Dairy Chemistry And Biochemistry

MILK PROTEINS 20 1

Table 4.8 Some important differences between bovine and human milk proteins

Constituent Bovine Human

Protein concentration (YO)

Casein : NCN

Casein types

/?-Lactoglobulin

Lactotransferrin

Lysozyme

GI ycopeptides

NPN (as YO TN)

Taurine

Lactoperoxidase

Immunoglobulins (Ig)

Ig type

(colostrum)

3.5

80: 20

50% of NCN

Trace

Trace

Trace

3

Trace

High

Very high

IgG, > IgG, > IgA

ZSl = p > a,, = K

1

40: 60

None

20% of total N

Very high (6% TN;

High

20

High

Low

Lower

IgA > IgG, > IgG,

p > K > CiSly

3000 x bovine)

NCN, Non-casein nitrogen; NPN, non-protein nitrogen; TN, total nitrogen.

"A low level of ?,,-casein has recently been demonstrated in human milk (Martin et al., 1996).

4.14 Synthesis and secretion of milk proteins

The synthesis and secretion of milk proteins have been studied in consider-
able detail; reviews include Mercier and Gaye (1983), Mepham (1987) and
Mepham et al. (1992).

4.14.1 Sources of amino acids

Arteriovenous (AV) difference studies and mammary blood flow measure-
ments (Chapter 1) have shown that in both ruminants and non-ruminants,
amino acids for milk protein synthesis are obtained ultimately from blood
plasma but that some interconversions occur. The amino acids can be
divided into two major groups:

1. those for which uptake from blood is adequate to supply the require-
   ments for milk protein synthesis and which correspond roughly to the
   essential amino acids (EAA); and


2. those for which uptake is inadequate, i.e. the non-essential amino acids
   (NEAA).

Studies involving AV difference measurements, isotopes and perfused gland
preparations indicate that the EAA may be subdivided into those for which
uptake from blood and output in milk proteins are almost exactly balanced
(Group I) and those for which uptake significantly exceeds output (Group
11). Group I1 amino acids are metabolized in the mammary gland and
provide amino groups, via transamination, for the biosynthesis of those