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1 Endocrine Research Unit, Mayo Clinic and Foundation, Rochester, Minnesota 55905; and 2 Department of Clinical Physiology, Karolinska Institute, S-171 76 Stockholm, Sweden
In human protein turnover studies with
isotopically labeled leucine (Leu) as a tracer, plasma ketoisocaproate
(KIC) enrichment is extensively used as a surrogate measure of
intracellular leucine enrichment. To test how accurately arterial
ketoisocaproate (A-KIC) represents leucine isotopic enrichment in the
hepatic (HV) and femoral veins (FV), which drain liver and muscle beds,
we measured Leu and KIC enrichments in samples collected from HV, FV,
and femoral artery (A) in 24 control and 6 type I diabetic subjects after a primed, continuous infusion of
L-[1-13C,15N]-Leu.
Studies were performed during insulin deprivation or insulin replacement in the diabetic group, whereas the effect of normal saline
or three different doses of insulin infusion (0.25, 0.50, and 1 mU · kg
1 · min1)
were assessed in healthy controls. The ratios of baseline isotopic enrichments of A-KIC to HV Leu and FV Leu were 0.93 ± 0.01 and 0.94 ± 0.02, respectively, in normal subjects and 1.07 ± 0.04 and
1.05 ± 0.03, respectively, in diabetic subjects (P < 0.01, diabetic vs. normal subjects). Insulin did not
change A-KIC-to-HV Leu ratios in either group, but the A-KIC-to-FV Leu
ratio decreased during insulin infusion in normal subjects
(P < 0.05). In conclusion, A-KIC
represents a reliable surrogate measure of HV Leu enrichment at
different levels of circulating insulin in humans. The present data
support the use of A-KIC as a surrogate precursor pool for hepatic
protein synthesis.
ketoisocaproic acid; protein synthesis; precursor pool
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