Farm Animal Metabolism and Nutrition

some disadvantages. The animal is often
killed to collect samples or you must be
able to take biopsies of the tissues, and the
procedures themselves may alter protein
turnover. Therefore, some researchers use
an arterio-venous difference technique to
define the net balances of amino acids. The
balance of any amino acid within an organ
or tissue is the result of the same processes
that apply to the body as a whole, where
amino acid net balance = input
amino
acid catabolism = protein synthesis

proteolysis.
All of the components of the equation
can be determined from the measurements
of amino acid concentration, label uptake
across a tissue, and blood flow. This
method requires the constant infusion of a
tracer amino acid. Both leucine (Pell et al.,
1986) and phenylalanine (Barrett et al.,
1987) tracers have been used, along with
measurements of arterial and venous iso-
tope enrichment, concentrations, metabolic
output and blood flow. The technique
measures the difference between total label
uptake and irrevocable catabolism to pro-
tein synthesis, the difference between net
uptake and irrevocable catabolism to pro-
tein deposition, and the difference between
protein synthesis and deposition to
degradation. Using phenylalanine meta-
bolism of the hindlimb as an example,

(EAconcAblood flow)


(EVconcVblood flow)

= total label uptake (2.4)

and

(concAblood flow)


(concVblood flow)

= net amino acid balance (2.5)

where EAand EVare the isotopic enrich-
ment of a phenylalanine tracer and concA
and concVare the concentration of the
tracee phenylalanine in arterial and venous
blood, respectively. Based on Equations 2.4
and 2.5, protein synthesis = total label
uptake ÷EA, and protein degradation =
(total label uptake ÷EA)
net amino acid
balance.
There are problems with this method.
One is the definition of the isotopic enrich-

ment of protein synthesis and oxidative

compartments. Another problem is the

accurate measurement of amino acid con-

centrations and isotopic enrichments and,

finally, the accurate measurement of blood

flow. However, despite these problems, this

method has great promise in large animals.

Indirect measurement of protein breakdown

The fractional breakdown rate of muscle

protein can be estimated if the fractional

accretion rate of muscle protein is known

(fractional breakdown rate = fractional

synthesis rate 
fractional accretion rate,

FBR = FSR 
FAR) (Millward et al., 1975).

This approach to the study of protein

degradation is somewhat unsatisfactory.

The main problem with this method arises

from the time scale of measurements.

Synthesis is measured over a period of

minutes or hours, while growth is

integrated over the day and measured over

a period of days or months; thus, estima-

tion of protein synthesis will vary over the

course of the day and before and after a

meal. These changes in protein synthesis

could, in turn, grossly over- or under-

estimate the degradation rate.

A more direct approach to measure

muscle degradation is the ‘tracee release

method’ (Zhang et al., 1996). This approach

involves infusing a labelled amino acid to

an isotopic equilibrium and then observing

the isotopic decay in arterial blood and the

muscle intracellular pool. The FBR is cal-

culated as the rate at which tracee dilutes

the intracellular enrichment. This method

can be combined with a tracer incorpora-

tion method in order to measure both the

FSR and FBR in the same study.

Measurement of muscle protein breakdown:

3-methylhistidine metabolism

3-Methylhistidine: historical background

Tallen et al.(1954) were the first to identify

3MH as a component in urine in 1954, but

they were not sure what the source of 3MH

was. The metabolism of 3MH was first

Measurement and Significance of Protein Turnover 33