NUTRITION IN SPORT

In vitromuscle amino acid metabolism

Rat muscles incubated in vitroare in net protein
breakdown (protein synthesis <protein degra-
dation) and release amounts of glutamine and
alanine in excess by far of the relative occurrence
of these amino acids in muscle protein. This sug-
gests that de novosynthesis of these amino acids
occurs (Chang & Goldberg 1978b). Ruderman
and Lund (1972) were the first to observe that
addition of BCAA to the perfusion medium of rat
hindquarters increased the release of alanine and
glutamine. The relationship between the metabo-
lism of BCAA on the one hand and the release of
alanine and glutamine has since been the subject
of many studies (for reviews, see Goldberg &
Chang 1978; Wagenmakers & Soeters 1995). Most
of this relationship today has been firmly estab-
lished. In the BCAA aminotransferase reaction,
the amino group is donated to a-ketoglutarate to
form glutamate and a branched-chain a-keto
acid (Fig. 9.1). In the reaction catalysed by gluta-
mine synthase, glutamate reacts with ammonia
to form glutamine. Alternatively, glutamate may

120 nutrition and exercise

donate the amino group to pyruvate to form

alanine and regenerate a-ketoglutarate. These

reactions provide a mechanism for the elimina-

tion of amino groups from muscle in the form of

the non-toxic nitrogen carriers alanine and gluta-

mine (Fig. 9.1).

Arteriovenous difference studies in

postabsorptive man

Muscle amino acid metabolism has also been

investigated in man in vivoin the resting state

and during exercise by measuring the exchange

of amino acids across a forearm or a leg (arteri-

ovenous difference multiplied by blood flow

gives the net exchange of amino acids; e.g. Felig

& Wahren 1971; Marliss et al. 1971; Wahren et al.

1976; Eriksson et al. 1985; Van Hall et al. 1995b;

Van Hall 1996). As muscle is the largest and most

active tissue in the limbs, the assumption that

limb exchange primarily reflects muscle metabo-

lism seems reasonable. After overnight fasting

there is net breakdown of muscle proteins as

protein synthesis is slightly lower than protein

Tricarboxylic acid

cycle

α-ketoglutarate

Succinyl-CoA

Valine

α-KIV

Isoleucine α-KMV

α-KG Glu

NH (^3) Aspartate
Asparagine Leucine
α-KIC
Acetyl-CoA
α-KG
Glu
Isoleucine
α-KMV
Alanine
Pyruvate
Glycogen
or glucose
Glutamate
Glutamine
NH 3
Source?
Oxaloacetate
Fig. 9.1Amino acid metabolism in
muscle.a-KG,a-ketoglutarate;
a- KIC, a-ketoisocaproate; a-KIV,
a-ketoisovalerate;a-KMV, a-keto-
b-methylvalerate; CoA, coenzyme
A.