Gel
Filtration - Enzymic characteristics of fat globule membranes from
bovine colostrum and bovine milk
JT Powell, U
Jarlfors and K Brew
Fat globule membranes have been isolated from bovine colostrum and
bovine milk by the dispersion of the fat in sucrose solutions at 4
degrees C and fractionation by centrifugation through discontinuous
sucrose gradients. The morphology and enzymic characteristics of the
separated fractions were examined. Fractions comprising a large proportion
of the total extracted membrane were thus obtained having high levels
of the Golgi marker enzymes UDP-galactose N- acetylglucosamine beta-4-galactosyltransferase
and thiamine pyrophosphatase. A membrane-derived form of the galactosyltransferase
has been solubilized from fat and purified to homogeneity. This enzyme
is larger in molecular weight than previously studied soluble galactosyltransferases,
but resembles in size the galactosyltransferase of lactating mammary
Golgi membranes. In contrast, when fat globule membranes were prepared
by traditional procedures, which involved washing the fat at higher
temperatures, before extraction, galactosyltransferase was not present
in the membranes, having been released into supernatant fractions,
When the enzyme released by this procedure was partially purified
and examined by gel filtration, it was found to be of a degraded form
resembling in size the soluble galactosyltransferase of milk. The
release is therefore attributed to the action of proteolytic enzymes.
Our observations contrast with previous biochemical studies which
suggested that Golgi membranes do not contribute to the milk fat globule
membrane. They are, however, consistent with electron microscope studies
of the fat secretion process, which indicate that secretory vesicle
membranes, derived from the Golgi apparatus, may provide a large proportion
of the fat globule membrane.
Articles
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