Department of Animal and Avian Sciences, University of Maryland, College Park, MD 20742, USA.
J Anim Sci. 2011 Dec;89(12):3945-53. doi: 10.2527/jas.2011-3985. Epub 2011 Jul 15.
The objective was to determine the contributions of glucose to glycogen synthesis and glycerol to glycogen, glucose, and nonessential AA (NEAA) synthesis on embryonic day (e) 14/15 and e19/20. Chicken embryos from small (56.6 ± 0.88 g) and large eggs (71.7 ± 1.09 g) were repeatedly dosed with either [(13)C(3)]glycerol (14 mg/d for 4 d) or [(13)C(6)]glucose (15 mg/d for 3 d) into the chorio-allantoic fluid before blood and tissue collection. (13)C-Mass isotopomer enrichments in blood glucose, liver, and muscle glycogen, and blood and tissue NEAA were analyzed by mass spectrometry. Glucose metabolism did not differ between small- and large-egg embryos. Although glucose entry was 60% less for e20 compared with e15 embryos, e20 embryos conserved glucose more efficiently as a result of 2- to 3-fold greater (P < 0.001) rates of glucose carbon recycling. Importantly, the direct contribution of glucose to liver glycogen synthesis was minimal on e15, and on e20 direct incorporation of glucose into liver glycogen was only 17%. By comparison, [(13)C(3)]glycerol dosing led to the appearance of [M + 1], [M + 2], and [M + 3] isotopomers in blood glucose and in liver and muscle glycogen on e14 and e19. Here, the (13)C-isotopomer enrichments in blood glucose were ~2-fold greater (P < 0.05) in small- than in large-egg embryos on e14 and e19. Furthermore, [(13)C(3)]glycerol dosing led to substantial labeling of [M + 1], [M + 2], and [M + 3] isotopomers of alanine, aspartate, and glutamate in blood and in tissues where (13)C enrichments were greater (P < 0.05) in liver of small-egg embryos. In summary, this study provides unequivocal evidence that glycerol is a precursor for glucose and NEAA synthesis. Furthermore, glycerol, but not egg-derived glucose, is a major substrate for synthesis of liver and muscle glycogen and is an important anaplerotic substrate for the tricarboxylic acid cycle of embryos during later development.
本研究旨在确定葡萄糖对糖原合成、甘油对糖原、葡萄糖和非必需氨基酸（NEAA）合成的贡献，研究时间分别为胚胎第 14/15 天和第 19/20 天。从小蛋（56.6 ± 0.88 g）和大蛋（71.7 ± 1.09 g）中取出鸡胚胎，在收集血液和组织之前，通过绒毛尿囊腔反复向鸡胚胎中注射[(13)C(3)]甘油（4 天，每天 14 mg）或[(13)C(6)]葡萄糖（3 天，每天 15 mg）。通过质谱分析血液葡萄糖、肝脏和肌肉糖原以及血液和组织中非必需氨基酸的（13）C-质量同位素丰度。小蛋和大蛋胚胎的葡萄糖代谢没有差异。尽管与 e15 胚胎相比，e20 胚胎的葡萄糖进入量减少了 60%，但由于葡萄糖碳循环的速率增加了 2-3 倍，e20 胚胎对葡萄糖的利用率更高（P < 0.001）。重要的是，e15 时葡萄糖对肝脏糖原合成的直接贡献微不足道，e20 时葡萄糖直接掺入肝脏糖原仅为 17%。相比之下，[(13)C(3)]甘油处理导致血液葡萄糖以及肝脏和肌肉糖原中出现[M + 1]、[M + 2]和[M + 3]同位素峰，在 e14 和 e19 时均出现。在此，e14 和 e19 时，小蛋胚胎血液葡萄糖中的（13）C-同位素丰度比大蛋胚胎高约 2 倍（P < 0.05）。此外，[(13)C(3)]甘油处理导致血液和组织中丙氨酸、天冬氨酸和谷氨酸的[M + 1]、[M + 2]和[M + 3]同位素峰显著标记，其中小蛋胚胎肝脏中的（13）C 丰度更高（P < 0.05）。综上所述，本研究提供了明确的证据表明甘油是葡萄糖和非必需氨基酸合成的前体。此外，甘油而不是来自鸡蛋的葡萄糖是肝脏和肌肉糖原合成的主要底物，也是后期胚胎三羧酸循环的重要补充剂。
