bell et al, 2003; Astudillo et al, 2012; Stenson, 2014; Xu et al, 2017; Turchini et al, 2022)。在海水草食性鱼类如褐篮子鱼(Siganus fuscescens)中观察到了非常高的ARA水平(Osakoa et al, 2006; Jiarpinijnun et al, 2017),但Shanab等(2018)研究发现,这可能是由于食物来源形成的,即藻类中的高ARA水平。在所研究的5种鱼类中,长绵鳚的营养级是最低的(3.2)。其体内高ARA含量可能是其摄食富含ARA的藻类。此外,ARA的抗逆调节作用也可能解释了一些鱼体中的高ARA含量的原因。例如,在河口环境的地中海沿岸泻湖中,观察到鱼类的高ARA含量(Koussoroplis et al, 2011),高ARA需求可能与环境抗逆需求相关。当花鲈养殖在波动大的温度、氧气和浑浊度下,研究发现,在饲料中添加ARA具有显著的必要性(Xu et al, 2010)。Norambuena等(2012)也有同样发现,在水温发生变化时,塞内加尔鳎鱼(Solea senegalensis)将优先选择富含ARA的饲料,均表明鱼体可能增加对ARA的需求以应对环境胁迫。长绵鳚经常生活在海浪较大的近岸岩石环境中,这一环节胁迫,这可能是这种鱼中ARA含量高的部分原因。

与LC-PUFA相比,主要由18:2n-6和18:3n-3组成的18C的多不饱和脂肪酸(18C-PUFA),在5种鱼中含量不高。18C-PUFA主要来自陆生植物(Griel et al, 2008),因此在海洋鱼类中并不丰富,当然,这些脂肪酸也不是鱼产品消费者的关注点。

关于MUFA和SFA,鲬在腹肌、肝脏、脑、眼和皮肤中的16:1n-7含量高得多,肝脏中18:1n-9含量高,背肌中16:0含量也比其他鱼高。黄和长绵鳚的背肌中18:1n-9的含量也很高。然而,相比之下,长绵鳚的16:1n-7含量非常低,而且在各种组织中16:0含量也很低。MUFA和SFA的相对含量对消费者来说不太重要,因为这些脂肪酸也可以从陆生油中获得,例如牛油、猪油和植物油(Griel et al, 2008)。然而,不同鱼种中,不同的MUFA和SFA的相对丰度表明了鱼类在储存能量和调动脂质方面的偏好。一般来说,脂肪酸动员进行β-氧化的一般优先顺序是MUFA和SFA > LC-PUFA (Turchini et al, 2022),在许多鱼种中,18:1n-9、16:1n-7和16:0可能是脂质分解代谢的最优先底物,而LC-PUFA通过β-氧化的分解代谢代谢的能力较弱(Henderson, 1996; Sargent et al, 2002; Turchini et al, 2022)。然而,在特定饲料和环境条件下,鱼类体内脂肪酸β-氧化的实际程度是复杂的,主要由每种脂肪酸的相对丰度决定(Torstensen et al, 2004; Stubhaug et al, 2007)。脂肪酸供应过剩时,甚至在某些情况下EPA和DHA也会更积极地进行β-氧化以产生能量(Tocher et al, 2002; Stubhaug et al, 2005a、b、20062007; Turchini et al, 2009; Teoh et al, 2011)。本结果表明,鲬进行β-氧化时优先选择C16:1n-7。与之前提到的红鳍东方鲀对SFA的优先选择一致(Xu et al, 2019b2021; Liao et al, 2021),但这类优先选择的机制目前还没有较合理的解释。

研究表明,鲆鲽鱼类鳍周围的皮下脂肪组织和鲬的腹腔脂肪组织是主要的脂质储存场所。鲬的腹腔脂肪组织具有较高的18:1n-9含量,该脂肪酸是已知的三酰甘油合成的主要底物,通常用于能量储存(Szabo et al, 2011)。然而,角木叶鲽和带纹条鳎的皮下脂肪组织中16:0含量比18:1n-9高,表明在这一部位的脂质储存中可能更优先选择16:0。16:0是鱼类β-氧化的首选底物之一(Stubhaug et al, 2006)。此外,16:0在磷脂中也很丰富,特别是在磷脂酰胆碱中,其次是在磷脂酰乙醇胺中(Trushenski et al, 2008),因此,16:0可能在某种程度上被选择性地保留在富含磷脂的组织。虽然没有进行测定,但角木叶鲽和带纹条鳎的皮下脂肪组织中磷脂含量可能很高。

不同组织的脂肪酸差异在不同底栖型鱼类中存在大致相同的趋势。大体上,肌肉中16:0、18:0、EPA和DHA含量较高(胡文静等, 2020; 王腾等, 2021);眼和脑等组织中富含DHA和EPA等长链多不饱和脂肪酸含量;而肝脏、皮下脂肪和腹腔脂肪组织作为脂质储存的场所,其18:1n-9含量较高。不同组织间的脂肪酸组成差别与特定脂肪酸在各组织中的功能密切相关,如肌肉需为游泳运动提供能量,作为氧化供能的重要底物16:0含量则高于其他组织(Stubhaug et al, 2006),此外,肌肉含有大量的磷脂,磷脂中富含16:0和18:0 (Trushenski et al, 2008),因此,16:0和18:0相对高于其他组织;而作为能量储存的组织,肝脏、皮下脂肪和腹腔脂肪组织中脂质以三酰甘油的形式存在,18:1n-9是三酰甘油的主要底物(Szabo et al, 2011),因此,这些组织中18:1n-9较多于其他组织。此外,DHA、EPA和ARA对于神经和视觉发育起着重要作用,由此这些脂肪酸在眼和脑组织中含量较高。

4 结论

本研究中,5种鱼中肌肉脂肪含量都很低,特别是黄和带纹条鳎(0.3%~0.4%)。黄、鲬和角木叶鲽的肝脏中脂质含量很高(19%~29%),但带纹条鳎和长绵鳚较低(4%~5%)。黄和鲬分别主要在肝脏和腹腔内脂肪组织中储存脂质。黄的n-3 LC-PUFA含量,特别是DHA,明显高于其他鱼,而长绵鳚和角木叶鲽的特点分别是20:4n-6含量高和22:5n-3含量高但EPA含量低。鲬背肌的16:0含量非常高,但黄和长绵鳚的18:1n-9含量高。本研究显示,即使均为底栖型鱼类,它们之间的脂肪和脂肪酸组成也存在很大差异。黄的DHA含量很高,具有较高的脂肪酸营养价值。另外,黄作为在脂肪存储部位和脂肪酸组成都具有独特特点的鱼类,为后续的脂肪和脂肪酸代谢多样化研究提供了较好的研究模型。

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