Membranes of mammary gland: X. Adenosine triphosphate dependent calcium accumulation by golgi apparatus rich fractions from bovine mammary gland

Abstract

Golgi apparatus rich fractions from lactating bovine mammary gland had an Mg 2+-dependent, Ca 2+-stimulated adenosine triphosphatase. These Golgi apparatus fractions also accumulated Ca 2+ in vitro. Accumulation of Ca 2+ required ATP and could be abolished by treatment either with low concentrations of deoxycholate followed by ultrasound, or by heating at 100 °C for 10 min. The adenosine triphosphatase activity of Golgi apparatus was strongly stimulated by low concentrations of Ca 2+ and moderately stimulated by high concentrations of K +. This activity was unaffected by Na + and was not inhibited by ouabain. The pH optimum for the Mg 2+-dependent hydrolysis of ATP was 7.5, the K m was 5 × 10 −5 M and the activation energy was 6 000 calories/mole. This Mg 2+-dependent adenosine triphosphatase activity was also found in rough endoplasmic reticulum, smooth microsomes and milk fat globule membrane, the latter membrane being derived directly from the apical plasma membrane. All of these membrane fractions had the ability to specifically accumulate Ca 2+. Specific accumulation was highest with smooth microsomes and lowest with milk fat globule membrane with Golgi apparatus and rough endoplasmic reticulum being intermediate. These observations provide one plausible explanation for intracellular Ca 2+ accumulation and secretion into milk. Further, these results help explain the ultrastructural observations of casein micelle formation in secretory vesicles elaborated by Golgi apparatus.