Lowering the lactose content of milk in vivo has
two potential benefits: to develop
milk products for lactose intolerant people (Delmont, 1983) and to produce
a more
concentrated milk, reducing milk storage volume and milking frequency without
affecting the overall fat and protein yields. Lactose synthesis in the
lactating
mammary gland results from the interaction of α-lactalbumin
(α-la) with a Golgian
UDP-galactosyltransferase (EC 2.4.1.22), and it has been suggested that
lactose and
α-la contents are directly related (Fitzgerald et al.
1970). Evidence that α-la is the
only protein responsible for the in vivo induction of
lactose synthesis came recently
from knock-out experiments of the relevant gene (Stinnakre
et al. 1994; Stacey et al.
1995), which disrupt lactation, as lactose-deprived milk of
α-la-deficient mice is
highly viscous. However, although the lactose, fat and protein contents
of the milk
were found to be directly related to the quantity of α-la present
(Stinnakre et al.
1994), only in mice homozygous for the α-la null allele was there
a significant
alteration of milk concentration (Stacey et al. 1995).
Stacey et al. (1995) also
demonstrated that human α-la could substitute for its mouse counterpart
via
homologous recombination.
In the present study, we report the generation of transgenic mice producing
only
bovine α-la by back-crossing animals from four independent
bovine α-la transgenic
lines with α-la-deficient mice. Our work confirms that foreign
α-la can substitute for
the mouse protein and suggests a negative correlation between the
α-la content and protein and fat concentrations in milk.