牛奶中的十八碳不饱和脂肪酸(C18 unsaturated fatty acid，C18 UFA)，如油酸(C18 ∶ 1 cis-9)、亚油酸(C18 ∶ 2 cis-9, 12)和亚麻酸(C18 ∶ 3 cis-9, 12, 15)具有多种生物学功能[1]。由于存在广泛的瘤胃氢化作用和独特的体内代谢特性且脂肪酸(FA)在体内的代谢存在交互组合效应[2]，通过饲粮途径提高乳脂中上述FA含量的方法并不高效，提高程度并不能达到预期。
FA之间的组合效应对乳脂FA的组成具有较大影响[3-4]，如外源补充不饱和脂肪酸(unsaturated fatty acid, UFA)对乳腺中短链脂肪酸(short and medium-chain fatty acid，SMCFA)合成产生的抑制作用。生产实践中，在饲粮中补充的油料籽实或油脂，其FA种类并不单一，所以FA之间的组合效应影响乳脂合成的现象广泛存在[5]。因此，了解FA之间的组合效应有利于在饲粮中合理添加脂肪，本文就FA之间的组合效应对奶牛乳脂合成的影响进行综述并阐述其机制。
1 乳FA来源及乳脂的合成泌乳奶牛乳腺合成乳脂所需的FA主要依赖从血液中摄取FA前体物。其中摄取的乙酸和β-羟丁酸主要用于从头合成SMCFA，包括4~14碳链长的FA和近1/2的16碳FA，而血液中脂蛋白所携带的脂类或白蛋白所携带的长链脂肪酸(long-chain fatty acid，LCFA)则可以部分被乳腺特异性识别并摄取利用。
乳中的脂类大约96%为甘油三酯。乳脂甘油三酯在乳腺的合成大致可以分解为：血液摄取前体物，SMCFA的从头合成及LCFA的转运，形成酯酰辅酶A，部分饱和脂肪酸(saturated fatty acid，SFA)的去饱和，酯化为甘油三酯，形成脂滴。上述过程需要大量的酶、转录因子的协同调控[6-8]且调控网络较为复杂，如过氧化物酶体增殖物激活受体γ调控途径的上、下游基因均对乳脂合成具有重要调控作用，而该途径中的任何微小变化均会影响乳脂FA[9-10]。
2 FA之间的组合效应对乳脂组成的影响 2.1 SMCFA与LCFA的组合效应从头合成的SMCFA对乳脂合成的贡献高于LCFA[11]。饲粮直接添加SMCFA可以提高乳脂率[12]，供给SMCFA的前体物乙酸和β-羟丁酸也可促进乳脂合成[3]。然而当外源供给SMCFA以增加乳脂产出时，虽然增加了乳SMCFA产量，却常伴随着LCFA含量的异常变化。饲粮添加十六烷酸(C16 ∶ 0)虽然增加了乳中C16 ∶ 0的含量，但降低了C18 ∶ 1 cis-9的含量[13-14]。因为乳中C16 ∶ 0有一部分来自于乳腺从血液中直接摄取，所以有研究固定C16 ∶ 0的添加量，即各组C16 ∶ 0添加量保持一致时在奶牛饲粮中增量添加其他SMCFA混合物，虽然显著增加了乳十四烷酸(C14 ∶ 0)产量，但对C18 ∶ 1 cis-9和C18 ∶ 3 cis-9, 12, 15的产量呈显著抑制作用，而C18 ∶ 2 cis-9, 12的产量并未受到显著影响[12]。
