degradation (Rennie et al. 1982; Cheng et al. 1987;
Pacyet al. 1994). This implies that those amino
acids that are not metabolized in muscle will be
released in proportion to their relative occur-
rence in muscle protein, while a discrepancy will
be found when amino acids are transaminated,
oxidized or synthesized. Human limbs release
much more glutamine (48% of total amino acid
release) and alanine (32%) than would be antici-
pated from the relative occurrence in muscle
protein (glutamine 7% and alanine 9%; Clowes
et al. 1980). This implies that glutamine with two
N-atoms per molecule is dominant for the amino
acid N-release from human muscle. The BCAA
(19% relative occurrence in muscle protein), glu-
tamate (7%), aspartate and asparagine (together,
9%), on the other hand, are not released or in
lower amounts than their relative ocurrence.
Glutamate, in fact, is constantly taken up from
the circulation by skeletal muscle. This suggests
that the BCAA, glutamate, aspartate and
asparagine originating from net breakdown of
muscle proteins and glutamate taken up from the
circulation are metabolized in muscle and used
forde novosynthesis of glutamine and alanine
after overnight starvation. All other amino acids
are released in proportion to their relative ocur-
rence in muscle protein, implying that little or no
metabolism occurs in muscle.
Source of alanine and glutamine carbon
and nitrogen
The next issue to address is whether the carbon
and nitrogen atoms from the six amino acids that
can be degraded in muscle (Fig. 9.1) can be used
for complete synthesis of both glutamine and
alanine or whether other precursors help to
provide some of the required building blocks.
Studies with [^15 N]-leucine have shown that the
amino group of the BCAA is indeed incorporated
in humans in vivoin the a-amino nitrogen of
alanine (Haymond & Miles 1982) and of gluta-
mine (Darmaun & Déchelotte 1991). As gluta-
mate is central in all aminotransferase reactions
in muscle (Fig. 9.1), this implies that the amino
group of all six amino acids is interchangeable
and can be incorporated in the a-amino nitro-
gen of alanine and of glutamine. The source of
ammonia in glutamine synthesis (incorporated
in the amide nitrogen) forms one of the puzzles
in muscle amino acid metabolism remaining
today. A small part is derived from the uptake of
ammonia from the circulation. The positive
femoral arteriovenous difference for ammonia in
man is between 5% and 10% of the glutamine
release in postabsorptive subjects at rest (Eriks-
sonet al. 1985; Van Hall et al. 1995b). Two intracel-
lular enzymatic reactions are main candidates
for the production of the remainder of the re-
quired ammonia. The adenosine monophos-
phate (AMP)-deaminase reaction is not only
involved in the breakdown of adenine
nucleotides to inosine monophosphate (IMP),
but, as proposed by Lowenstein and colleagues,
also in the deamination of aspartate via the reac-
tions of the purine nucleotide cycle (Lowenstein
& Goodman 1978). A second possible source of
ammonia production in muscle is the reaction
catalysed by glutamate dehydrogenase:
glutamate+NAD+ ́a-ketoglutarate+NH 4 +
+NADH
The BCAA indirectly can also be deaminated by
these reactions after transfer via transamination
of the amino group to glutamate and aspartate.
However, both AMP deaminase and glutamate
dehydrogenase have been suggested to have
very low activities in muscle both in vivoand
in vitro(Lowenstein & Goodman 1978). Esti-
mates of limb production rates in the fed and
fasted state nevertheless indicate that between 10
and 25 g of glutamine is synthesized in the com-
bined human skeletal muscles per 24 h, much
more than any other amino acid. This also
implies that there must be a corresponding rate
of ammonia production in muscle.
In vitromuscle incubations and perfusions
with [U-^14 C]-amino acids have led to the general
consensus that the carbon skeletons of the six
indicated amino acids (Fig. 9.1) are used for de
novosynthesis of glutamine (Chang & Goldberg
1978b; Wagenmakers et al. 1985; Lee & Davis
1986). This has been confirmed more recently in