Farm Animal Metabolism and Nutrition

red blood cell. The distribution of amino
acids between the two blood compartments
may vary across tissues. For example, it
has been suggested that amino acids
absorbed across the intestinal wall are
transported to the liver in the plasma pool
(Elwyn et al., 1972; Houlier et al., 1991;
Lobley et al., 1996) whereas erythrocytes
may be more important in the transport of
amino acids from the liver in non-ruminant
species (Elwyn et al., 1972). For ruminant
animals, the data are conflicting; Houlier et
al. (1991) have shown a similar differential
distribution across the liver of cattle which
has not been observed in studies with
sheep (Lobley et al., 1996).

The role of low-molecular weight peptides

The importance of low-molecular weight
peptides in the transport of free amino
acids (FAAs) is equivocal. Various
analytical techniques have been employed
to determine the concentration of peptide-

bound amino acids (PBAAs) in plasma,

and the results of these procedures indicate

that the PBAA:FAA ratio in plasma varies

between 0.4 and 3.6 depending on the

species and the analytical technique used

(Seal and Parker, 1997). There is consider-

able interest in the potential use of low-

molecular weight peptides as nutritional

substrates in parenteral nutrition (Stehle et

al., 1997) and there is convincing evidence

that mixtures of peptides administered in

this way can be hydrolysed and utilized by

different tissues in several species (Albers

et al., 1988; Druml et al., 1991; Stehle et

al., 1997). These studies suggest that

hydrolysis of small synthetic peptides and

utilization of the constituent amino acids

can occur. It may be possible for PBAAs to

be hydrolysed within specific tissues or

organs, with the constituent amino acids

then being available for intracellular use.

The release of peptides back out of the cell

during the process of intracellular protein

turnover may also account for the apparent

change in peptide composition observed

50 C.J. Seal and D.S. Parker

Table 3.1.Distribution of individual amino acids between plasma and erythrocytes.

Whole blood:plasma ratio
Elwyn Felig et al. Aoki et al. Koeln et al. Heitmann and Lobley et al.
(1966)a (1973) (1976)b (1993)c Bergman (1980) (1996)a
Amino acid Dog Humans Humans Calves Sheep Sheep

Histidine 2.21 ND 1.67 1.38 ND 2.55
Isoleucine 1.51 1.06 1.63 0.11 0.95 0.95
Leucine 1.54 1.03 1.65 0.20 1.11 1.48
Lysine 3.59 ND 1.59 0.44 1.13 2.75
Methionine 1.19 0.84 ND 0.56 ND 0.34
Phenylalanine 1.88 1.02 1.63 0.08 1.26 1.03
Threonine 1.59 1.23 1.73 0.40 1.20 1.16
Valine 1.42 1.06 1.64 0.11 0.99 1.02
Alanine 1.44 1.29 1.98 0.91 0.95 1.26
Arginine 4.08 ND 1.49 ND 0.72 ND
Aspartate 9.85 ND ND 1.63 ND 4.24
Glutamate 4.11 ND 2.14 0.20 0.83 3.77
Glycine ND 1.68 2.49 0.42 1.24 1.63
Proline 1.62 1.15 1.61 ND ND 1.41
Serine 2.08 1.38 2.11 0.28 1.26 1.53
Tyrosine 3.34 1.13 1.80 0.34 1.19 1.17

aErythrocyte:plasma ratio.
bData from fasted humans.
cData from fed calves.

ND, not determined.