外源供给LCFA对SMCFA的合成也具有抑制作用[15]。有学者总结分析并指出饲粮C18 ∶ 1 cis-9的含量与乳中SMCFA的产量呈负相关关系[5]。饲粮添加试验表明，葵花籽或葵花籽油(富含C18 ∶ 2 cis-9, 12)的饲喂降低了放牧奶牛乳中SMCFA尤其是十二烷酸(C12 ∶ 0)、C14 ∶ 0、C16 ∶ 0的含量[16]。富含C18 ∶ 1 cis-9和C18 ∶ 2 cis-9, 12的大豆油是常用的饲粮LCFA补充剂。饲粮添加4%的豆油降低了乳脂中SMCFA的含量[17]。进一步研究表明，饲粮添加大豆油对乳SMCFA产生的抑制作用具有广泛性[18]。避开瘤胃“发酵”可以避免UFA在瘤胃的大量氢化，从而在一定程度上提高了饲粮UFA向乳UFA的转化效率，然而瘤胃LCFA的供给对SMCFA的产出同样具有显著抑制作用。在灌注试验中，泌乳中期奶牛皱胃增量灌注游离C18 ∶ 1 cis-9或富含C18 ∶ 1 cis-9的菜籽油，线性提高了乳中C18 ∶ 1 cis-9的含量，显著降低了C14 ∶ 0和C16 ∶ 0的含量，而十八烷酸(C18 ∶ 0)含量无显著变化[19]。十二指肠灌注C18 ∶ 3 cis-9, 12, 15或皱胃灌注C18 ∶ 2 cis-9, 12时[20-21]，在提高目标FA含量的同时，均降低了乳中SMCFA的含量。不同的C18 UFA所产生的抑制效应不尽相同，C18 ∶ 2 cis-9, 12对SMCFA的抑制程度比C18 ∶ 1 cis-9更显著，且当二者混合供给时对SMCFA的影响存在交互效应[22]。近年来，有学者提出饲粮中的UFA对SMCFA的抑制不仅发生在外源额外补充，而且广泛存在。换言之，即便不额外补充UFA，饲粮本身包含的UFA也可能对SMCFA的合成具有一定的抑制作用，且这种抑制作用随饲粮中脂肪含量的增加而增加[5]。
2.2 LCFA之间组合效应奶牛乳中含量较高的FA是C16 ∶ 0、C18 ∶ 0和C18 ∶ 1 cis-9，尽管在奶牛体内具有不同的功能和代谢途径，上述FA之间可能具有竞争或互补的互作关系[23]，主要体现在影响奶牛体内能量的分配[24]，同时互作调控乳脂的合成[25]。也有学者证实，不同C18 UFA在乳腺的代谢虽不相同，但其对乳脂合成的贡献并不独立，可能存在互作效应[26-27]。目前关于提高乳C18 UFA的研究，主要聚焦于提高单一目标FA的产出[28-29]，但在增加目标FA的同时却常常伴随着其他C18 UFA含量的不规律变化。瘤胃增量灌注高油酸葵花籽油，灌注剂量从0 g/d升高到500 g/d时，线性提高乳中C18 ∶ 1 cis-9含量的同时也线性降低了其他FA的含量，但C18 ∶ 2 cis-9, 12的含量并没有受到显著影响，且乳脂产量无显著变化[30]。皱胃增量灌注高亚油酸大豆油脂线性提高了乳中C18 ∶ 2 cis-9, 12的含量，尽管大豆油脂中同时包含了23.60%的C18 ∶ 1 cis-9，但并没有改变乳中C18 ∶ 1 cis-9的含量[21]。有研究给奶牛颈静脉混合灌注等量的C18 ∶ 1 cis-9、C18 ∶ 2 cis-9, 12、C18 ∶ 3 cis-9, 12, 15并逐一回撤，发现外源供给的C18 ∶ 2 cis-9, 12降低了乳脂中C18 ∶ 1 cis-9的含量，尽管同时灌注了C18 ∶ 1 cis-9，因此提出C18 ∶ 2 cis-9, 12可能对C18 ∶ 1 cis-9具有抑制效应[4]。也有研究指出，乳C18 ∶ 3 cis-9, 12, 15的产出受饲粮补充C18 ∶ 2 cis-9, 12和鱼油的交互作用影响[16]。
