Barrett E J, Revkin J H, Young L H, Zaret B L, Jacob R, Gelfand R A
Department of Medicine, Yale University School of Medicine, New Haven, CT 06510.
Biochem J. 1987 Jul 1;245(1):223-8. doi: 10.1042/bj2450223.
In eight anaesthetized post-absorptive dogs we measured the concentration and specific radioactivity of phenylalanine and leucine in arterial and femoral-venous plasma, together with hindlimb flow during a continuous infusion of L-[ring-2,6-3H]phenylalanine and [1-14C]leucine. The femoral-venous plasma concentration was greater than arterial for both phenylalanine and leucine (P less than 0.05 for each). Despite net amino acid release there was a significant removal of both labelled phenylalanine and labelled leucine. Consequently, a significant dilution of specific radioactivity was observed between artery and vein for both radio-tracers. The uptake of leucine from the arterial circulation by the hindlimb exceeded by 2.6-fold that of phenylalanine; the measured molar ratio of leucine to phenylalanine in hindlimb muscle protein averaged 2.4 +/- 0.1. Since phenylalanine is neither synthesized nor degraded by muscle tissue, the measured removal of tracer and the dilution of tracer specific radioactivity across the hindlimb can be used to estimate rates of phenylalanine incorporation into, and release from, tissue protein. The estimated rate of protein synthesis by hindlimb averaged 644 +/- 250 nmol of phenylalanine/min. This was exceeded by the rate of tissue protein degradation (987 +/- 285 nmol of phenylalanine/min). The present results demonstrate that the dilution of the specific radioactivity of labelled phenylalanine can be readily measured across dog hindlimb. This measurement, coupled with an estimate of tissue blood flow, can provide a readily measured, non-destructive, method for estimation of protein turnover in specific muscle beds in vivo. Measurements can be made repeatedly over time in a single experiment, allowing the study of factors which regulate protein turnover. The method developed here in dogs can be readily extended to clinical studies.
在八只麻醉状态下的空腹犬中，我们在持续输注L-[环-2,6-³H]苯丙氨酸和[1-¹⁴C]亮氨酸的过程中，测量了动脉和股静脉血浆中苯丙氨酸和亮氨酸的浓度及比放射性，同时还测量了后肢血流量。苯丙氨酸和亮氨酸的股静脉血浆浓度均高于动脉血浆浓度（每种氨基酸的P值均小于0.05）。尽管有氨基酸净释放，但标记的苯丙氨酸和标记的亮氨酸均有显著清除。因此，两种放射性示踪剂在动脉和静脉之间均观察到比放射性的显著稀释。后肢从动脉循环中摄取亮氨酸的量比摄取苯丙氨酸的量超出2.6倍；后肢肌肉蛋白中亮氨酸与苯丙氨酸的测量摩尔比平均为2.4±0.1。由于肌肉组织既不合成也不降解苯丙氨酸，因此测量到的示踪剂清除以及后肢示踪剂比放射性的稀释可用于估计苯丙氨酸掺入组织蛋白和从组织蛋白中释放的速率。后肢蛋白合成的估计速率平均为644±250 nmol苯丙氨酸/分钟。这一速率低于组织蛋白降解速率（987±285 nmol苯丙氨酸/分钟）。目前的结果表明，在犬后肢可以很容易地测量标记苯丙氨酸比放射性的稀释情况。这种测量方法，结合组织血流量的估计，可为体内特定肌肉床中蛋白质周转的估计提供一种易于测量的、非侵入性的方法。在单个实验中可以随时间重复进行测量，从而能够研究调节蛋白质周转的因素。这里在犬中开发的方法可以很容易地扩展到临床研究。
