Farm Animal Metabolism and Nutrition

reaminated to leucine or it may leave the
cell. Because KIC is only found in the cell
from leucine transamination, plasma KIC
reflects the intracellular KIC enrichment
and can be used as an index of intracellular
leucine tracer enrichment. This approach
expands the single-pool model into a four-
pool model. The calculations are made by
substituting the leucine isotopic enrich-
ment for the KIC enrichment (Fig. 2.7).
This approach is commonly called the
‘reciprocal-pool’ approach (measurement
of the tracer in the metabolite opposite to
the infused) (Schwenk et al., 1985). There
are still drawbacks that remain: (i) there are
still multiple intracellular sites in the body,
and it is not known what contribution they
make to plasma KIC, and (ii) there are a
variety of proteins in the body turning over
at different rates.
Finally, Cobelli et al. (1991) designed a
ten-compartment model, which involves
simultaneously infusing dual tracers of
leucine and KIC and measuring the result-
ing four enrichment curves in plasma and
one in expired air. This model accounted
for the complexity of the leucine system,
but the authors failed to adopt a multi-
tissue scheme for slow and fast kinetic

events (liver as compared with muscle).

These data demonstrated that there was no

single intracellular pool for leucine and

KIC. However, this model is mathematically

difficult and is not solved easily. This

model has not been used in an experimental

design where protein metabolism is altered.

Its value may be to test the structural errors

of simpler but commonly used models.

Measurement of Tissue Protein

Metabolism in Vivo

Currently, protein synthesis can be

measured directly by two approaches

(Garlick et al., 1994; Rennie et al., 1994), i.e.

constant infusion or the ‘flooding dose’ of a

labelled amino acid. The fractional rate of

synthesis of a protein or mixture of proteins

can be measured from an estimate of the

change in incorporation of a labelled amino

acid into protein over time in tissues.

Measurement and Significance of Protein Turnover 31

Protein

B S

I C

Leucine

Protein

S B

[1
^13 C]KIC

[1
^13 C]Leucine

[1
^13 C]Leucine

[1
^13 C]KIC

(^13) CO 2
Isovaleryl CoA
Tracer Infusion
([1
^13 C]leucine)
Intracellular Plasma
Fig. 2.6.Original model for leucine kinetics of an
essential amino acid and its relationship to protein
turnover. B = breakdown; S = synthesis (non-
oxidative disposal); I = dietary intake; C =
oxidation. Under steady-state conditions, leucine
flux is Q = I + B = S + C. When I = O (post-
absorptive), B = Q. Whole-body synthesis is
calculated as S = Q
C.
Fig. 2.7.The current model of whole-body
synthesis of [1
^13 C]leucine. The infused tracer
reached an equilibrium with intracellular leucine
and KIC. The use of plasma KIC enrichment allows
for the calculation of the total intracellular rate of
appearance and the correct calculation of leucine
oxidation. Breakdown (B) = F/[1
^13 C]KIC
enrichment; oxidation (C) =^13 CO 2 /[1
^13 C]KIC
enrichment; synthesis (S) = B
C.