Iverson S J, Kirk C L, Hamosh M, Newsome J
Department of Pediatrics, Georgetown University Medical Center, Washington, D.C. 20007.
Biochim Biophys Acta. 1991 Apr 24;1083(1):109-19. doi: 10.1016/0005-2760(91)90131-z.
Intragastric lipolysis may be particularly important for the digestion of milk lipid since milk fat globules are resistant to pancreatic lipase without prior disruption; milk bile salt stimulated lipase (BSSL) may supplement further intestinal hydrolysis. Previous information on gastric lipolysis has been based primarily on in vitro studies using artificial lipid emulsions containing a single component fatty acid and have focused on the preferential release of medium-chain fatty acids. The actual contribution of these enzymes to overall fat digestion in vivo on natural substrates has rarely been studied, however. The neonatal dog is an excellent model in the study of lipid digestion because, like the human, milk lipids are high in long-chain unsaturated fatty acids, milk contains BSSL and gastric lipase is the predominant lipolytic enzyme acting in the stomach. We used a combination of in vivo studies with in vitro incubations to investigate digestion of milk lipid by gastric and milk (BSSL) lipases in the suckling dog. In the first 4 weeks postpartum, 14-41% and 42-60% of milk triacylglycerol was hydrolyzed to primarily diacylglycerol and free fatty acid (FFA) in the first 30 and 60 min in the stomach, respectively. Milk lipid contained high levels (63%) of long-chain unsaturated fatty acids, which were preferentially released as FFA during in vivo gastric lipolysis, consistent with the actions and stereospecificity of gastric lipase. While levels of hydrolysis in gastric aspirates were significantly different (by age and time in stomach) at the start of in vitro studies, total hydrolysis in all incubation systems plateaued at about 65%, suggesting product inhibition by the long-chain FFA, but to a much lesser degree than previously expected from in vitro studies. The magnitude of in vivo intragastric lipolysis was 3- to 6-times greater than that predicted by in vitro assays using either milk lipid or labeled emulsion as substrate, respectively. Prior exposure to intragastric lipolysis resulted in 30% hydrolysis by BSSL compared to 5% hydrolysis without prior exposure. We suggest that previous in vitro studies have largely underestimated the actual degree of intragastric lipolysis that can occur and its activity on long-chain fatty acids; this study indicates the importance of the combined mechanisms of gastric lipase and BSSL to fat digestion in the suckling neonate.
胃内脂肪分解对于乳脂的消化可能尤为重要，因为乳脂肪球在未经预先破坏的情况下对胰脂肪酶具有抗性；乳胆汁盐刺激脂肪酶（BSSL）可能会进一步补充肠道水解作用。先前关于胃内脂肪分解的信息主要基于使用含单一成分脂肪酸的人工脂质乳剂的体外研究，并且集中于中链脂肪酸的优先释放。然而，这些酶对天然底物在体内整体脂肪消化的实际贡献鲜有研究。新生犬是脂质消化研究中的一个优秀模型，因为与人类一样，乳脂富含长链不饱和脂肪酸，乳汁中含有BSSL，且胃脂肪酶是在胃中起作用的主要脂肪分解酶。我们结合体内研究与体外孵育来研究哺乳犬中胃脂肪酶和乳（BSSL）脂肪酶对乳脂的消化作用。产后前4周，分别在胃内的最初30分钟和60分钟内，14% - 41%和42% - 60%的乳三酰甘油被水解为主要是二酰甘油和游离脂肪酸（FFA）。乳脂含有高水平（63%）的长链不饱和脂肪酸，这些脂肪酸在体内胃内脂肪分解过程中优先以FFA形式释放，这与胃脂肪酶的作用和立体特异性一致。虽然在体外研究开始时胃吸出物中的水解水平因年龄和在胃内的时间而有显著差异，但所有孵育系统中的总水解率在约65%时趋于平稳，这表明长链FFA存在产物抑制作用，但程度远低于先前体外研究的预期。体内胃内脂肪分解的程度分别比使用乳脂或标记乳剂作为底物的体外测定预测值大3至6倍。与未经预先暴露相比，预先暴露于胃内脂肪分解会使BSSL导致30%的水解，而未经预先暴露时为5%。我们认为，先前的体外研究在很大程度上低估了可能发生的胃内脂肪分解的实际程度及其对长链脂肪酸的活性；本研究表明胃脂肪酶和BSSL联合机制对哺乳新生儿脂肪消化的重要性。
