Department of Biochemistry and Molecular Biomedicine, University of Barcelona, Faculty of Biology,08028 Barcelona, Spain.
Institute of Biomedicine, University of Barcelona, 08028 Barcelona, Spain.
Nutrients. 2019 Feb 1;11(2):316. doi: 10.3390/nu11020316.
Nitrogen balance studies have shown that a portion of the N ingested but not excreted is not accounted for. We compared several diets (standard, high-fat, high-protein, and self-selected cafeteria) to determine how diet-dependent energy sources affect nitrogen handling, i.e., the liver urea cycle. Diet components and rat homogenates were used for nitrogen, lipid, and energy analyses. Plasma urea and individual amino acids, as well as liver urea cycle enzyme activities, were determined. Despite ample differences in N intake, circulating amino acids remained practically unchanged in contrast to marked changes in plasma urea. The finding of significant correlations between circulating urea and arginine-succinate synthase and lyase activities supported their regulatory role of urea synthesis, the main N excretion pathway. The cycle operation also correlated with the food protein/energy ratio, in contraposition to total nitrogen losses and estimated balance essentially independent of dietary energy load. The different regulation mechanisms observed have potentially important nutritional consequences, hinting at nitrogen disposal mechanisms able to eliminate excess nitrogen under conditions of high availability of both energy and proteins. Their operation reduces urea synthesis to allow for a safe (albeit unknown) mechanism of N/energy excess accommodation.
氮平衡研究表明，摄入的氮有一部分未被排出而无法解释。我们比较了几种饮食（标准、高脂肪、高蛋白和自选自助餐厅），以确定饮食相关的能量来源如何影响氮处理，即肝脏尿素循环。饮食成分和大鼠匀浆用于氮、脂质和能量分析。测定了血浆尿素和个别氨基酸以及肝脏尿素循环酶的活性。尽管氮的摄入量有很大差异，但与血浆尿素的显著变化相比，循环氨基酸实际上保持不变。循环尿素与精氨酸-琥珀酸合成酶和裂解酶活性之间存在显著相关性的发现支持了它们对尿素合成的调节作用，尿素合成是主要的氮排泄途径。循环操作还与食物蛋白质/能量比相关，与总氮损失和估计的平衡形成对比，这些平衡基本上与饮食能量负荷无关。观察到的不同调节机制具有潜在的重要营养意义，表明在能量和蛋白质都高度可用的情况下，存在能够消除多余氮的氮处理机制。它们的运作减少了尿素的合成，从而为过量的氮/能量提供了一种安全（尽管未知）的适应机制。
