Farm Animal Metabolism and Nutrition

They analysed the data using the modified
three-compartment model depicted in Fig.
2.3.
Improvements were made to the model
by Waterlow and colleagues (Picou and
Taylor-Roberts, 1969; Waterlow et al.,
1978) by introducing the constant infusion
steady-state approach. This approach
reduces the number of samples and simpli-
fies the mathematics required. The general
concept is depicted in Fig. 2.4. There is a
metabolic pool of N into which amino
acids enter from the diet (I) and from the

breakdown (B) of body protein. N in the

form of amino acids is synthesized into

protein(s) or can be excreted into the urine.

The [^15 N]glycine model is based on the

following assumptions: (i) the metabolic

pool of N remains constant during tracer

infusions; (ii) [^15 N]glycine is not recycled;

(iii) the three-pool model is correct; (iv)

exogenous^15 N is metabolized in a similar

manner to endogenous and exogenous N;

(v) synthesis and excretion are the major

pathways of N disposal; and (vi) amino

acids from breakdown and the diet are

handled in the same way.

The method involves the administra-

tion of [^15 N]glycine (intravenously or

orally) at a continuous rate until a plateau

in the^15 N enrichment is achieved. This

usually takes 20–40 h. However, the time

to reach plateau is greatly reduced with a

primed constant infusion (Jeevanandam et

al., 1985). Urine samples are taken at the

plateau. The enrichment of [^15 N]urea is

then determined. At steady state (when the

rate of amino acids entering the metabolic

pool is equal to the rate at which they

leave), the equation for the model is

Q = I + B = S + E. (2.1)

Measurement and Significance of Protein Turnover 29

Protein

S

Free AA pool

BUE

Excretion

Total body protein

S

Metabolic pool N

B

(^15) N-glycine (F)
E (Urea + non-urea)
Food I
Protein
S
Free AA pool
B
D
E
Fig. 2.2.Simple model of whole-body protein
metabolism. S = protein synthesis; B = protein
breakdown; D = dietary intake; E = nitrogen
excretion.
Fig. 2.4.Picou and Taylor-Roberts (1969) model of [^15 N]glycine protein metabolism. I = intake of dietary
protein; F = infusion of [^15 N]glycine; B = protein breakdown; S = protein synthesis; E = excretion of both
urinary urea and non-urea end-products.
Fig. 2.3.Three-pool model of Rittenberg to describe protein metabolism. S = protein synthesis; B = protein
breakdown; U = urea synthesis; E = nitrogen excretion.