Study of B Particles in Stomach Milk of Nursing Mice2

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Abstract
With procedures effective for isolation of B particles from breast milk, no particles were found in extracts of coagulated stomach milk from suckling mice nursing on mothers containing the mammary tumor agent. Treatment at 37° C for ½ hour of coagulated stomach milk slurries with stomach juices, trypsin, chymotrypsin, or pepsin yielded extracts from which B particles could be sedimented. Morphologically and in their buoyant densities in cesium chloride these B particles were indistinguishable from those obtained from breast milk. The activity of stomach milk was associated with the supernatant fraction after ultracentrifugation of buffered extracts of stomach milk. The washed sediments containing B particles were inactive. These results indicate the active principle in stomach milk is a molecule unsedimentable after 1 hour at 100,000 × g.