The amino flux (Q) is equal to the infusion
rate (F) divided by the enrichment of urea
or ammonia (d), Q = F d^1. Therefore,
synthesis (S) = Q E, where E is the total
excretion and breakdown (B) = Q I, where
I is intake.
Constant infusion of [^13 C]leucineThe [^13 C]leucine method was developed
based on common characteristics of the
[^15 N]glycine method (Golden and Waterlow,
1977). The model is illustrated in Fig. 2.5.
Leucine is an essential amino acid, there-
fore it is not produced in vivo. The first
metabolite of leucine is -ketoisocaproate
(KIC). Leucine and KIC are intercon-
vertable by a reversible transamination
reaction. Both leucine and KIC have meta-
bolic pools in plasma and inside the cell.
Leucine enters the cell from protein break-
down and leaves through oxidative
disposal (CO 2 ) and non-oxidative disposal
(protein synthesis). These processes take
place in all tissues and may exhibit
different characteristics.
The difference between the leucine
protein turnover model and the [^15 N]-
glycine model are that the kinetics of the
amino acid are measured directly. The dif-
ficulty with this model is the extrapolation
of leucine kinetics to rates of protein break-
down and synthesis. For a further discus-
sion, the advantages, limitations and
difficulties associated with leucine meta-
bolism can be found elsewhere (Waterlow
et al., 1978; Matthews and Bier, 1983; Bier,
1989). The original model proposed by
Waterlow is illustrated in Fig. 2.6. Leucine
was selected as the essential amino acid of
choice because it is readily available
cheaply in a pure form (L-leucine). In addi-
tion, when leucine is isotopically labelled
as [1^13 C]leucine or [1^14 C]leucine, the
label is completely removed as CO 2 (the
first irreversible step). The model in Fig.
2.6 is resolved by infusing (constant) label
leucine (L-[1^13 C]leucine) into the blood
stream until an isotopic steady state is
reached in plasma. The measurements
taken are the dilution of tracer by
unlabelled leucine and the rate of labelled
CO 2 excretion in the breath. The dilution of
tracer defines the rate of appearance of
leucine in plasma. The labelled CO 2 excre-
tion divided by the leucine tracer infusion
rate defines the oxidation rate (C). The
breakdown rate (B) in the post-absorptive
rate is equal to Q (leucine flux, leucine
infusion/isotopic enrichment), and syn-
thesis(s) is S = Q C. In summary, neither
Q or C protein are measured directly, and B
and S are extrapolated from them. Other
difficulties associated with this simple
model are that the body does not have a
single leucine pool or a single pool of
protein entering and leaving it. In addition,
the leucine tracer is infused into and
sampled from blood, but leucine protein
metabolism occurs within the cell. Leucine
is transanimated (a reversible reaction)
inside cells to KIC. The KIC may suffer one
of three fates: it may be decarboxylated,30 J.A. Rathmacher
Fig. 2.5.Illustration of metabolism of leucine and -ketoisotocaproate (KIC).
Plasma LeucineIntracellular LeucineProteinOxidationKICKICCO 2