Farm Animal Metabolism and Nutrition

Chapter 3

Inter-organ Amino Acid Flux

C.J. Seal and D.S. Parker^1

Department of Biological and Nutritional Sciences, University of Newcastle,

Newcastle upon Tyne, UK

(^1) Present address: Novus Europe s.a./n.v., Brussels, Belgium

Introduction

The integration of nitrogen metabolism in
mammalian systems involves the complex
interaction of several key processes which
link together in a coordinated manner
between, within and across individual
tissues and organs. The principal role of
these processes is the maintenance of
finely tuned nitrogen cycles to meet the
body’s requirements for tissue mainten-
ance, turnover and growth for any fixed
level of dietary nitrogen intake. These
processes include tissue protein turnover
and degradation, salvage of the end-
products of nitrogen metabolism, and the
transport and redistribution of the end-
products of nitrogen metabolism around
the body. This chapter focuses on the role
of amino acids in these processes and will
consider transport of amino acids between
tissues and organs in addition to flux
across tissue beds. Quantitatively, muscle
protein constitutes the largest pool of
amino acids present in the body, in the
form of protein and peptide-bound amino
acids. The concentration of free amino
acids in extracellular and intracellular
compartments and blood is much smaller,

but nevertheless contributes the major

route by which amino nitrogen is trans-

ferred between metabolic compartments.

Transport of Amino Acids

Distribution between erythrocytes

and plasma

Analysis of the distribution of individual

amino acids between plasma and erythro-

cytes (Table 3.1) shows that the concentra-

tion of many amino acids is similar in both

compartments and that there is generally

good agreement between species in the

relative distribution of individual amino

acids. For some amino acids, notably

aspartate, glutamate, glycine, histidine and

lysine, the concentration of each amino acid

in plasma is considerably less than that

inside the red blood cell. Since Na+-depen-

dent concentrative uptake is not present in

red blood cells (Young and Ellory, 1977),

this indicates net production or synthesis

of these amino acids within the erythro-

cyte. In contrast, arginine and glutamine

are more concentrated in plasma, suggest-

ing catabolism of these amino acids in the

© CAB International2000. Farm Animal Metabolism and Nutrition

(ed. J.P.F. D’Mello) 49