Sources and upstream processing - Pharmaceutical Biotechnology: Concepts and Aapplications - page 116

Table 5.7 Typical annual milk yields as well as time lapse between generation of
the transgene embryo and fi rst product harvest (fi rst lactation) of indicated species

Species Annual milk yield (l) Time to fi rst production batch (months)

Cow 6000-9000 33-36

Goat 700-800 18-20

Sheep 400-500 18-20

Pig 250-300 16-17

Rabbit 4-5 7

• Ongoing supply of product is guaranteed (by breeding).

• Milk is biochemically well characterized, and the physicochemical properties of the major na-

tive milk proteins of various species are well known. This helps rational development of appro-
priate downstream processing protocols (Table 5.8).

Despite the attractiveness of this system, a number of issues remain to be resolved before it is
broadly accepted by the industry. These include:

• Variability of expression levels. Although in many cases the expression levels of heterologous

proteins exceed 1 g l 1 , expression levels as low as 1.0 mg l 1 have been obtained in some in-
stances.

• Characterization of the exact nature of the post-translational modifi cations the mammary sys-

tem is capable of undertaking. For example, the carbohydrate composition of tPA produced in
this system differs from the recombinant enzyme produced in murine cell culture systems.

• Signifi cant time lag between the generation of a transgenic embryo and commencement of

routine product manufacture. Once a viable embryo containing the inserted desired gene is
generated, it must fi rst be brought to term. This gestation period ranges from 1 month for rabbits
to 9 months for cows. The transgenic animal must then reach sexual maturity before breeding
(5 months for rabbits, 15 months for cows). Before they begin to lactate (i.e. produce the recom-
binant product), they must breed successfully and bring their offspring to term. The overall time

Table 5.8 Some physicochemical properties of the major (bovine) milk proteins

Serum
albumin Protein Caseins β-Lactoglobulin α-Lactalbumin IgG

Concentration
(g l 1 )

25 (total) 2-4 0.5-1.5 0.4 0.5-1.0

Mass (kDa) 20-25 18 14 66 150

Phosphorylated? Yes No No No No

Isoelectric point Vary 5.2 4.2-4.8 4.7-4.9 5.5-8.3

Glycosylated? K-casein only No No No Yes