饲粮C18 UFA在瘤胃氢化的中间产物，如不同构型的反式C18 ∶ 1或不同构型的共轭亚油酸(CLA)，代谢至乳腺后，对乳脂的正常合成产生较大影响。研究表明，奶牛皱胃持续6周单独灌注逐步增量的必需脂肪酸(富含C18 ∶ 2 cis-9, 12和C18 ∶ 3 cis-9, 12, 15)、CLA(C18 ∶ 2 cis-9, trans-11和C18 ∶ 2 trans-10, 12)或二者的混合油脂，在灌注第6周时混合灌注组乳中C18 ∶ 3 cis-9, 12, 15的含量显著高于单独灌注必需脂肪酸组，而在第2、4周无此现象。而乳中C18 ∶ 2 cis-9, 12的含量在灌注第4周时，即呈现混合灌注显著高于单独灌注的现象并且持续到第6周。结果提示C18 ∶ 2 cis-9, 12和C18 ∶ 3 cis-9, 12, 15含量与CLA含量之间存在互作效应[31]，但上述研究无法区分C18 ∶ 2 cis-9, 12含量与CLA含量、C18 ∶ 3 cis-9, 12, 15含量与CLA含量或C18 ∶ 2 cis-9, 12含量与C18 ∶ 3 cis-9, 12, 15含量之间可能存在的互作效应。此外，有报道指出，C18 UFA之间的互作效应影响FA在体内的进一步碳链延长和去饱和过程，研究指出，C18 ∶ 2 cis-9, 12摄入过高会抑制C18 ∶ 3 cis-9, 12, 15向超长链n-3 FA的转化，同样的，C18 ∶ 3 cis-9, 12, 15也会抑制C18 ∶ 2 cis-9, 12向超长链n-6 FA的转化[32]，但此报道针对单胃动物猪，而在奶牛的相关研究中未见上述现象的报道，因此需进一步探究。
3 FA之间组合效应的可能机制 3.1 瘤胃氢化过程饲粮中的UFA会改变瘤胃的发酵和氢化过程，这与参与氢化的菌群丰度变化或FA氢化途径改变有关，进而影响氢化产物的组成[22]。有学者提出UFA在瘤胃的氢化过程存在交互作用[33]，如C18 ∶ 1 cis-9可以降低C18 ∶ 3 cis-9, 12, 15在瘤胃的氢化率，C18 ∶ 2 cis-9, 12对二十碳五烯酸(C20 ∶ 5)和二十二碳六烯酸(C22 ∶ 6)的氢化具有抑制作用。上述研究提示若混合供给多种UFA，则氢化产物的组成可能并非遵循多供给即多氢化的预期。氢化过程中的UFA交互作用的机制可能与氢化菌群的变化有关，瘤胃细菌对UFA的氢化作用可能是一种“减毒”行为，且瘤胃细菌对不同UFA如C18 ∶ 2 cis-9, 12和C18 ∶ 3 cis-9, 12, 15的氢化，具有“菌株特异性”[34]。研究指出，与单独添加C18 ∶ 3 cis-9, 12, 15相比，C18 ∶ 1 cis-9和C18 ∶ 3 cis-9, 12, 15的混合添加降低了氢化C18 ∶ 3 cis-9, 12, 15的菌群丰度，进而增加了C18 ∶ 3 cis-9, 12, 15的瘤胃流出率[35]。但也有研究指出，奶牛饲粮混合供给鱼油和植物油，与单独供给鱼油相比改变了UFA的氢化途径，但关键细菌种群并未发生改变[36]。由于涉及氢化的菌群结构复杂，且氢化过程的中间产物繁多，因此瘤胃氢化过程中UFA的交互作用机制需深入全面研究。
