tissues might contribute a significant
amount to the daily production. Haverberg
et al. (1975) showed that the mixed
proteins in all of the organs sampled
contained detectable levels of bound 3MH.
However, when examining each organ as a
whole, skeletal muscle contained the
majority (98%) of the total amount.
Nishizawa et al. (1977) concluded that the
skin and intestine contributed up to 10%
of the total body pool of 3MH. A study of
humans with short-bowel syndrome
indicated that skeletal muscle was the
major source of urinary 3MH (Long et al.,
1988). In human patients with varying
degrees of infection (Sjölin et al., 1989), it
was concluded that urinary 3MH was a
valid marker of myofibrillar protein break-
down, because it was correlated with the
release of 3MH from the leg. Furthermore,
it was shown later in additional patients
(Sjölin et al., 1990) that there was a signifi-
cant linear relationship between the leg
effluxes of tyrosine, phenylalanine and
3MH and the resulting urinary excretion of
3MH. Therefore, urinary 3MH excretion is
associated with net skeletal muscle protein
breakdown. Our data using the 3MH
kinetic model in portal vein cannulated
swine (van den Hemel-Grooten et al., 1997)
suggest that 3MH production from the
gastrointestinal tract is not increased in
swine fed a protein-free diet. The FBR of
the whole body in swine was 2.16 and 2.56
for controls and those fed a protein-free
diet, respectively, and the percentage from
the gastrointestinal tract was <6% for both
treatments. In conclusion, based on
previous studies, it is reasonable to assume
that changes in 3MH production largely
reflect muscle metabolism.
3-Methylhistidine metabolism in cattle
Cattle, like humans and rats, quantitatively
excrete 3MH in urine. Harris and Milne
(1981b) demonstrated that between 82 and
99% of a [^14 C]3MH dose was recovered
after 6 days in 21- to 98-month-old non-
lactating cows, steers and a bull. Similarly,
McCarthy et al. (1983) recovered 90% of
the injected tracer dose after 120 h in two
heifers. 3MH is excreted in the urine
unchanged, and occurs in muscle extracts
both in the free form (4–10 nmol g^1
muscle) and as a perchloric acid-soluble,
acid-labile form which account for 85% of
the total non-protein-bound 3MH. This
compound was later identified as balenine
(Harris and Milne, 1987), a dipeptide com-
posed of equal molar amounts of -alanine
and 3MH. Balenine was later identified in
muscle extracts of sheep and pigs. There
appears to be an age-related decline in the
concentration of balenine in muscle, but
this did not produce a measurable change
in the recovery of radioactivity in urine.
3MH is present in whole blood of cattle at
concentrations ranging from 2 to 6 nmol
ml^1 blood.
The distribution of 3MH in organs has
been determined for cattle (Holstein)
(Nishizawa et al., 1979). Skeletal muscle
contained 93.4% of the total 3MH in the
analysed cattle tissues. The concentration
of 3MH was 3.5106 μmol of 3MH g^1
muscle protein or 0.587 μmol g^1 wet
muscle in growing steers. This value is
used commonly to calculate the protein-
bound pool of muscle 3MH, when deter-
mining the fractional breakdown rate of
skeletal muscle. Other values for the level
of protein-bound 3MH in the muscle of
cattle have been reported: 5.6 μmol g^1
protein and 1.8 μmol g^1 protein. This
variability is of some concern because, in
most studies, the amount of protein-bound
3MH is not determined.3-Methylhistidine metabolism in sheep
Sheep are unlike cattle in that urinary 3MH
is not a reliable index of muscle protein
breakdown (Harris and Milne, 1980). After
an intravenous dose of^14 C-labelled 3MH,
only 25–50% of the label was recovered
after 7 days. The recovery progressively
increased with the age (4 weeks–7 years) of
the animal, becoming almost quantitative
in the older animals after 3 weeks. The
incomplete recovery was not due to excre-
tion in the faeces or elimination as expired
gas, but was related to the presence of a
muscle pool of non-protein-bound 3MH
which was several times larger than the
expected daily urinary excretion. TheMeasurement and Significance of Protein Turnover 35