concentration of free 3MH in muscle
ranged from 17 to 120 nmol g^1 of muscle.
This pool in newly synthesized muscle
tissue was maintained by retention of some
of the 3MH released by breakdown of
muscle protein. Only a small proportion of
the 3MH released from protein breakdown
was available for excretion, and the propor-
tion excreted in the urine increased with
the age of the animal. Another non-protein-
bound component of 3MH in muscle is the
dipeptide, balenine (Harris and Milne,
1987), which comprises on average 82%
of the total non-protein-bound 3MH in
muscle.
This percentage does not seem to
change as the animal ages, and the same
proportion was seen in cattle muscle. This
dipeptide appeared to be synthesized in
muscle from free 3MH and was not a
terminal product of protein breakdown.
The enzyme system responsible for the
synthesis of the analogous peptides
carnosine and anserine shows a broad
specificity for histidine and histidine
derivatives (Kalyankar and Meister, 1959).
The occurrence of balenine in sheep
muscle should be considered the norm
rather than a rarity. Sheep also have a
much higher concentration of 3MH in
blood than cattle, with values ranging from
17 to 50 nmol ml^1 of whole blood. The
acid-labile form is present in blood in a
higher proportion than was observed in
cattle, averaging 30% of the total non-
protein-bound 3MH in sheep. The concen-
tration of protein-bound 3MH in sheep was
similar to that in cattle, with an average
concentration from the longissimus dorsi
and leg being 5.8 μmol g^1 protein. Buttery
(1984) reported a value of 0.6 μmol g^1
muscle.
3-Methylhistidine metabolism in pigs
The pig is another species in which the
recovery of radiolabelled 3MH is less than
quantitative (Harris and Milne, 1981a).
From five animals, <21% of the tracer dose
was recovered after 7 days, after which the
recovery was <0.3% day^1. The incomplete
recoveries of radiolabelled 3MH were asso-
ciated with the presence of a large pool of
non-protein-bound 3MH in muscle, the
concentration of which increased with age.
The 3MH in the muscle pool was present
as free 3MH, with values ranging from 4 to
8 nmol g^1 muscle, and as a dipeptide
which constituted >90% of the total non-
protein-bound 3MH. The contribution of
the dipeptide, balenine (Harris and Milne,
1987), increased with age, reaching 99.8%
in older animals, which was 2 μmol of the
total non-protein-bound 3MH g^1 of
muscle tissue at 9 months of age. The
concentration in blood was not as high as
in sheep, but was comparable with that in
cattle, with values in pigs ranging from 6 to
19 nmol ml^1 of blood.
In summary, urinary excretion of 3MH
cannot be used as an index of myofibrillar
protein breakdown in sheep and pigs,
because 3MH is not excreted quantitatively
in the urine. Sheep have elevated levels of
3MH in plasma and muscle, and a dipep-
tide of 3MH is also present at a high
concentration. In pigs, the pool of non-
protein-bound 3MH was maintained and
increased in both established and newly
synthesized tissue by retention of some of
the 3MH released by muscle protein break-
down, only a proportion of which was
available for excretion.
The traditional approach (Fig. 2.8) to
quantitating 3MH production requires
collecting total urinary output for 1–3 days.
The fractional breakdown (day^1 ) rate of
muscle protein rate can be determined by
total urinary 3MH excretion divided by the
total body 3MH muscle pool: 3MH excretion
day^1 ÷ total body 3MH muscle pool. An
estimate of skeletal mass is often difficult to
obtain. The rate of muscle protein break-
down can also be calculated as the rate of
urinary 3MH excretion and its known con-
centration in muscle protein: 3MH excretion
μmol kg^1 day^1 ÷ 3.63 μmol g^1 of muscle
protein = B (g of protein kg^1 of BW day^1 ).
Another approach is to express the data on a
urinary creatinine basis since urinary creati-
nine excretion is a satisfactory estimate of
muscle protein.
The initial studies showing the inade-
quacy of 3MH as an index of muscle
protein breakdown required the intravenous36 J.A. Rathmacher