1,721,030 research outputs found
A fast high-performance liquid chromatographic method for the measurement of plasma concentration and specific activity of phenylalanine.
No full textA fast high-performance liquid chromatographic method for the measurement of plasma concentration and specific activity of phenylalanine.
No full textA fast high-performance liquid chromatographic (HPLC) method for the measurement in plasma of phenylalanine concentration and specific activity is reported. One-to-two mL of acidified plasma are applied to an ion-exchange resin. The eluted amino acids are enzymatically converted into the corresponding alpha-ketoacids, i.e. phenylalanine is converted into phenylpyruvic acid. After a two-step extraction, phenylpyruvic acid is separated by reverse phase chromatography within 8-10 min. The use of an internal standard allows precise quantitation of plasma concentrations. The radioactivity eluted from the HPLC is divided by the amount injected to yield the specific activity. Concentration and rate of appearance of phenylalanine in man, calculated with the L-[2,6-H-3]phenylalanine tracer, are in the range of published data
Effects of acute systemic hyperinsulinemia on forearm muscle proteolysis in healthy man.
No full textTo investigate the mechanism(s) of insulin-induced suppression of plasma amino acid concentration and release, we studied forearm as well as whole-body leucine and phenylalanine uptake and release during a peripheral insulin infusion in postabsorptive normal subjects using isotope-dilution methods. Before insulin, leucine and phenylalanine release exceeded uptake (P < 0.01 and P < 0.07, respectively). A net output of alpha-ketoisocaproate (KIC) was also observed. During insulin, arterial plasma leucine, KIC and phenylalanine concentrations decreased (P < 0.05 or less vs. basal), despite ongoing net output of these substrates by the forearm, that persisted after correction for the mean transit time spent through the extracellular muscular space. By the end of insulin, whole-body leucine and phenylalanine concentrations and rate of appearance were decreased (P < 0.01 vs. basal). However, release and uptake of both amino acids by the forearm were not significantly decreased vs. the preinsulin values. These data indicate that systemic hyperinsulinemia acutely decreases plasma amino acid concentrations by acting primarily at sites other than skeletal muscle
Effects of acute systemic hyperinsulinemia on forearm muscle proteolysis in healthy man.
No full textRegulation of postprandial whole-body proteolysis in insulin-deprived IDDM.
No full textSuppression of tissue proteolysis is an important mechanism of postprandial protein anabolism, and it may be mediated by insulin, hyperaminoacidemia, or both. To evaluate whether insulin is essential in the regulation of this process, we have investigated the effect of mixed-meal ingestion on whole-body protein breakdown in insulin-deprived insulin-dependent diabetes mellitus (IDDM) patients and normal control subjects. Endogenous phenylalanine and leucine rate of appearance (Ra) from proteolysis were measured at steady-state conditions using a multiple stable isotope technique before and after the constant administration of a synthetic mixed meal. In the postabsorptive state, the IDDM patients exhibited accelerated intracellular leucine Ra (IDDM, 2.64 +/- 0.19 mumol.min-1.kg-1; control, 2.02 +/- 0.08 mumol.min-1.kg-1; P < 0.05) and plasma phenylalanine Ra (IDDM, 0.73 +/- 0.03 mumol.min-1.kg-1; control, 0.61 +/- 0.04 mumol.min-1.kg-1; P < 0.05). During meal ingestion, endogenous phenylalanine and leucine Ra values were suppressed in both the insulin-deficient IDDM (P < 0.05) and control subjects (P < 0.05). Although postmeal endogenous leucine and phenylalanine Ra values remained greater (P < 0.05) in IDDM, the delta changes from the basal endogenous leucine Ra (IDDM, -0.56 +/- 0.11 mumol.min-1.kg-1; control, -0.56 +/- 0.09 mumol.min-1.kg-1) and phenylalanine Ra (IDDM, -0.13 +/- 0.01 mumol.min-1.kg-1; control, -0.14 +/- 0.02 mumol.min-1.kg-1) were similar in both groups. In the IDDM patients, the postmeal increases from the basal leucine concentration were onefold greater (P < 0.05) than in the control-subject
A model of skeletal muscle leucine kinetics measured across the human forearm.
No full textWe propose a new six-compartment model of intracellular muscle kinetics of
leucine and of its transamination product alpha-ketoisocaproic acid (KIC) by
combining systemic tracer infusions of [14C]- and [15N]leucine with the
arterial-deep venous catheterization of the human forearm. Venous [14C]KIC
specific activity (SA) is taken as representative of intracellular [14C]leucine
SA, whereas net [15N]leucine disposal is used to calculate leucine inflow and
outflow across forearm cell membrane(s). In post-absorptive normal subjects,
model-derived rates of intracellular leucine release from and incorporation into
protein were approximately 32% (P = 0.03) and approximately 37% greater (P =
0.025), respectively, than those calculated using a conventional arteriovenous
approach. Forearm fasting proteolysis exceeded protein synthesis (P < 0.025),
whereas leucine oxidation was greater than zero (P < 0.01), suggesting a net
negative leucine (i.e., protein) balance. Leucine inflow from blood to cell
represented approximately 30% of arterial leucine delivery; therefore
approximately 70% of arterial leucine bypassed intracellular metabolism. This
model provides a comprehensive description of regional leucine and KIC kinetics
and new estimates of protein degradation and synthesis across the human forearm
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