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8705189 
Journal Article 
A high affinity calcium-stimulated magnesium-dependent adenosine triphosphatase in rat adipocyte plasma membranes 
Pershadsingh, HA; McDonald, JM 
1980 
Yes 
Journal of Biological Chemistry
ISSN: 0021-9258
EISSN: 1083-351X 
255 
9 
4087-4093 
English 
Two different calcium-stimulatable adenosine triphosphatase (ATPase) activities were identified in rat adipocyte plasma membranes. One was a calcium-stimulated magnesium-dependent ATPase ((Ca2+ + Mg2+)-ATPase) which possessed a high affinity for calcium and characteristics consistent with those of a calcium extrusion pump. The other was a kinetically distinct low affinity nonspecific divalent cation ATPase. The (Ca2+ + Mg2+)-ATPase had an apparent half-saturation constant of 0.14 ± 0.02 μM for calcium, a maximum velocity of 97 ± 23 nmol of P(i) released/mg of protein/min, and a Hill number of 1.5 ± 0.2. Maximum activity was obtained between 0.5 and 1.0 μM free calcium. The distribution of this enzyme among the particulate membranous fractions was similar to that of the plasma membrane marker enzyme 5'-nucleotidase, indicating that it is located in the plasma membrane. The magnesium requirement of the (Ca2+ + Mg2+)-ATPase was maximal at 5 μM magnesium and was demonstrated by utilizing ion buffers containing trans-cyclohexane-1,2-diamine-N,N,N',N'-tetraacetic acid which has high affinities for both calcium and magnesium. Other properties of the high affinity (Ca2+ + Mg2+)-ATPase include: 1) high and low affinity sites for ATP,2) a physiological pH optimum of 7.5,3) insensitivity to sodium (20 mM), potassium (20 mM), and ouabain (0.5 mM), and 4) complete inhibition by 0.1 mM lanthanum. The characteristics of this enzyme bear a close resemblance to those of the high affinity (Ca2+ + Mg2+)-ATPase of human erythrocyte plasma membranes which is known to drive a calcium extrusion pump. By analogy, it is proposed that the high affinity (Ca2+ + Mg2+)-ATPase of adipocyte plasma membranes represents a similar calcium extrusion pump which could play a critical role in cellular calcium homeostasis.