Measurement of leucine metabolism in man from a primed, continuous infusion of L-[1-3C]leucine

Abstract

Leucine metabolism in vivo can be determined from a primed, continuous infusion of L-[1-13C]leucine by measuring, at isotopic steady state, plasm [-13C]leucine enrichment, expired 13CO2 enrichment, and CO2 production rate. With an appropriate priming dose of L-[1-13C]leucine and NaH13CO3, isotopic steady state is reached in less than 2 h, and the infusion is completed in 4 h. The method can determine rates of leucine turnover, oxidation, and incorporation into protein with typical relative uncertainties of 2, 10, and 4%, respectively. The method requires no more than 1 ml of blood and uses stable isotope rather than radioisotope techniques. Thus, the method is applicable to studies of human beings of all ages. L-[1-13C]leucine may be infused with a second amino acid labeled with 15N for simultaneous determination of the kinetics of two amino acids.

Elevations of plasma methylarginines in obesity and ageing are related to insulin sensitivity and rates of protein turnover

“…Glucose turnover was studied using a primed (814 kBq), continuous infusion (8.51 kBq/min) of [3-3 H]glucose, started 180 min prior to insulin and maintained throughout the clamp. At the same time, a primed (0.5 mg/kg), constant infusion of [1-13 C]leucine was started at 0.008 mg kg −1 min −1 , after an oral bolus of 0.1 mg/kg of NaH 13 CO 3, to study leucine kinetics [24]. A primed infusion of human insulin (Humulin R; Eli Lilly Canada, Toronto, ON, Canada) was started at 0 min and continued at a rate of 40 mU m −2 min −1 for at least 210 min.…”

“…Expired air samples were collected into a collection balloon, and transferred to 10-ml tubes (Vacutainer; Becton Dickinson Vacutainer Systems, Franklin Lakes, NJ, USA). Leucine kinetics were calculated according to the stochastic model [24], using plasma α-ketoisocaproic acid as an index of the precursor pool enrichment (reciprocal model). In the postabsorptive state, the endogenous leucine rate of appearance determines protein catabolism and is equivalent to total flux.…”

Protein metabolism in rheumatoid arthritis and aging. Effects of muscle strength training and tumor necrosis factor α

“…Leucine metabolism was calculated using the stochastic model of Matthews et a1 (21). Under this model,…”

Perioperative catabolism

“…the difference between the body's total nitrogen intake and its total nitrogen loss; however, retention of nitrogen within the body and underestimation of nitrogen excretion in urine and other routes (feces, skin, and wound secretion) invariably lead to false positive values. 135,136 Novel tracer methods using metabolic substrates (glucose, amino acids) labelled with stable isotopes ( 2 H, 13 C, 15 N) are considered the technique of choice for the global assessment of catabolism in humans and its relation to protein and energy intake. 137 They provide a dynamic picture about the kinetics of glucose and amino acids on the whole body (protein breakdown, oxidation and synthesis, glucose production and utilization) and the organ-tissue level.…”