Ekberg, Karin

To determine the source(s) of blood and very low density lipoprotein (VLDL)-triglyceride glycerol during fasting, four men ingested2H2O from 14 to 20 h into a 60-h fast to achieve ∼0.5% body water enrichment. At 60 h of fasting, glycerol flux was measured using [2-14C]glycerol. Blood was taken for measurement of2H enrichment at carbon 6 of glucose and at carbon 3 of free glycerol and VLDL-triglyceride glycerol.2H enrichment of the 2 hydrogens bound to carbon 3 of VLDL-triglyceride glycerol was 105 ± 2% of the2H enrichment of the 2 hydrogens bound to carbon 6 of glucose, indicating isotopic equilibrium between hepatic glyceraldehyde 3- P and glycerol 3- P. The2H enrichment of the 2 hydrogens bound to carbon 3 of free glycerol was 17 ± 3% of VLDL-triglyceride glycerol, indicating that a significant percentage of free glycerol in blood originated from the hydrolysis of circulating VLDL-triglyceride or a pool of glycerol with similar2H enrichment. Glycerol flux was 6.3 ± 1.1 μmol · kg−1· min−1. Glycerol appearing from nonadipose tissue sources was then ∼1.1 μmol · kg−1· min−1. Seven other subjects were fasted for 12, 42, and 60 h. A small percentage of glycerol in the circulation after 12 h of fasting was enriched with2H. The enrichment of the 2 hydrogens bound to carbon 3 of free glycerol in the longer periods of fasting was ∼16% of the enrichment of the 2 hydrogens bound to carbon 6 of glucose. Therefore, as much as 15–20% of systemic glycerol turnover during fasting is not from lipolysis of adipose tissue triglyceride.

These studies were conducted to understand the relationship between measures of systemic free fatty acid (FFA) reesterification and regional FFA, glycerol, and triglyceride metabolism during fasting. Indirect calorimetry was used to measure fatty acid oxidation in six men after a 60-h fast. Systemic and regional (splanchnic, renal, and leg) FFA ([3H]palmitate) and glycerol ([3H]glycerol) kinetics, as well as splanchnic triglyceride release, were measured. The rate of systemic FFA reesterification was 366 ± 93 μmol/min, which was greater ( P < 0.05) than splanchnic triglyceride fatty acid output (64 ± 6 μmol/min), a measure of VLDL triglyceride fatty acid export. The majority of glycerol uptake occurred in the splanchnic and renal beds, although some leg glycerol uptake was detected. Systemic FFA release was approximately double that usually present in overnight postabsorptive men, yet the regional FFA release rates were of the same proportions previously observed in overnight postabsorptive men. In conclusion, FFA reesterification at rest during fasting far exceeds splanchnic triglyceride fatty acid output. This indicates that nonhepatic sites of FFA reesterification are important, and that peripheral reesterification of FFA exceeds the rate of simultaneous intracellular triglyceride fatty acid oxidation.

Phenylalanine (Phe) kinetics are increasingly used in studies of amino acid kinetics, because the metabolic fate of Phe is limited to incorporation into protein (protein synthesis, Sp) and catabolism via hydroxylation ( Q pt) to tyrosine (Tyr). Besides an infusion of labeled Phe to measure Phe flux ( Q p), a priming dose of Tyr and an independent Tyr tracer are used to measure Tyr flux ( Q t) and Q pt. Alternatively, Q t, Q pt, and Sp can be approximated by using equations, based on Phe and Tyr concentrations in body proteins, that eliminate the need for a Tyr tracer. To evaluate the accuracy of this approach, data were obtained from 12 type I diabetic patients and 24 nondiabetic control subjects who were studied with the full complement of tracers both with and without insulin infusion. Sp approximations closely matched measured values in both groups (mean difference <2%, all values <5%), but the agreement was poor for Q pt (error range = −8 to +43%) and Q t (error range −22 to +41%). Insulin status had no effect on these comparisons. The lower approximation error for Sp vs. Q pt is due to the small contribution (∼10%) of Q pt to Q p. Approximation error for Q pt( r > 0.99) can be explained by variability in the ratio of Tyr to Phe coming from protein breakdown, ( Q t − Q pt)/ Q p. Ideally, all fluxes should be directly measured, but these data suggest that whole body Sp can be approximated with an acceptably small margin of error. However, the same equations do not yield reliably accurate values for Q pt or Q t.

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 ofl-[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⋅ min−1) 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.