Imura K, Walser M
Department of Pharmacology and Molecular Sciences, John Hopkins University School of Medicine, Baltimore, MD 21205.
Metabolism. 1988 Jun;37(6):591-6. doi: 10.1016/0026-0495(88)90177-1.
In the steady state, the ratio of the rate of utilization for whole-body protein synthesis of any essential amino acid to its rate of oxidation should be the same as the ratio of the peak fraction of a dose of tracer incorporated into protein (F) to the fraction oxidized (1-F) provided that negligible tracer remains in the free amino acid pool or remains unabsorbed in the gut. The total rate of amino acid catabolism (C) can be estimated from the rate of urinary excretion of urea nitrogen (N) plus ammonia N. Hence the rate of whole-body protein synthesis (S) can be estimated as CF/(1-F). This method, which is not new, was explored as follows: (1) Radioactivity in the leucine of whole-body protein of rats after intravenous (IV) injection of labeled leucine was shown to plateau from three to nine hours. (2) The fractions of labeled leucine, valine, and methionine remaining in the gut six hours after enteral injection were 1.2 +/- 0.4% (SD), 1.2 +/- 0.4%, and 7.1 +/- 2.9%, respectively; thus, enterally administered methionine cannot be used for this purpose. (3) Oxidation of [1-14C]-labeled leucine or valine, measured from 14CO2 excretion, was found to be the same whether these isotopes were given IV or enterally. (4) Negligible radioactivity remained in the free leucine of plasma within one hour after injection of labeled leucine.(ABSTRACT TRUNCATED AT 250 WORDS)
在稳态下，任何必需氨基酸用于全身蛋白质合成的利用率与其氧化速率之比，应与掺入蛋白质的示踪剂剂量的峰值分数（F）与氧化分数（1 - F）之比相同，前提是游离氨基酸池中残留的示踪剂可忽略不计或在肠道中未被吸收。氨基酸分解代谢的总速率（C）可根据尿素氮（N）加氨氮的尿排泄率来估算。因此，全身蛋白质合成速率（S）可估算为CF/(1 - F)。这种方法并非新方法，其探究如下：（1）静脉注射标记亮氨酸后，大鼠全身蛋白质中亮氨酸的放射性在3至9小时达到平稳。（2）肠内注射6小时后，肠道中残留的标记亮氨酸、缬氨酸和蛋氨酸的分数分别为1.2±0.4%（标准差）、1.2±0.4%和7.1±2.9%；因此，肠内给予的蛋氨酸不能用于此目的。（3）通过14CO2排泄测量，发现无论[1-14C]标记的亮氨酸或缬氨酸是静脉注射还是肠内给予，其氧化情况相同。（4）注射标记亮氨酸后1小时内，血浆游离亮氨酸中的放射性可忽略不计。（摘要截断于250字）