3.2 乳腺摄取利用环节乳腺对血液中FA的摄取具有选择性[37]，乳腺脂蛋白脂酶识别血液LCFA时，对乳糜微粒和极低密度脂蛋白所携带的甘油三酯上的FA特异性识别。在多数情况下，饲粮中的C18 ∶ 2 cis-9, 12等多不饱和脂肪酸被肠道吸收后在血液中更多的是以磷脂或胆固醇酯的形式通过脂蛋白运输。然而，当外源额外添加C18 UFA时，则会改变肠道吸收的形式，使进入小肠的C18 UFA在肠上皮细胞更多地被酯化为甘油三酯进而被吸收[38]。此时不同C18 UFA被酯化为甘油三酯的效率并不可知，因此当乳腺特异性识别血液中脂类时，可能因额外供给的不同C18 UFA酯化在甘油三酯的比例不同而出现摄取量不一的情况。此外，乳腺脂蛋白酯酶对血液中脂类sn-1和sn-3位点的FA具有立体选择性[38]，而植物油脂中的UFA则更多的存在于脂类的sn-2位点，因此更易被乳腺舍弃进而以残留脂滴的形式被运输至肝脏或肝脏外组织[39]。不仅如此，乳腺对乳脂前体物的摄取还与FA本身有关。如n-3和n-6系列的多不饱和脂肪酸在乳腺上皮细胞摄取时的细胞表面受体、通路等并不相同[40]，但无法明确不同摄取机制之间是否存在交互关系。由于饲粮添加的脂类中FA种类多且脂类类型不同，因此上述机制均可能影响乳腺对其的摄取利用，但尚需多方面研究证实。
此外，有学者提出，不适宜的饲粮FA组成，即便供给量很少，也可能会诱导乳脂产出降低，若优化饲粮FA组成，则可能有效降低乳脂降低综合征的发生[41]。这表明乳腺在摄取利用前体物合成乳脂的过程可能存在最优摄取利用模式，若外源额外供给大量FA，则会打破该模式进而出现乳中FA非目标性变化。已有研究表明，供给与乳脂FA组成相同的黄油，乳中FA的组成并无显著差异[12, 42]。
3.3 乳腺内乳脂合成环节虽然大量研究提出供给C18 UFA对SMCFA具有抑制作用，但尚无一致的结论来解释其机理。可能的机制是：1)改变了瘤胃发酵状态。饲粮额外供给油脂会抑制瘤胃微生物对纤维的利用，降低了乙酸的产出进而降低了乳腺SMCFA的从头合成[18, 43]。2)油脂中富含的C18 UFA在瘤胃的氢化产生的不完全氢化产物如异构化的反式油酸和亚油酸，其在乳腺通过抑制乙酰辅酶A羧化酶(ACACA)、脂肪酸合成酶(FASN)和固醇调节元件结合蛋白-1进而降低SMCFA的从头合成[44]。有研究侧面证实了SMCFA的受抑制现象发生于乳腺内部，并非血液中SMCFA的前体物(乙酸和β-羟丁酸)供给不足[45]。而且瘤胃后供给试验同样呈现C18 UFA对SMCFA的抑制效应，因此第2种机制得到了较多学者的支持。体外研究也指出，在奶牛乳腺上皮细胞中添加LCFA时，从头合成重要调控基因ACACA和FASN的mRNA表达量显著下调[7]。不仅如此，外源的C18 UFA不仅抑制SMCFA在乳腺的从头合成，还抑制SMCFA酯化为乳甘油三酯[11]。值得注意的是, 有学者指出丁酸(C4 ∶ 0)在乳腺的合成独立于从头合成过程，因此外源LCFA通过抑制从头合成关键酶基因而抑制SMCFA时并不影响C4 ∶ 0的产出[46]。
乳腺硬脂酰辅酶A去饱和酶(SCD)位于乳腺上皮细胞内质网，对甘油三脂合成和维持乳脂流动性具有重要贡献，因此被认为是调控乳FA合成的核心基因之一[6]。细胞中添加C18 ∶ 1 cis-9对SCD基因的表达具有显著下调作用，这可能是因为产物增加所发生的负反馈调节机制所致[7]。然而另有研究报道，奶牛乳腺中SCD对饲粮供给的C18 ∶ 2 cis-9, 12较为敏感，强于C18 ∶ 1 cis-9和C18 ∶ 3 cis-9, 12, 15[47]，相比于饲粮供给菜籽油和亚麻籽油，供给大豆油显著下调了SCD的mRNA表达量。