Lane M A, Baldwin R L, Jesse B W
Department of Animal Sciences, Rutgers--The State University of New Jersey, New Brunswick 08903, USA.
J Anim Sci. 2002 Jun;80(6):1538-44. doi: 10.2527/2002.8061538x.
Ketogenesis is the conversion of acetyl-CoA to the ketone bodies acetoacetate and beta-hydroxybutyrate (BHBA). In hepatic ketogenesis, which occurs during fasting in both nonruminant and ruminant animals, the source of acetyl-CoA is the mitochondrial oxidation of predominantly long-chain fatty acids. In the mature, fed ruminant animal, the ruminal epithelium is also capable of producing ketone bodies. In this case, the source of acetyl-CoA is the mitochondrial oxidation of butyrate produced by the microbial fermentation of feed. The purposes of this study were to determine ontogenic and dietary effects on ketogenic enzyme gene expression in developing lamb ruminal epithelium. Twenty-seven conventionally reared lambs and twenty-seven milk-fed lambs were slaughtered between 1 and 84 d of age. Six additional milk-fed lambs were weaned (the fed group) or maintained on milk replacer with a volatile fatty acid gavage (the VFA group) until 84 d of age. At slaughter, total RNA was extracted from samples of ruminal epithelium. The expression of the genes encoding acetoacetyl-CoA thiolase, the first enzyme in the ketogenic pathway, and 3-hydroxy-3-methylglutaryl-CoA synthase, the rate-limiting enzyme in the ketogenic pathway in nonruminant liver, were examined. Acetoacetyl-CoA thiolase and 3-hydroxy-3-methylglutaryl-CoA synthase mRNA concentrations increased with age independent of diet. 3-Hydroxy-3-methylglutaryl-CoA synthase mRNA levels in ruminal epithelium obtained from milk-fed lambs were low before 42 d of age, but a marked increase occurred by 42 d of age. At 84 d of age, there were no differences in acetoacetyl-CoA thiolase and 3-hydroxy-3-methylglutaryl-CoA synthase expression due to diet. The pattern of the expression of these genes, in particular, 3-hydroxy-3-methylglutaryl-CoA synthase, parallels the rate of production of BHBA by rumen epithelial cells isolated from the same lambs, which increased to conventionally reared adult levels at 42 d of age and did not differ with diet. In conclusion, development of the ketogenic capacity of the ruminal epithelium occurs as the animal ages, regardless of dietary treatment. Thus, the expression of the genes encoding the ketogenic enzymes are not affected by the presence of VFA in the ruminal lumen.
生酮作用是指将乙酰辅酶A转化为酮体乙酰乙酸和β-羟基丁酸（BHBA）。在非反刍动物和反刍动物禁食期间发生的肝脏生酮作用中，乙酰辅酶A的来源主要是长链脂肪酸的线粒体氧化。在成熟的、进食后的反刍动物中，瘤胃上皮也能够产生酮体。在这种情况下，乙酰辅酶A的来源是饲料微生物发酵产生的丁酸的线粒体氧化。本研究的目的是确定发育中的羔羊瘤胃上皮中生酮酶基因表达的个体发育和饮食影响。27只常规饲养的羔羊和27只人工哺乳的羔羊在1至84日龄之间屠宰。另外6只人工哺乳的羔羊断奶（喂食组）或用挥发性脂肪酸灌胃维持人工乳喂养（VFA组）至84日龄。屠宰时，从瘤胃上皮样本中提取总RNA。检测了编码生酮途径中的第一种酶乙酰乙酰辅酶A硫解酶以及非反刍动物肝脏生酮途径中的限速酶3-羟基-3-甲基戊二酰辅酶A合酶的基因表达。乙酰乙酰辅酶A硫解酶和3-羟基-3-甲基戊二酰辅酶A合酶的mRNA浓度随年龄增加而增加，与饮食无关。42日龄前，人工哺乳羔羊的瘤胃上皮中3-羟基-3-甲基戊二酰辅酶A合酶的mRNA水平较低，但到42日龄时显著增加。在84日龄时，饮食对乙酰乙酰辅酶A硫解酶和3-羟基-3-甲基戊二酰辅酶A合酶的表达没有影响。这些基因，特别是3-羟基-3-甲基戊二酰辅酶A合酶的表达模式，与从同一只羔羊分离的瘤胃上皮细胞产生BHBA的速率相似，后者在42日龄时增加到常规饲养成年水平，且不受饮食影响。总之，无论饮食处理如何，瘤胃上皮生酮能力的发育随着动物年龄的增长而发生。因此，编码生酮酶的基因表达不受瘤胃腔中挥发性脂肪酸存在的影响。
