Vincent N, Martin G, Baverel G
INSERM U 80, Laboratoire de Physiologie Rénale et Métabolique, Faculté de Médecine Alexis Carrel, Lyon, France.
Biochim Biophys Acta. 1989 Nov 20;1014(2):184-8. doi: 10.1016/0167-4889(89)90032-3.
When hepatocytes suspensions obtained from whole livers of 48-h-fasted rats were incubated in Krebs-Henseleit buffer with a near-physiological concentration (1 mM) of L-[1-14C]glutamine as substrate, the apparent removal of glutamine was low, but the release of 14CO2 was much larger than the enzymatically measured removal of glutamine. This indicates that glutamine was metabolized at rates much higher than those accounted for by the apparent removal of glutamine. This also suggests that glutamine utilization was, at least in part, masked by concomitant synthesis of glutamine from endogenous substrates via glutamine synthetase. Evidence that such synthesis occurred was obtained by: (i) addition of methionine sulfoximine, an inhibitor of glutamine synthetase, which caused a large increase in the apparent removal of glutamine; and (ii) measurement of the specific radioactivity of L-[1-14C]glutamine which was shown to decrease during incubation. Addition of vasopressin (10(-7) M) led to a marked increase in glutamine removal by a dual mechanism: it accelerated flux through glutaminase, the enzyme which initiates the hepatic degradation of glutamine, and inhibited flux through glutamine synthetase.
当用1 mM这种接近生理浓度的L-[1-¹⁴C]谷氨酰胺作为底物，在Krebs-Henseleit缓冲液中孵育从禁食48小时大鼠的全肝中获得的肝细胞悬液时，谷氨酰胺的表观清除率较低，但¹⁴CO₂的释放量远大于通过酶法测定的谷氨酰胺清除量。这表明谷氨酰胺的代谢速率远高于谷氨酰胺表观清除率所反映的速率。这也表明，谷氨酰胺的利用至少部分被内源性底物通过谷氨酰胺合成酶合成谷氨酰胺的过程所掩盖。通过以下方式获得了这种合成发生的证据：（i）添加谷氨酰胺合成酶抑制剂甲硫氨酸亚砜胺，这导致谷氨酰胺的表观清除率大幅增加；（ii）测量L-[1-¹⁴C]谷氨酰胺的比放射性，结果显示在孵育过程中其比放射性降低。添加血管加压素（10⁻⁷ M）通过双重机制导致谷氨酰胺清除率显著增加：它加速了通过谷氨酰胺酶的通量，谷氨酰胺酶是启动肝脏中谷氨酰胺降解的酶，并且抑制了通过谷氨酰胺合成酶的通量。
