Comparative properties of microsomes from cow's milk and from mammary gland. 1. Enzymic activities.

It has been well established that the occurrence of cancer of the mammary gland of certain strains of mice is greatly influenced by a factor which is transmitted to the young via the mother's milk (Bittner, 1936; see also Dmochowski, 1953; Greenstein, 1954). The nature of this 'nmilk factor' is still uncertain. Huseby, Barnum & Bittner (1950) found that a large proportion of the milkfactor activity of dispersions of mouse mammary gland was sedimented by centrifuging at 23 000 g for 1 hr. Particles of the size-range of viruses and of microsomes were observed in electron micrographs of the precipitate from the mammary gland and also in a precipitate obtained under similar conditions from mouse milk containing the milk factor. Huseby et al. (1950) therefore suggested that the milk factor might be associated with microsomes. However, they did not report any examination of milk known to be free of the factor, and they were apparently unaware of the particulate nature of casein in milk. Morton (1953, 1954a) showed that in addition to casein, which represents the major particulate component, normal cow's milk also contains enzymically active lipoprotein particles which he called 'milk microsomes'. Preliminary observations of the chemical and enzymic properties of microsomes from cow's milk and from mammary gland indicated that the milk particles probably originated directly from the secretory cells of the gland (Morton, 1954a). It was therefore suggested that the milk factor could be associated with microsomes and thus pass into mouse milk from the mammary gland (Morton, 1953). This and the following paper report investigations related to this hypothesis. The cow was chosen as the experimental animal because material could be obtained in relatively large quantities. It has been found that, although microsomes from milk and mammary gland are not identical, milk microsomes are probably derived from gland nicrosomes.