Studies on [formula omitted] ATPase XXXII. Occurrence and properties of [formula omitted] in immature, lactating and involuted guinea pig mammary gland

Abstract

In homogenates of immature, lactating and involuted mammary gland tissue of the guinea pig the presence of (Na +−K +)-ATPase activity (ATP phosphohydrolase, EC 3.6.1.3) has been established in activities of 0.09, 0.23 and 0.20 mole/kg dry weight per h, respectively. The properties of the enzyme have been determined in a microsomal fraction of the lactating mammary gland tissue. Half-maximal activities are 1.6 mM K +, 8 mM Na + and 1 mM Mg 2+, respectively. Optimal activities are obtained at 7.8 and 2 mM ATP and K +, Na + and Mg 2+ concentrations of 4 mM 40 mM and 4 mM, respectively. The (Na +−K +)-ATPase activity is totally inhibited by 10 −4 M ouabain, while 50% inhibition occurs with 2.5·10 −6 M ouabain. In the immature, lactating and involuted gland the DNA, phospholipid and protein contents have been determined on a dry weight basis. All three parameters are lowest in the immature gland due to the presence of a large amount of fat tissue. During involution the DNA content increases by 60%, while the phospholipid content and to a lesser extent the protein content decrease, indicating cell shrinkage and loss of membrane material. The (Na +−K +)-ATPase activity, expressed on a DNA basis (proportional to activity per cell) is the same in the immature and in the lactating gland. It is 43% lower in the involuted gland, indicating a lower cation pump activity in the resting cell. The Mg 2+-ATPase activity on DNA basis is highest in the lactating gland, and about double that in the immature and involuted gland. Total phospholipids and individual phospholipids, excluding phosphatidylserine and sphingomyelin, decrease by 60–75% upon involution, indicating a decrease in mitochondrial membrane content and little change in the plasma membrane content of the cell. Mg 2+-ATPase activity, which is present in plasma membranes and mitochondrial membranes, is about the same in all three states when expressed on a total phospholipid basis. During involution (Na +−K +)-ATPase activity, which is present in plasma membranes only, decreases to the same extent when expressed on a DNA basis as on a phosphatidylserine or sphingomyelin basis.