Chemistry Reference
In-Depth Information
Husband (1922) were probably the first to suggest that the quantity of lactose
produced by the mammary gland controls the daily volume of the milk.
Koestler (1920) used the ratio of lactose and chloride as a method to indicate
normal and mastitic (abnormal) milk. The Koestler number is given by (100
chlorine %)/lactose %. Normal milk has a Koestler number less than 3 while
mastitic milk is considerably higher (e.g. 15). One of the first studies of the
possible mechanisms involved in the secretion of Ca and phosphate in milk was
reported by Wright (1928).
The large amounts of phosphate required by the suckling for normal
growth and development are also supplied through the milk in at least three
chemical forms, namely free inorganic orthophosphate in solution, colloidal
phosphate associated with Ca in micellar Ca phosphate and the ester phos-
phate of the caseins. The major pathway for phosphate secretion into milk is
believed to be the Golgi vesicle route by a Na
+
-P
i
co-transport mechanism
(Shennan and Peaker, 2000). Holt (1985) described another possible mechan-
ism by which phosphate is generated in the Golgi lumen by hydrolysis of
UDP during lactose synthesis (Kuhn and White, 1977). This uridine nucleo-
tide cycle involves UDP-galactose and glucose. Within the vesicle, these
precursors form UDP and lactose. The UDP cannot cross the vesicle mem-
brane unless hydrolysed to UMP and inorganic phosphate, both of which can
re-enter the cytosol, avoiding product inhibition of lactose synthetase. How-
ever, the widely varying concentrations of lactose found in milks of different
mammalian groups suggest that other routes for P
i
transport across the Golgi
membrane may exist conjointly with the UDP hydrolysis mechanism. These
may involve the ATP-driven Ca
2+
pump discussed above, driving the accu-
mulation of Ca
2+
and actively participating in the phosphorylation of casein.
Here, P
i
is a by-product of the hydrolysis of ADP produced in that phosphor-
ylation reaction (Shennan and Peaker, 2000).
Citrate concentration in milk varies widely throughout lactation (Banks
et al., 1984). In general, citrate levels are higher during the grazing season
(Holt and Muir, 1979) and during early lactation (Braunschweig and Puhan,
1999; Garnsworthy et al., 2006). In studies on the goat, Linzell et al. (1976)
found that the mammary epithelium is impermeable to citrate in both direc-
tions, suggesting that citrate is synthesized within the secretory cells and
released into milk after exocytosis of Golgi vesicles. Citrate has an indirect
role in fat synthesis by providing reducing equivalents in the form of
NADPH, which are required for de novo synthesis of fatty acids (Faulkner
and Peaker, 1982). Citrate is in equilibrium with iso-citrate which is converted
to
-ketoglutarate in the production of NADPH. Thus, increased de novo
synthesis of fatty acids is predicted to lead to a decrease in citrate concentra-
tion. Such a correlation was found in the studies of Banks et al. (1984) who
