Farm Animal Metabolism and Nutrition

Chapter 2

Measurement and Significance of

Protein Turnover

J.A. Rathmacher

Metabolic Technologies Inc., Ames, Iowa, USA

Introduction

The accretion of body proteins is the net
result of both the synthesis and breakdown
of protein. The dynamic nature of protein
metabolism has been known for 60 years
thanks to the pioneering work of
Schoenheimer and others (Schoenheimer
et al., 1939). Using stable isotopes of amino
acids, they demonstrated that proteins con-
tinually were being broken down and
resynthesized. In addition, they reported
that different organs have different rates of
protein synthesis. The dynamic process by
which body proteins are continually
synthesized and broken down is protein
turnover.
Studies on the growth of body protein
stores and the metabolism of protein have
been a major area of research. For example,
a major reason for this is the dramatic fact
that up to 20–25% of the muscle protein
can be broken down per day early in the
life of humans and farm animals. This rate
slows with age to 1–2% day
^1 in adults.
Rates of synthesis and breakdown are influ-
enced not only by age, but by plane of
nutrition, stress, disease, hormones, exer-
cise and inactivity.

Nomenclature

The nomenclature employed in studying

growth and protein metabolism is rela-

tively straightforward, but it is a worth-

while exercise to define briefly the terms

that will be used in this chapter.

● Synthesis. The conversion of amino

acids into proteins by the protein

synthetic apparatus in the cytoplasm in

the cell.

● Breakdown. The proteolysis of polypep-

tides by different proteinases within

both the cytoplasmic and lysosomal

compartments of the cell. The terms

breakdown and degradation are used

interchangeably.

● Growth. The net accumulation of

protein that occurs when the rate of

synthesis is greater than the rate of

breakdown. The term growth is often

used interchangeably with accretion or

net synthesis.

● Wasting. The loss of protein that occurs

when the rate of breakdown is greater

than the rate of synthesis.

● Turnover. A general term that involves

both synthesis and breakdown. However,

© CAB International2000. Farm Animal Metabolism and Nutrition

(ed. J.P.F. D’Mello) 25