BIOSYNTHESIS AND SECRETION OF CERULOPLASMIN BY MAMMARY-GLAND CELLS INLACTATING RATS

This work was intended to isolate and characterize ceruloplasmin (CP) found in rat milk and to determine the site of its synthesis. Electrop horesis and immunoelectrophoresis showed that rat milk contains CP at a concentration of 20 mg per 100 mi. The milk contained only one molec ular form of CP whose molecular weight, enzymatic activity, and antige nic properties are fully identical with serum CP. Dot-hybridization sh owed the concentration of CP mRNA in the cells of adult rat mammary gl ands to be similar to 10-fold lower than in liver. The analysis of pol yribosomal RNA by RNA-cDNA blot-hybridization showed that mammary CP m RNA exists in a single 3.5 kb molecular form whose molecular weight is equal to that of rat liver predominant CP mRNA. The course of appeara nce of newly synthesized CP in rat milk suggests a serum-independent w ay of synthesis and secretion of the milk CP by mammary cells, as indi cated by pulse labeling of proteins in vivo and in vitro.