PROPERTIES OF [CA2+ + MG2+]-ADENOSINE TRIPHOSPHATASES IN THE GOLGI-APPARATUS AND MICROSOMES OF THE LACTATING MAMMARY-GLANDS OF COWS

The [Ca2+ + Mg2+]-ATPase activity of bovine lactating mammary gland is associated with membranes. This study compares the ATPase activity in microsomal membranes to that of the Golgi apparatus. The enzyme activity in both fractions hydrolyzed Ca2+-ATP and Mg2+-ATP. The ATPase activities were inhibited by p-chloromercuribenzoate, indicating the involvement of a sulfhydryl group for activity. Although calmodulin had no effect on the ATPase activities of the two fractions, calmodulin antagonists (chlorpromazine, fluphenazine, and trifluoperazine) were inhibitory. Strong inhibitors of the ATPase activities were vanadate, dicyclohexylcarbodiimide, La3+, and Zn2+. There were some differences in the activities from two membrane fractions. Although both fractions could hydrolyze all of the triphosphonucleotides, cytidine-5'-triphosphate and uridine-5'-triphosphate were poor substrates for the Golgi enzyme. Detergents diminished the activity of the microsomal enzyme to a much greater extent than the ATPase of the Golgi apparatus. Thus, the intact membrane may be more critical to microsomal activity. The role of these enzymes in Ca2+ accumulation in milk is discussed.