Mammary origin of rat milk ribonuclease

When assayed under a variety of ionic conditions rat milk RNAase activity was highest under added Ca2+, while the serum enzyme was highest in no added ion, suggesting that milk was higher in the mammary-specific, Ca2+-stimulated, RNAase than was the serum of lactating rats. The acrylamide gel electrophoresis results demonstrated that milk RNAase differed from serum RNAase both in distribution and ionic preference; all milk RNAase species strongly preferred Ca2+ while serum RNAase species had no ionic preference. The Sephacryl S200 separation accentuated these differences by showing that 98% of the total milk RNAase activity was Ca2+-stimulated and clustered in a region of higher molecular weight than the serum enzyme which again had no ionic preference. From these results, we conclude that the vast majority of milk RNAase molecules are not transported by the serum, but must originate in the alveolar cells.