但此现象可能与FA的添加剂量有关，也可能与添加形式有关。饲粮中的UFA在瘤胃的不完全氢化产物C18 ∶ 2 trans-10, cis-12对乳腺SCD基因具有明显的抑制作用，进而降低SCD所介导的SFA去饱和，如降低乳腺C18 ∶ 1 cis-9的产出，反向增加C18 ∶ 0的积聚[44]。此外，添加混合C18 UFA对SCD的影响较添加单一C18 UFA更为复杂，混合添加时，SCD基因表达的变化可能与FA之间的互作效应有关[48]。
乳腺进行甘油三酯合成时，不同FA在甘油三酯不同位点的酯化顺序并不一致且具有一定规律，同时存在不同FA竞争性酯化在同一位点的现象。如C18 ∶ 0、C18 ∶ 1 cis-9易酯化在sn-1, 3位点[49]，而C18 ∶ 2 cis-9, 12和C18 ∶ 3 cis-9, 12, 15在3个位点均易酯化但更多酯化在sn-1, 3位点[50]。绝大部分C4 ∶ 0和己酸(C6 ∶ 0)酯化在sn-3位点[49]。因此，若外源供给额外的FA，则在乳腺甘油三酯酯化环节易出现竞争性的互作效应。C16 ∶ 0在酯化过程中扮演重要角色，作为在sn-1位酯化的首选FA，C16 ∶ 0优先于C18 ∶ 0和C18 ∶ 1 cis-9在该位点的酯化[25]，其还具有刺激甘油-3-磷酸酰基转移酶的活性进而刺激甘油三酯第1步合成的作用[11, 42]，这也是外源供给C16 ∶ 0常能提高乳脂合成的原因之一。由于UFA抑制SMCFA的从头合成，若将C16 ∶ 0与UFA混合添加，可以有效抵消由于UFA带来的乳脂合成降低的现象[51]。
乳腺合成乳脂可能具有自平衡机制，当某些FA如LCFA含量过高时，会自平衡调节SMCFA的产出，其机制可能是为了保持乳的理化特性[52-53]。乳脂熔点的高低可能直接影响乳脂的产出，当乳脂中FA组成变化而导致熔点升高时，可能导致维持乳脂流动性的能力降低进而降低乳脂的合成[34]。乳腺的自平衡机制受LCFA添加剂量的影响，当LCFA含量过高时，平衡机制被打破，则乳中目标LCFA的含量可无限量增加[30]，从而影响乳的基本理化特性。有研究表明，给泌乳奶牛阴外动脉供给混合C18 UFA可以显著改变乳脂FA组成，但乳中C18 ∶ 1 cis-9含量对血液低剂量额外供给表现较不敏感，表观转运效率较低，而C18 ∶ 2 cis-9, 12含量则表现极为敏感，表现为供给即增加的现象[54]。这样的变化可能是由于乳腺对不同FA的耐受剂量不同导致，给奶牛灌注高剂量(500 g/d)富含C18 ∶ 1 cis-9的植物油后，乳中含量可增加至39.23%且乳脂产量无显著变化，该结果提示C18 ∶ 1 cis-9的产出在乳腺生理范围内可能是不受限制的[30, 55]。乳腺可能存在某种阈值，在低于此阈值时，C18 ∶ 1 cis-9摄取的增加并不会改变其合成和分泌，而当摄取量超过此阈值，则其在乳中的含量可以增加到一个较高的水平。
4 小结在实际生产中，为提高乳中功能性FA产量而在饲粮中额外添加油脂或油料籽常发生FA之间的组合效应，该效应广泛存在但关于其机制的研究较少。奶牛乳腺乳脂的合成过程较为复杂，该组合效应的机制研究还需将各环节因素结合起来系统、深入地分析。
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