Farm Animal Metabolism and Nutrition

it is sometimes used to represent break-
down. In this chapter, it will be used to
describe protein metabolism, which
includes both protein synthesis and
protein breakdown.
● Fractional synthesis and breakdown.
When quantitating protein synthesis
and breakdown, one often expresses the
rates as per cent per day. This allows
one to compare different organs of
different sizes or different muscles of
different sizes.

Three intrinsic problems are associ-
ated with measuring protein synthesis and
breakdown in body tissues. Details of these
problems can be found reviewed elsewhere
(Bier, 1989). The goal of this chapter will
be to describe the methodology used to
quantitate indirect whole-body protein
turnover and direct measurement of tissue
protein synthesis and breakdown.
One of the key problems in studying
protein synthesis and degradation is that
labelled (radio-labelled or stable isotopes)
amino acids which are to measure their
incorporation into protein, or conversely,
release of amino acids from labelled
proteins, and the true enrichment or
specific activity of the amino acid or

metabolite are difficult to measure in an

experiment. The labelled amino acid has

various states in a cell, summarized in Fig.

2.1.

The amino acid can be transported

into a free amino acid pool. Once in the

cell, it can be transported back out,

degraded to other metabolites (depending

on the amino acid) or linked to a specific

aminoacyl-tRNA. From the tRNA pool, it

can be translated into protein by ribosomes

(detailed below) and eventually degraded

to free amino acids (mechanism described

below).

Even though it is known that protein

tissue accretion is influenced by both the

synthesis and degradation of protein,

protein synthesis and degradation are not

always measured simultaneously. However,

mechanisms controlling protein synthesis

and degradation are distinct (Reeds, 1989)

and, therefore, can be influenced indepen-

dently. Often breakdown is ignored,

especially when measuring direct tissue

protein turnover. For example, very signifi-

cant gains in muscle protein can be

increased by decreasing protein break-

down, if protein synthesis remains the

same. A 10% decrease in the fractional

breakdown rate of muscle will result in a

26 J.A. Rathmacher

Fig. 2.1.Pathway of uptake, utilization and reutilization of amino acids for protein synthesis and
breakdown.

Cell membrane

Extracellular pool

Free IC

amino acids

tRNA

amino acids

Protein

amino acids

Protein

Intracellular pool

AA AA AA-tRNA

Protein breakdown and

AA reutilization

α-OXO-derivative

CO 2 Etc.